INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY
ENVIRONMENTAL HEALTH CRITERIA 151
SELECTED SYNTHETIC ORGANIC FIBRES
This report contains the collective views of an international group of
experts and does not necessarily represent the decisions or the stated
policy of the United Nations Environment Programme, the International
Labour Organisation, or the World Health Organization.
Published under the joint sponsorship of
the United Nations Environment Programme,
the International Labour Organisation,
and the World Health Organization
First draft prepared by Dr M.E. Meek,
Environmental Health Directorate, Health
and Welfare, Ottawa, Canada
World Health Orgnization
Geneva, 1993
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WHO Library Cataloguing in Publication Data
Selected synthetic organic fibres.
(Environmental health criteria ; 151)
1.Carbon - adverse effects 2.Nylons - adverse effects
3.Polyenes - adverse effects 4.Environmental exposure
I.Series
ISBN 92 4 157151 9 (NLM Classification: QV 627)
ISSN 0250-863X
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CONTENTS
ENVIRONMENTAL HEALTH CRITERIA FOR SELECTED SYNTHETIC ORGANIC FIBRES
INTRODUCTION
1. SUMMARY
1.1. Identity, physical and chemical properties
1.2. Sources of human and environmental exposure
1.3. Environmental levels and human exposure
1.4. Deposition, clearance, retention, durability and
translocation
1.5. Effects on experimental animals and in vitro
test systems
1.6. Effects on humans
1.7. Summary of evaluation
2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, AND ANALYTICAL
METHODS
2.1. Identity, physical and chemical properties
2.1.1. Carbon/graphite fibres
2.1.2. Aramid fibres
2.1.3. Polyolefin fibres
2.2. Production methods
2.2.1. Carbon/graphite fibres
2.2.2. Aramid fibres
2.2.3. Polyolefin fibres
2.3. Sampling and analytical methods
3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE
3.1. Production
3.1.1. Carbon/graphite fibres
3.1.2. Aramid fibres
3.1.3. Polyolefin fibres
3.2. Uses
3.2.1. Carbon/graphite fibres
3.2.2. Aramid fibres
3.2.3. Polyolefin fibres
3.3. Emissions into the environment
3.3.1. Fibre emissions
3.3.2. Decomposition products
4. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE
4.1. Occupational environment
4.1.1. Carbon/graphite fibres
4.1.1.1 Production
4.1.1.2 Processing of composites
4.1.2. Aramid fibres
4.1.2.1 Production
4.1.2.2 End-use processing and processing
of composites
4.1.3. Polyolefin
4.1.3.1 Production
4.2. General environment
5. DEPOSITION, CLEARANCE, RETENTION, DURABILITY AND TRANSLOCATION
5.1. Introduction
5.2. Studies in experimental animals
5.2.1. Carbon/graphite fibres
5.2.2. Aramid fibres
5.2.3. Polyolefins
5.3. In vitro solubility studies
6. EFFECTS ON EXPERIMENTAL ANIMALS, AND IN VITRO TEST SYSTEMS
6.1. Experimental animals
6.1.1. Introduction
6.1.2. Carbon/graphite fibres
6.1.2.1 Inhalation
6.1.2.2 Intratracheal administration
6.1.2.3 Intraperitoneal administration
6.1.2.4 Dermal administration
6.1.3. Aramid fibres
6.1.3.1 Inhalation
6.1.3.2 Intratracheal administration
6.1.3.3 Intraperitoneal administration
6.1.4. Polyolefin fibres
6.1.4.1 Inhalation
6.1.4.2 Intratracheal administration
6.1.4.3 Intraperitoneal administration
6.2. In vitro studies
6.2.1. Carbon fibres
6.2.2. Aramid fibres
6.2.3. Polyolefin fibres
7. EFFECTS ON HUMANS
7.1. Carbon/graphite fibres
7.2. Aramid fibres
8. EVALUATION OF HUMAN HEALTH RISKS
8.1. Exposure
8.2. Health effects
9. CONCLUSIONS AND RECOMMENDATIONS FOR PROTECTION OF HUMAN HEALTH
10. FURTHER RESEARCH
10.1. Sampling and analytical methods
10.2. Exposure measurement and characterization
10.3. Human epidemiology
10.4. Toxicology studies
REFERENCES
APPENDIX 1. SUMMARY OF PATHOLOGY WORKSHOP ON THE LUNG
EFFECTS OF PARAARAMID FIBRILS AND TITANIUM
DIOXIDE
RESUME
RESUMEN
WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR SELECTED
SYNTHETIC ORGANIC FIBRES
Members
Dr D.M. Bernstein, Geneva, Switzerland
Dr J.M. Dement, Office of Occupational Health and Technical
Services, National Institute of Environmental Health Sciences,
Research Triangle Park, North Carolina, USA
Dr P.T.C. Harrison, Department of the Environment, London, United
Kingdom
Professor T. Higashi, Department of Work Systems & Health,
Institute of Industrial Ecological Sciences, University of
Occupational & Environmental Health, Kitakyishu-shi, Japan
Dr I. Mangelsdorf, Institute for Toxicology, GSF München,
München-Neuherberg, Germany
Dr E. McConnell, Raleigh, North Carolina, USA
Mrs M. Meldrum, Health and Safety Executive, Bootle, Merseyside,
United Kingdom
Professor M. Neuberger, Institute of Environmental Hygiene,
University of Vienna, Kinderspitalgasse, Vienna, Austria
(Chairman)
Dr R.P. Nolan, Environmental Sciences Laboratory, Brooklyn College,
Brooklyn, New York, USA
Dr V. Vu, Health Effects Branch, Health and Environmental Review
Division, Office of Pollution Prevention and Toxics, US
Environmental Protection Agency, Washington, DC, USA
Mr R. Zumwalde, Division of Standards Development and Technology
Transfer, National Institute for Occupational Safety and Health,
Robert A. Taft Laboratories, Cincinnati, Ohio, USA
Observers
Dr T. Hesterberg, Health Safety and Environment Department, Mountain
Technical Center, Schuller International Inc., Littleton,
Colorado, USA
Dr E.A. Merriman, DuPont Fibers, Wilmington, Delaware, USA
Dr J.W. Rothuizen, Rothuizen Consulting, Genolier, Switzerland
Dr D.B. Warheit, E.I. Du Pont de Nemours and Co., Haskell Laboratory
for Toxicology and Industrial Medicine, Newark, Delaware, USA
Secretariat
Dr M.E. Meek, Environmental Health Directorate, Health Protection
Branch, Health and Welfare, Tunney's Pasture, Ottawa, Ontario,
Canada (Rapporteur)
Professor F. Valic, IPCS Consultant, World Health Organization,
Geneva, Switzerland; also Vice-Rector, University of Zagreb,
Zagreb, Croatia (Responsible Officer and Secretary)
NOTE TO READERS OF THE CRITERIA MONOGRAPHS
Every effort has been made to present information in the
criteria monographs as accurately as possible without unduly
delaying their publication. In the interest of all users of the
Environmental Health Criteria monographs, readers are kindly
requested to communicate any errors that may have occurred to the
Director of the International Programme on Chemical Safety, World
Health Organization, Geneva, Switzerland, in order that they may be
included in corrigenda.
* * *
A detailed data profile and a legal file can be obtained from
the International Register of Potentially Toxic Chemicals, Case
postale 356, 1219 Châtelaine, Geneva, Switzerland (Telephone
No. 9799111).
* * *
This publication was made possible by grant number 5 U01
ES02617-14 from the National Institute of Environmental Health
Sciences, National Institutes of Health, USA.
ENVIRONMENTAL HEALTH CRITERIA FOR SELECTED
SYNTHETIC ORGANIC FIBRES
A Task Group on Environmental Health Criteria for Selected
Synthetic Organic Fibres met at the British Industrial and
Biological Research Association (BIBRA), Carshalton, Surrey, United
Kingdom, from 28 September to 2 October 1992. Dr. S.E. Jaggers
opened the meeting on behalf of the host institute and greeted the
participants on behalf of the Department of Health. Professor F.
Valic welcomed the participants on behalf of the heads of the three
cooperating organizations of the IPCS (UNEP/ILO/WHO). The Task
Group reviewed and revised the draft monograph, made an evaluation
of the direct and indirect risks for human health from exposure to
the synthetic organic fibres reviewed, and made recommendations for
health protection and further research.
The first draft was prepared by Dr M.E. Meek, Environmental
Health Directorate, Health and Welfare, Ottawa, Canada. Professor
F. Valic was responsible for the overall scientific content and for
the organization of the meeting, and Dr P.G. Jenkins, IPCS, for the
technical editing of the monograph.
ABBREVIATIONS
CKSCC cystic keratinizing squamous cell carcinoma
FEV forced expiratory volume
GC gas chromatography
MMAD mass median aerodynamic diameter
MMMF man-made mineral fibre
MS mass spectrophotometry
PAN polyacrylonitrile
PCOM phase contrast optical microscopy
SEM scanning electron microscopy
SPF specific pathogen free
TGA thermogravimetric analysis
TWA time-weighted average
INTRODUCTION
This document is a review of occupational and environmental
exposure to selected synthetic organic fibres. The particular
fibres covered in this document have been selected because they are
the only ones for which some toxicological data are available, and
they are probably of most importance in terms of production volumes
and potential for human exposure. Considerations relating to
possible future uses and applications of synthetic organic fibres
have not been discussed, nor have the possible consequences of
exposure to secondary products, combustion products, etc. The Task
Group noted the complexity of the working environment in the
manufacture and use of synthetic organic fibres, which may involve
exposures to a range of chemical substances and non-fibrous dusts.
1. SUMMARY
1.1 Identity, physical and chemical properties
Carbon/graphite fibres are filamentary forms of carbon produced
by high-temperature processing of one of three precursor materials:
rayon (regenerated cellulose), pitch (coal tar or petroleum residue)
or polyacrylonitrile (PAN). Nominal diameters of carbon fibres range
between 5 and 15 µm. Carbon fibres are flexible and electrically
and thermally conductive, and in high performance varieties have
high Young's modulus (coefficient of elasticity measuring the
softness or stiffness of the material) and tensile strength. They
are corrosion resistant, lightweight, refractile and chemically
inert (except to oxidation), and have a high degree of stability to
traction forces, low thermal expansion and density, and high
abrasion and wear resistance.
Aramid fibres are formed by the reaction of aromatic diamines
and aromatic diacid chlorides. They are produced as continuous
filaments, staple and pulp. There are two main types of aramid
fibres, para- and meta-aramid, both with a nominal diameter of
12-15 µm. Para-aramid fibres can have fine-curled, tangled fibrils
within the respirable size range (< 1 µm in diameter) attached to
the surface of the core fibre. These fibrils may be detached and
become airborne upon abrasion during manufacture or use. Generally,
aramid fibres exhibit medium to very high tensile strength, medium
to low elongation, and moderate to very high Young's modulus. They
are resistant to heat, chemicals and abrasion.
Polyolefin fibres are long-chain polymers composed of at least
85% by weight of ethylene, propylene or other olefin units;
polyethylene and polypropylene are used commercially. Except for
some types such as microfibre, the nominal diameters of most classes
of polyolefin fibres are sufficiently large that few are within the
respirable size range.
Polyolefins are extremely hydrophobic and unreactive. Their
tensile strengths are considerably less than those of carbon or
aramid fibres and they are relatively flammable, melting at
temperatures between 100 and 200 °C.
Methods developed for counting mineral fibres have been used
for industrial hygiene monitoring of synthetic organic fibres.
However, these methods have not been validated for this purpose.
Factors such as electrostatic properties, solubility in mounting
media and refractive index may cause difficulties when using such
methods.
1.2 Sources of human and environmental exposure
The estimated worldwide production of carbon and graphite
fibres was in excess of 4000 tonnes in 1984. For aramid it was more
than 30 000 tonnes in 1989, and for polyolefin fibres more than
182 000 tonnes (USA only). Carbon and aramid fibres are used
principally in advanced composite materials in aerospace, military
and other industries to improve strength, stiffness, durability,
electrical conductivity or heat resistance. Polyolefin fibres are
typically used in textile applications.
Exposures to synthetic organic fibres have been documented in
the occupational environment. Synthetic organic fibres can be
released into the environment during production, processing or
combustion of composites and during disposal. Very few data are
available concerning actual releases of these materials into the
environment.
Available data on the transport, distribution and
transformation of organic fibres in the environment are restricted
to identification of products of municipal incineration of refuse
from carbon fibre-containing composites and pyrolysis decomposition
products of carbon fibre and aramids. During simulation of municipal
incineration, both the diameters and lengths of carbon fibres were
reduced. Principal pyrolysis decomposition products of carbon and
aramid fibres include aromatic hydrocarbons, carbon dioxide, carbon
monoxide and cyanides.
1.3 Environmental levels and human exposure
Synthetic organic fibre dusts are released in the workplace
during operations such as fibre forming, winding, chopping, weaving,
cutting, machining and composite formation and handling.
In the case of carbon/graphite fibres, respirable fibre
concentrations are generally less than 0.1 fibres/ml but
concentrations of up to 0.3 fibres/ml have been measured close to
chopping or winding operations. Fibres may also be released during
machining (drilling, sawing, etc.) of carbon fibre composites,
although most of the respirable material thus produced is
non-fibrous.
Average airborne concentrations of para-aramid fibrils in the
workplace are reported to be less than 0.1 fibrils/ml in filament
operations and less than 0.2 fibrils/ml in floc cutting and pulp
operations. During staple yarn processing, average airborne fibril
concentrations are typically below 0.5 fibrils/ml, but levels as
high as approximately 2.0 fibrils/ml have been reported. Other
end-use workplace exposures are typically below 0.1 fibrils/ml on
average with peak exposures of 0.3 fibrils/ml. Special potential for
exposure was demonstrated for waterjet cutting of composites, levels
being as high as 2.91 fibrils/ml. Particles of mean aerodynamic
diameter of 0.21 µm have been generated during laser cutting of
epoxy plastics reinforced with aramid fibres, but the fibre content
of the dust was not reported. Certain volatile organic compounds
(including benzene, toluene, benzonitrile and styrene) and other
gases (hydrogen cyanide, carbon monoxide and nitrogen dioxide) are
also produced during such operations.
Limited air monitoring data from a facility producing
polypropylene fibres indicate maximum airborne levels for fibres
longer than 5 µm of 0.5 fibres/ml, with most values being less than
0.1 fibres/ml. Scanning electron microscopy showed that airborne
fibre sizes range from 0.25 to 3.5 µm in diameter and 1.7 to 69 µm
in length. In a single ambient sample collected near a carbon fibre
weaving plant, a concentration of 0.0003 fibres/ml was detected.
The average dimensions of the fibres were 706 µm by 3.9 µm. The
release of carbon fibre at the crash site of two military aircraft,
following combustion of carbon fibre composite used in construction,
has also been reported. No other relevant information on
concentrations in the environment was identified.
1.4 Deposition, clearance, retention, durability and translocation
Few data on specific synthetic organic fibres have been ident
ified. The data on para-aramid fibres (Kevlar) indicate that, when
inhaled, these fibres are deposited at alveolar duct bifurcations.
There is also evidence of translocation to the tracheobronchial
lymph nodes.
1.5 Effects on experimental animals and in vitro test systems
For the synthetic organic fibre types reviewed here, there is a
dearth of good quality data from relevant experimental studies.
There are no adequate studies in which the fibrogenic or
carcinogenic potential of carbon/graphite fibres has been examined.
Effects following short-term inhalation exposure (days) of rats to
respirable-size pitch-based fibres included inflammatory responses,
increased parenchymal cell turnover and minimal type II alveolar
cell hyperplasia. Available data from an intratracheal instillation
and an intraperitoneal injection study are considered inadequate for
assessment owing to the lack of characterization of the test
materials and lack of adequate documentation of protocol and
results. A mouse skin painting study on four carbon fibre types
suspended in benzene was inadequate for the evaluation of
carcinogenic activity.
In the case of paraaramid fibres, the majority of data is
derived from experiments on Kevlar. Short-term (2 week) inhalation
studies of Kevlar dust have resulted in a pulmonary macrophage
response which decreased in severity after exposure ceased.
Short-term studies of ultrafine Kevlar fibrils have shown a similar
macrophage reaction and patchy thickening of the alveolar ducts.
Both lesions again decreased after exposure, but a minimal amount of
fibrosis was present 3-6 months later. A two-year inhalation study
of Kevlar fibrils in rats induced exposure-related lung fibrosis (at
> 25 fibres/ml) and lung neoplasms (11% at 400 fibres/ml and 6% at
100 fibres/ml in female rats; 3% at 400 fibres/ml in male rats) of
an unusual type (cystic keratinizing squamous cell carcinoma).
Increased mortality due to lung toxicity was observed at the highest
concentration, indicating that the Maximum Tolerated Dose had been
exceeded. There is consider able debate concerning the biological
potential of these lesions and their relevance to humans. The full
carcinogenic potential of the fibrils may not have been revealed in
this study because it was terminated after 24 months.
Intratracheal instillation of a single dose of shredded Nomex
paper (2.5 mg) containing fibres with diameters of 2 to 30 µm
resulted in a non-specific inflammatory response. A granulomatous
reaction developed two years post-exposure. Intratracheal
instillation of a single dose of 25 mg Kevlar resulted in a
non-specific inflammatory response which subsided within about one
week. A granulomatous reaction and a minimal amount of fibrosis were
observed later.
In three studies, intraperitoneal injection of Kevlar fibres
(up to 25 mg/kg) resulted in a granulomatous response but no
significantly increased incidence of neoplasms. It was suggested by
the authors of these investigations that the lack of neoplastic
response was possibly due to the agglomeration of the Kevlar fibrils
in the peritoneal cavity.
There are no adequate studies in which the fibrogenic or
carcinogenic potential of polyolefin fibres has been examined. A
90-day inhalation experiment in rats with respirable (46% < 1 µm)
polypropylene fibres (up to 50 fibres/ml) indicated dose- and
duration-dependent changes characterized by increased cellularity
and bronchiolitis. No relevant data on intratracheal instillation
are available. In intraperitoneal injection studies on polypropylene
fibres or dust in rats, there was no significant increase in
peritoneal tumours.
There are inadequate data on which to make an assessment of the
in vitro toxicity and genotoxicity of synthetic organic fibres.
For aramids, studies have shown that short and fine para-aramid
fibrils have cytotoxic properties. With regard to polyolefin fibres,
there is some evidence of cytotoxicity of polypropylene fibres.
Mutagenicity tests on extracts of polyethylene granules gave
negative results.
1.6 Effects on humans
In a cross-sectional study of 88 out of 110 workers in a
PAN-based continuous filament carbon fibre production facility,
there were no adverse respiratory effects, as assessed by
radiographic and spirometric examination and administration of
questionnaires on respiratory symptoms. In other less well
documented studies, adverse effects have been reported in workers
involved in the production of both carbon and polyamide fibres; data
presented in the published accounts of these investigations,
however, were insufficient to assess the validity of the reported
associations.
1.7 Summary of evaluation
Data concerning the exposure levels of most synthetic organic
fibres are limited. Those data available generally indicate low
levels of exposure in the occupational environment. There is a
possibility of higher exposures in future applications and uses.
Virtually no data are available with respect to environmental fate,
distribution, and general population exposures.
On the basis of limited toxicological data in laboratory
animals, it can be concluded that there is a possibility of
potential adverse health effects following inhalation exposure to
these synthetic organic fibres in the occupational environment. The
potential health risk associated with exposure to these synthetic
organic fibres in the general environment is unknown at this time,
but is likely to be very low.
2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, AND ANALYTICAL METHODS
2.1 Identity, physical and chemical properties
2.1.1 Carbon/graphite fibres
Carbon and graphite fibres are filamentary forms of carbon
produced by high temperature transformation of one of three organic
precursor materials: rayon fibres (regenerated cellulose), pitch
(coal tar or petroleum residue) or polyacrylonitrile (PAN) fibres.
Graphite fibres are materials characterized by a three-dimensional
polycrystalline structure (Volk, 1979). The typical temperature
range for graphite fibre production is 1900 to 3000 °C, while
carbon fibre is manufactured at about 1200 to 1300 °C (Martin et
al., 1989). In much of the literature the terms carbon and
graphite fibre are used interchangeably. Trade names of
carbon/graphite fibres include WCA, CCA-4, Magnamite, Thornel-T,
Celion, Panex, HITEX, Fortafil, Thornel-P and Carboflex (ITA, 1985;
ICF, 1986).
Rayon-based carbon fibres are very stable at high temperatures
and consist of over 99% carbon. PAN-based carbon fibres have a lower
carbon content (92 to 95%) and undergo further changes when heated
beyond the original process temperature. The carbon content of
pitch-based carbon fibres varies from 75% upward. Pitch-based
fibres with a Young's modulus of over 345 GPa are almost pure (over
99.5%) carbon.
The nominal diameters of carbon fibres have been reported to
range from 5 to 15 µm (Volk, 1979) and from 7 to 10 µm (Jones et
al., 1982; ICF, 1986) according to end use. The distribution of
fibre diameters is not available from the literature. Carbon fibres
are lightweight, refractile, and have high tensile strengths, high
Young's modulus (a measure of the stiffness of the material), a high
degree of dimensional stability and low thermal expansion. These
fibres are chemically inert and corrosion resistant, but oxidization
may occur at high temperatures. They have moderate electrical and
thermal conductivity (Volk, 1979). Carbon fibres are, however,
vulnerable to shearing due to a decrease in interlamellar shear
strength with increasing Young's modulus. The Young's modulus,
electrical and thermal conductivity, and tensile strength increase
with the degree of molecular orientation along the fibre axis (Volk,
1979). Strength and modulus values are greater at higher production
temperatures. However, fibres with the highest Young's modulus may
not have the highest tensile strength (Jones et al., 1982).
Information on the density, tensile strength and Young's
modulus of carbon and graphite fibres are presented in Table 1
(Volk, 1979).
2.1.2 Aramid fibres
Aramid fibres (aromatic polyamides) are produced by a two step
process involving production of the polymer followed by spinning.
The polymer is typically produced by the reaction of aromatic
diamines and aromatic diacid chlorides in an amide solvent. Aramid
fibres are defined as fibres in which the base material is a
long-chain synthetic polyamide in which at least 85% of the amide
linkages are attached directly to two aromatic rings (Preston,
1979). Two types of aramid fibres are produced by the DuPont
Company: Kevlar (para-aramid) and Nomex (meta-aramid), which differ
primarily in the substitution positions on the aromatic ring
(Preston, 1979; Reinhardt, 1980; Galli, 1981). Kevlar is made of
poly(p-phenyleneterephthalamide) and Nomex is made of
poly(m-phenyleneisophthalamide) (Preston, 1979). Fibres named Conex
and Fenilon, which have compositions similar to that of Nomex, have
also been developed in Japan (by Teijin) and the Russian Federation,
respectively (Preston, 1979). Arenka, now marketed as Twaron
(Holland/Germany), is a para-aramid fibre produced by Akzo. A
para-aramid fibre called Arimid is produced in the Russian
Federation.
Para-aramid fibres such as Kevlar/Twaron are produced as
continuous filament yarn, cut (staple) fibre (38-100 mm in length),
short fibre (6-12 mm in length) or pulp (2-4 mm in length), all with
a nominal diameter of 12-15 µm. However, abrasive processing will
produce some fibrils from para-aramid fibre. Pulp has many
fine-curled, tangled fibrils within the respirable size range
attached to the surface of the core fibre, and some of these fibrils
will break off and become airborne during manufacture or use. These
fibrils have a ribbon-like morphology and may have widths less than
1 µm. Figure 1 shows a scanning electron microscopy of ultrafine
Kevlar fibrils peeling off a 12-µm Kevlar fibre (Warheit et al.,
1992).
Meta-aramid fibres such as Nomex/Conex are manufactured as
continuous filament, staple fibre and short fibre, all with a
nominal diameter of approximately 12 µm. Unlike para-aramid fibres,
meta-aramids have no fibrillar substructure and do not tend to
produce smaller diameter fibrils upon abrasion (ICF, 1986).
Table 1. Physical and chemical properties of carbon/graphite fibresa
Fibre Density (g/cm3) Tensile strength (MPa) Young's modulus (GPa)
Rayon-based carbon fibres (low modulus) 1.43-1.7 345-690 21-55
Rayon-based carbon fibres (high modulus) 1.65-1.82 - 345-517
PAN-based carbon fibres 1.7-1.8 2400-2750 193-241
Pitch-based carbon fibres (filament yarn) 2.0 2000 345
a From: Volk (1979)
Generally, aramid fibres have medium to very high tensile
strength, medium to low elongation-to-break, and moderate to very
high modulus (Preston, 1979; Brown & Power, 1982). Meta-aramid fibre
of low orientation has a density of approximately 1.35 g/cm3 and
the hot-drawn fibre has a density of approximately 1.38 g/cm3.
Para-aramid fibres of relatively high crystallinity have a density
of 1.44 g/cm3. The volume resistivities and dielectric strengths
of these fibres are high, even at elevated temperatures (Preston,
1979). Aramid fibres are heat resistant, with mechanical properties
being retained at temperatures up to 300-350 °C (Preston, 1979).
Whole aramid fibres are generally resistant to chemicals, with the
exception of strong mineral acids and bases (Preston, 1979; Galli,
1981; Chiao & Chiao, 1982). The strength to weight ratio of Kevlar
is high; on a weight basis, it is five times as strong as steel, ten
times as strong as aluminum and up to three times as strong as
E-glass. Aramid fibres have excellent toughness, and withstand
continuous heat at temperatures in the 160-205 °C range. Aramids
will not melt or support combustion, and carbonization will not be
appreciable under 400 °C. Para-aramid fibres have a small negative
coefficient of longitudinal thermal expansion, similar to graphite,
making them suitable for joint usage in composites (Galli, 1981;
Hanson, 1980). Some physical and chemical properties of aramid
fibres are presented in Table 2 (Hodgson, 1989).
2.1.3 Polyolefin fibres
Polyolefin fibres are long-chain polymers composed (at least
85% by weight) of ethylene, propylene or other olefin units
(Buchanan, 1984). Although many different types of polyolefins are
produced, this term is commonly used only for hydrocarbon
polyolefins, polyethylene and polypropylene (Hartshorne & Laing,
1984). These are the fibres considered in this monograph.
Approximately 95% of all polyolefin fibres are polypropylene (Ahmed,
1982).
Structurally, the high-density polyethylenes consist of linear
methylene chains usually terminated by vinyl but occasionally by
methyl groups. The low-density analogue also contains methylene
chains, but has frequent branches. Chains are usually terminated by
methyl groups. An intermediate density polyethylene is also
available, together with blends of polyethylene with polypropylene
or polyisobutene for use mainly in ropes or twines. Trade names of
polyolefin fibres include BDH Low Density, Courlene C3, Courlene X3,
Courlene Y3, Downspun 82, Dyneema, Fibrite MF, Meraklon BCF,
Meraklon DO, Novatron, Polyolefine, Sanylene, and Spratra
(Hartshorne & Laing, 1984).
Table 2. Physical and chemical properties of aramid fibresa
Fibre type Density (g/cm3) Tensile strength Specific strength Young's modulus Flammability Approximate
(MPa) (MPa/d) (GPa) (LOI) at room thermal
temperature degradation (°C)
Kevlar/Twaron 1.44-1.45 2790-3000 2000 120-124 24.5 > 400
Nomex 1.38 - - - 26 > 370
a From: Hodgson (1989)
Polyolefin fibres and products are produced in eight different
forms (ICF, 1986):
1) monofilament yarn typically having a diameter of 150 µm or
more;
2) multifilament yarn similar in structure to monofilament yarn
but for which the diameter of individual filaments is generally
in the range of 5 to 20 µm has also been reported;
3) staple fibre, which is multifilament yarn cut into varying
lengths of up to a few millimeters and which may be used in
either woven or discontinuous form;
4) tape and fibrillated film yarn with a large rectangular
diameter resembling a ribbon; the thickness typically varies
from 2.5 to 12.7 µm for tape and 2.5 to 6.4 µm for fibrillated
film yarn;
5) spun-bonded fabric, i.e. non-woven fibrous structures produced
in the form of flat fabric in one continuous process directly
from the molten resin;
6) synthetic pulp - a relatively new class of discontinuous fibres
with diameters ranging from 5 to 40 µm and maximum lengths of
2.5 to 3 mm;
7) meltblown or microfibre - a relatively new type of polyolefin
fibre produced by the meltblown process for which the diameter
typically ranges from 0.1 to 2 µm and the length is a few
centimetres;
8) high strength, high modulus, highly oriented, high molecular
weight polyethylene fibres formed by spinning, drawing a
partially polymerized gel.
The specific gravities of polyethylene and polypropylene are
low and they are highly hydrophobic; properties of these materials
in wet conditions are, therefore, similar to those measured at
standard temperature and humidity (21 °C; 65% relative humidity).
The polyolefins are unaffected by a wide variety of inorganic acids
and bases and organic solvents at room temperature. The melting
point of low-density polyethylenes is less than 116 °C and of
high-density ones is above 131 °C. Medium-density polyethylene is
reported to melt at 125 °C, whereas polypropylenes melt at between
167 and 179 °C (Hartshorne & Laing, 1984). Some physical and
chemical properties of the polyolefins are presented in Table 3
(Hodgson, 1989).
2.2 Production methods
2.2.1 Carbon/graphite fibres
In general, the process of carbon and graphite fibre production
involves three distinct steps: preparation and heat treating,
carbonization, and optional high-temperature annealing (Volk, 1979).
Initially, the precursors (PAN, pitch or rayon) are oxidatively
stabilized and dehydrogenated at moderate tempera tures
(200-300 °C). For pitch-based fibres, the commercial coal or
petroleum pitch is converted through heat treatment into a mesophase
or liquid crystal state. The fibre is then carbonized at
temperatures of 750-1375 °C in a non-oxidizing atmosphere, and
production may involve a secondary heating phase, known as
graphitization, at approximately 1400 °C in an inert atmosphere.
Mesophase pitch-based fibres having a Young's modulus exceeding
690 GPa, made by heating without stretching to 3000 °C, are the most
graphitic in nature (Volk, 1979). To produce high modulus fibres,
yarn is stretched during the last heat treatment. Depending on the
configuration of the final product (e.g., chopped fibre, continuous
strand, felt or fabric) and intended use, the fibre may be treated
with a sizing material to improve its handling characteristics and
compatibility with various matrices. The formulation of sizing
compounds (protective coatings) is considered proprietary; it has
been reported, however, that they are typically compounds with
epoxy-based functionalities, resins, epoxides and aqueous systems
(ICF, 1986). Fibres may be chopped to lengths in the range of 3 to
25 mm for use in reinforced composite materials or milled down to
200 µm for special conductive applications. For composites, the
fibrous material is embedded in a matrix, such as an epoxy resin, to
form strong, lightweight engineering materials (Martin et al.,
1989).
2.2.2 Aramid fibres
Aramid polymers are made by solution polymerization. This
involves low temperature polycondensation of diacid chlorides and
diamines in amide solvents. Meta-aramid fibre is spun from the
polymerization solution of dimethylacetamide after neutralization.
Para-aramid polymer must first be neutralized and isolated from the
polymerization solution; it is then re-dissolved in a spinning
solution of concentrated sulfuric acid. This liquid crystalline
solution is then extruded through a spinneret, and the acid is
extracted and neutralized to form a highly-oriented fibre.
Table 3. Physical and chemical properties of polyolefin fibresa
Polymer Fibre type Density Young modulus Tensile strength Elongation to
(g/cm3) (GPa) (MPa) break (%)
Low-density polyethylene monofilament 0.92 0.8-1.0 - -
High-density polyethylene monofilament 0.95-0.96 1.7-4.2 - -
Polyethylene high density 0.96 1.7-4.2 290-570 10-45
filament
Gel-spun polyethylene filament - 44-77 2580-5500 -
Polypropylene staple and tow 0.90-0.96 0.28-3.3 - -
monofilament 0.90-0.91 1.6-4.8 - -
multifilament 0.90-0.91 1.2-3.2 - -
Polypropylene filament 0.91 1.6-4.8 270-540 14-30
high tenacity - - 811 -
a From: Hodgson (1989)
2.2.3 Polyolefin fibres
Low-density polyethylene is manufactured by a high-temperature
and high-pressure polymerization process, whereas the high-density
form is made at low temperatures and pressures using a more
efficient catalyst (e.g., the Marlex process). Intermediate density
polyethylene and polypropylene are produced with a catalyst by the
Ziegler process (Hartshorne & Laing, 1984).
There are several processes for the production of continuous
and discontinuous polyolefin fibres (ICF, 1986). Mono-filament and
multi-filament yarns are formed by the continuous extrusion of the
molten polymer through a spinneret, solidification by heat transfer,
and winding onto packages. Processing may include drawing of the
fibre to up to six times its original length, as well as heat
treatment to relieve thermal stress. Staple fibre is cut from
multi-filament yarn. Yarns are also formed by splitting a highly
oriented film. Flash spun fibres are formed by dissolving polyolefin
polymer under pressure and flashing from a spinneret to form fibres.
If done at high pressure the fibres are much shorter and can be used
as a synthetic pulp. Melt blown fibres are formed by drawing a spun
fibre in high pressure steam or air.
2.3 Sampling and analytical methods
Sampling and analytical methods for organic fibres include the
measurement of total airborne or respirable mass concentration and
the determination of airborne fibre number concentrations by phase
contrast optical microscopy (PCOM). Sampling methods used for
organic fibres are similar to those used for inorganic fibres such
as asbestos or man-made mineral fibres. These methods typically
involve drawing a measured volume of air through a filter mounted in
a holder that is located in the breathing zone of the subject. For
measurement of mass concentrations, either polyvinyl chloride or
glass fibre filters are normally used. The filters are stabilized in
air and weighed against control filters, both before and after
sampling, to permit correction of weight changes caused by varying
humidity. Cellulose ester membrane filters are usually used for
assessing fibre number concentrations. In this case, the filter is
made optically transparent with one of several clearing agents
(e.g., triacetin, acetone or ethylene glycol monomethyl ether), and
the fibres present in random areas are counted and classified using
PCOM (WHO, 1985; NIOSH, 1989 [Method 7400]). Different fibre
counting rules have been used in various countries and these may
give somewhat different values.
Although the basic methods for the determination of total
airborne mass and fibre number concentrations are similar in most
countries, differences in the sampling procedure, the filter size
and type, the clearing agent and the microscope used, and subjective
errors in sampling and counting all contribute to variations in
results. Specific reference methods for the determination of organic
fibres have not been developed. However, the methods mentioned
above have been used for routine industrial hygiene monitoring. A
WHO project is in progress to develop a reference method for the
measurement of health risk-related fibres in workplace air.
Several potential problems exist with respect to use of PCOM
sampling and analytical methods for organic fibres. Firstly, these
fibres have significant electrostatic charges that could affect
sampling efficiency. Secondly, some of these fibres may be soluble
in the microscope slide mounting medium, and their visibility in
PCOM has not been evaluated. Lastly, PCOM methods lack specificity
for counting organic fibres.
The improved resolution of electron microscopy and the
identification capacity particularly of the analytical transmission
electron microscope (TEM), with selected area electron diffraction
(SAED) and energy dispersive X-ray analysis (EDXA), make these
methods particularly suitable for more complete characterization of
the fibre size distribution and analysis of fibres of small diameter
(NIOSH, 1989 [Method 7402]). However, these methods have so far
rarely been used for analyses of organic fibres.
For analysis by scanning electron microscopy (SEM), fibres
collected on polycarbonate filters can be examined directly. This
avoids the need for transfer techniques that may affect the fibre
size distribution. For TEM, direct transfer preparation techniques
involving carbon coating of particles on the surface of a
polycarbonate or membrane filter and indirect transfer methods in
which attempts have been made to retain the fibre size distribution
are the most widely accepted.
3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE
3.1 Production
3.1.1 Carbon/graphite fibres
The principal producers of carbon and graphite fibres are
Japan, the USA and the United Kingdom, although these materials are
also manufactured in France, Germany and Israel (ITA, 1985; ICF,
1986). The raw material most commonly used is polyacrylo nitrile
(PAN); in Japan, the world's largest producer of PAN-based fibres,
the precursor material is obtained from the excess acrylic fibre
capacity of the Japanese petrochemical industry. The USA is the main
world producer of carbon/graphite fibres from rayon and pitch (ICF,
1986).
World production capacity was estimated to be 4173 tonnes in
1984. Based on information provided by manufacturers in the USA,
estimated world consumption for 1984 and 1987 was 889 tonnes and
2046 tonnes, respectively. Similar estimates based on information
provided by companies in Japan were 1068 tonnes and 1756 tonnes,
respectively. The USA and Japan are the principal consumers,
followed by western Europe (ITA, 1985). More recently, it has been
reported that the total world capacity for carbon fibres is 10 000
tonnes per annum, although the author noted that it was difficult to
assess what this means in terms of real annual output (Hodgson,
1989).
The estimated consumption of rayon-based carbon fibres from
1970 to 1976 was 50 to 125 tonnes (Volk, 1979). It has also been
estimated that the combined production of PAN-based carbon fibres in
the three principal producing countries (USA, Japan and the United
Kingdom) rose from 10 tonnes in 1970 to 250 tonnes in 1976 (Volk,
1979).
3.1.2 Aramid fibres
Kevlar and Nomex which are both produced by DuPont have been
sold commercially since 1970 and 1965, respectively (Reinhardt,
1980). The production capacity for Kevlar in 1978 was reported to be
approximately 3400 tonnes (Galli, 1981). In 1975, the production
capacity for Nomex was expanded from 4500 tonnes to more than 9100
tonnes (Preston, 1979). More recently, it has been reported that the
production capacity per year of Kevlar (USA) and Twaron
(Holland/Germany) is 20 000 and 5000 tonnes, respectively (Hodgson,
1989). Plants in Northern Ireland and Japan with annual production
capacities of about 5000 tonnes per year each have been brought on
line since then.
3.1.3 Polyolefin fibres
Production of polyolefin fibres has been increasing owing to
greater demand for continuous filament yarn, monofilaments and
staple fibres (Buchanan, 1984). Production and consumption in the
USA in 1983 were approximately 182 500 and 175 000 tonnes,
respectively (SRI International, 1985). The annual polyolefin fibre
production capacities of producers in the USA have been reported to
range from approximately 4500 kg to more than 134 000 kg (SRI
International, 1987).
3.2 Uses
Carbon and aramid fibres are used principally in advanced
composite materials to improve strength, stiffness, durability,
electrical conductivity or heat resistance. Since these fibres
improve properties such as these without adding much weight, they
are used principally in the aerospace industry, for military reasons
and in sports equipment manufacture (ITA, 1985). Polyolefin fibres
are typically used in textile applications, although gel spun fibres
are used for high tensile strength, but low temperature,
applications.
3.2.1 Carbon/graphite fibres
In addition to the general categories mentioned above, carbon
fibre in felt form is used in high temperature insulation,
principally in inert atmospheres due to the tendency of the
material to oxidize.
Rayon-based carbon fibres are used primarily in aerospace
applications, such as phenolic-impregnated heat shields and
carbon-carbon composites for missile parts and aircraft brakes
(Volk, 1979). PAN-based carbon fibres are used in structural
applications in the aerospace industry, sports goods (golf clubs,
fishing rods, tennis rackets, bows and arrows, skis, sailboat masts
and spars), reinforcement for moulded plastics, prostheses,
artificial joints and dental bridges (Volk, 1979; Bjork et al.,
1986). Pitch-based carbon fibres are used in applications to
increase electrical conduction and resistance to heat distortion and
to improve wear and stiffness (e.g., as veil mat for sheet moulding,
milled mat in injection moulding) (Volk, 1979).
3.2.2 Aramid fibres
Properties of aramid fibres, such as heat and flame resistance,
dimensional stability, ultra-high strength and high modulus,
electrical resistivity, chemical inertness and permselective
properties, make them useful in many applications (Preston, 1979).
Para-aramids are used principally as a strengthening and
reinforcing material in composite structures due to their low
density, high specific strength and stiffness, greater vibration
damping and better resistance to crack propagation and fatigue than
those of inorganic fibrous materials. Para-aramids are used
primarily for tyre cords, protective clothing, industrial fabrics,
high performance (sports and aerospace) composites, high-strength
ropes, cables, friction materials and gaskets (ILO, 1989).
Meta-aramids are primarily used for their heat and corrosion
resistance (Brown & Power, 1982). The paper form is used as
electrical insulation in motors and transformers. Staple and
continuous filaments are used in protective clothing and coated
fabrics, and industrial filter bags for hot gas emissions.
Aramid fibres are sometimes combined in various applications
with other fibrous materials such as carbon and graphite to reduce
costs and increase impact strength (Delmonte, 1981).
3.2.3 Polyolefin fibres
Apart from their traditional use in carpet backings, polyolefin
fibres are being used increasingly in other household furnishings
such as upholstery, bedding, curtains, wall-covering materials and
carpet pile (Hartshorne & Laing, 1984; Buchanan, 1984). The largest
use of polyolefin fabric in clothing is in disposable nappies
(diapers) and athletic socks.
Polyolefin fibres are also used in ropes, cordage and twine,
webbing, synthetic turf, agricultural fabrics, commercial fishing
lines and nets, sewing thread and book binders (Buchanan, 1984).
They are also used in the medical field in sutures and as matrices
for tissue ingrowth and anchoring (Hoffman, 1977).
Microfibres are used in a variety of applications including
filtration, medical/surgical fabrics, sanitary products, sorbents,
wipes, apparel insulation, protective clothing and battery
separators.
3.3 Emissions into the environment
3.3.1 Fibre emissions
Carbon fibres may be released into the environment during
production, processing or combustion of materials made of or
containing carbon composites. These may arise during the
incineration of waste material containing carbon fibres and
aircraft fires. Light microscopy analysis of emissions from several
key operations in the manufacture and processing of carbon fibres
(weaving, prepregging, machining and incineration of composites)
showed release rates (fibre mass released per unit of material
processed) varying over several orders of magnitude, with weaving
and incineration being the greatest (Gieseke et al., 1984). Fibre
diameters were found to be reduced only during incineration.
Based on laboratory studies in which composites containing
carbon fibres were burnt, it was concluded that the rate of
emissions and lengths of fibres released in municipal incineration
are dependent on air flow rates and mechanical agitation within the
combustion chamber. The diameters of emitted fibres were dependant
upon burning rate, temperature and oxygen levels, and on fibre
residence time in the incinerator. It was estimated that a typical
emission rate of carbon fibres from the burning zone in a municipal
incinerator would probably be in the order of 1% of the composite
being burnt. It was also concluded that composites made with epoxy
binder materials release more fibres during incineration than those
made with phenolic binder materials (Gieseke et al., 1984).
In studies conducted to simulate municipal incineration of
refuse from composites containing carbon fibres (combustion rate of
34 kg/h, flame temperature of 925 °C, 450 kg Celion unidirectional
laminate fed over a 1-min period), stack emission rates were 0.840
to 0.998 mg/dscm carbon fibre. Emission rates decreased with time
(0 to 60 min) following introduction of the refuse from the carbon
fibre composite (Henry et al., 1982; Gieseke et al., 1984).
In laboratory and field tests conducted by the United States
National Aeronautics and Space Administration, it was estimated
that approximately 38.4-49.5% of the fibres released during the
burning of composite materials containing PAN-based carbon/graphite
fibres in aircraft (spoilers, cockpits, and various structural
elements) were respirable (i.e. aspect ratio of > 3:1; < 80 µm in
length and < 3 µm in diameter). Based on analysis by optical
microscopy, the average length of emitted fibres was 30 µm
(Sussholz, 1980).
Based on data collected in his previous studies, the same
investigator estimated airborne fibre concentrations during the
burning of carbon fibre composites. For a fire resulting from an
aircraft crash, it was estimated that 1% of the carbon fibre would
be released, constituting about 5 x 1011 respirable fibres per kg.
Based on these data, it was further estimated that the airborne
concentration of fibres in the densest part of the smoke would be
5 fibres/ml.
The by-products of carbonization that are captured following
incineration of fibres include hydrogen cyanide from PAN-based
fibres and polynuclear aromatic compounds from pitch-based fibres.
By-products of graphitization include ammonia, cyanide and
hydrocarbons; these materials are decomposed during the heating
process (ICF, 1986).
3.3.2 Decomposition products
In studies conducted to simulate municipal incineration of
refuse from composites containing carbon fibres (combustion rate of
34 kg/h, flame temperature of 925 °C, 450 kg Celion unidirectional
laminate fed over 1-min period), observation by light microscopy
indicated that fibres are oxidized during incineration and that
fibre diameters are reduced. It was also observed that as oxidation
proceeds, there is notching and subsequent breaking of the fibres.
Thus, oxidized fibres that have small diameters are often relatively
short (Henry et al., 1982).
The burning or pyrolysis of organic fibres produces a wide
variety and quantity of emitted gases depending on the numerous
variables of the decomposition/oxidation. Simultaneously burning
materials, temperature, heating rate, burning time, humidity and
oxygen availability are just a few of the important variables.
Consequently, the data from a given experiment are uniquely
dependent on the conditions of that experiment.
Pyrolysis decomposition products of carbon and Kevlar fibres
have been identified by thermogravimetric analysis/gas
chromatography/mass spectrometry (TGA/GC/MS) (Razinet et al.,
1976). The products identified in the off-gasses of pyrolysis of 11
different types of carbon fibre at 1000 °C included carbon dioxide
(32-100%), methane (2-23%), propane (0.5-19%), propene (2-20%),
benzene (7-14.4%), toluene (2.0-8.3%), dimethylbenzene (2.9%),
naphthalene (2.9%) and methylnaphthalene (2.8%). The carbon dioxide
content of the off-gasses (expressed as a percentage of the weight
of the sample used in the decomposition) ranged from 0.02 to 2.34%.
The products identified after fast pyrolysis of Kevlar (650 °C)
were carbon monoxide (63%), benzene (35%) and toluene (2%).
Following slow pyrolysis (at 650 °C for 15 min), many compounds,
including nitrogen, carbon monoxide, carbon dioxide, methane,
ethylene, propene, benzene, toluene, benzonitrile, aniline,
methylaniline, phenylacetonitrile, phthalonitrile, phenyldiamine,
biphenyl, benzimidazole, fluorene and benzanilide, were identified.
4. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE
4.1 Occupational environment
4.1.1 Carbon/graphite fibres
4.1.1.1 Production
Carbon fibres are released in the workplace during operations
such as fibre forming, winding, chopping and composite formation.
Analysis by light microscopy of emissions in the immediate vicinity
of each of several key operations in the manufacture and processing
of carbon fibre (including weaving, handling, prepregging, and
machining of composites) demon strated that release rates (fibre
mass released per unit of material processed) were greatest for
weaving (Gieseke et al., 1984).
The data on aerosol exposures to carbon fibres in the
occupational environment are presented in Table 4. Due to the lack
of available data and limited information on methods for sampling
and analysis, only tentative conclusions concerning airborne
particle or fibre concentrations can be drawn.
4.1.1.2 Processing of composites
Some data are available concerning airborne concentrations and
fibre dimensions resulting from the machining of carbon fibre
composites. Following drilling, an analysis of the settled dust was
conducted using PCOM and scanning electron microscopy (SEM). Fibres
50 to 100 µm in length with diameters 6 to 8 µm were observed and
found to have dimensions similar to those of fibres in the composite
material. Following sawing, settled dust samples were also
collected and analysed, and this revealed longitudinal breaking of
fibres. Some fibres were observed to have fibre diameters less
than those in the parent material; fibres were also shorter in
length than those in the dust produced by drilling (Wagman et al.,
1979).
Limited data are available concerning the preparation and
machining of carbon fibre composites. PCOM analyses of five
samples from one facility yielded concentrations ranging from
< 0.001 fibres/ml to approximately 0.01 fibres/ml. The average
length of fibres in these samples ranged from approximately 31 µm to
749 µm and average diameter from 3.9 µm to 6.7 µm (Henry et al.,
1982).
Table 4. Airborne concentrations of carbon fibre in the workplacea
Sampling and analysis Results Reference
Light microscopic analysis of unspecified 0.0002 fibres/ml in prepregging, mean fibre diameter 6.1 µm and mean Henry et
number of samples (possibly one each) of fibre length 213 µm; 0.0009 fibres/ml in shuttle loom weaving, mean al. (1982)
"outside emissions" at prepregging machining fibre diameter 6.7 µm and mean fibre length 749 µm; 0.003 fibres/ml
and weaving operations in PAN-based production in rapier weaving, mean diameter 3.9 µm and mean fibre length 706 µm;
facility in the USA 0.01 and 0.001 fibres/ml in machining, mean fibre diameters 6.2 and
6.6 µm, and mean fibre lengths 32.8 and 30.9 µm, respectively
Light microscopic analysis of 4 samples of mean concentration of 0.0005 fibres/ml (range, 0.00002-0.006 Henry et
either "outside emissions" or worker exposure fibres/ml); average fibre diameter 6.5 µm and average fibre al. (1982)
(not clearly specified) in winding operations length 48-60 µm
in PAN-based production facility in the USA
Gravimetric analysis of 38 samples in mean concentration of 0.08-0.39 mg/m3 total dust with 40% respirable; Jones et
pitch-based continuous filament production concentrations highest in the laboratory owing to cutting, grinding al. (1982)
facility and milling of carbon fibre; reinforced resins (mean, 0.39 mg/m3),
concentration twice as high in winding (mean, 0.19 mg/m3) as in
production (mean, 0.08 mg/m3)
Table 4 (contd).
Sampling and analysis Results Reference
Light microscopic analysis of emissions at 0.003-0.029 fibres/ml in prepegging, mean fibre diameter 5.5-6 µm and Gieseke et
several locations each in various phases of mean fibre length 915-2399 µm; 0.001-0.05 fibres/ml in weaving, mean al. (1984)
carbon fibre production fibre diameter 5.5-7.8 µm and mean fibre length 945-2342 µm
Gravimetric analysis of 6 breathing zone 0.10-0.80 mg/m3; concentrations much higher for chopping and winding Gilliam
samples in PAN-based production facility in operators (0.54-0.8 mg/m3) than for line operators (0.1-0.12 mg/m3) (1986)b
the USA
PCOM analysis (NIOSH 239) of all areas of a 0.11-0.27 fibres/ml Familia
pitch-based production facility in the USA (1986)c
where exposure was deemed likely (1985 to
May 1986)
a This study was designed to estimate emissions from carbon fibre processing. The Task Group noted that these data may be of
limited value for predicting occupational exposure levels and airborne fibre characteristics.
b Gilliam, H.K. Great Lakes Carbon Corporation, Rockwood, TB 37854-0810. Letter providing monitoring data and product
literature to James C. Chang, ICF Inc., Washington, DC, April 28, 1986.
c Familia, R.A. Union Carbide/Amoco. Greenville, SC 29602. Letter providing sampling data to James Chang, ICF Inc.,
Washington, DC, (1986).
The mean airborne concentration determined by PCOM and SEM was
0.03 fibres/ml in fourteen samples taken during the machining of
fibre composites at the Air Force Aerospace Materials Research
Laboratory. Particles with an aspect ratio > 3 constituted less
than 0.1% of the released material. The diameters (> 7 µm) of the
airborne fibres were similar to those in the composites (Lurker &
Speer, 1984).
Airborne particulate samples were collected during trimming
operations on a wing panel composed of carbon fibre composites.
Based on analysis by PCOM, less than 8% of the airborne particulates
were fibrous. The diameters of at least 80% of the fibres were
approximately 7 µm, i.e. similar to those in the composites
(Dahlquist, 1984).
The physical, morphological and chemical characteristics of
machined graphite or fibrous glass composites have been determined.
Bulk and fractionated samples were examined by light and electron
microscopy and analysed chemically by GC/MS (Boatman et al., 1988).
The test materials were designated as "graphite-p", two graphite
materials manufactured from PAN, one pitch-based graphite and
PAN-graphite/Kevlar. The machining operations were spindle shaping
(at 3450 or 10 000 rpm), hand routing (at 23 000 rpm) and use of a
saber saw. Dusts were collected with a high efficiency vacuum
cleaner and resuspended in a dust generator to create an aerosol in
which 90% of particles were < 10 µm in aerodynamic diameter. The
relative proportion of respirable to total mass of bulk samples was
< 3%; aerodynamic diameters of fractionated samples collected at
the tool face ranged from 0.8 to 2.0 µm. Particles in bulk samples
ranged from 7 to 11 µm in diameter, and mean aspect ratios ranged
from 4:1 to 8:1 (26:1 for the fibrous glass composite). The mean
diameter of fractionated particles collected at the tool face was
2.7 µm, with 74% being less than 3.0 µm in diameter. There was no
evidence of longitudinal splitting of fibres, and volatilization of
chemicals from machined composites was low. The authors reported
that the respirable fractions in dusts collected at the tool face
were only 2 to 3% by weight. However, since large particles that
would settle rapidly and not normally reach the breathing zone were
included in samples taken at the tool face, these values are not
indicative of personal exposure.
4.1.2 Aramid fibres
4.1.2.1 Production
Available data on airborne concentrations of polyamide fibres
in the occupational environment are restricted to brief summaries
of unpublished data collected within the industry. In many cases,
methods of sampling and analysis have not been described.
Verwijst (undated report) described exposure monitoring during
para-aramid fibre and pulp manufacturing and during laboratory
operations. Air concentrations ranged from 0.01 to 0.1 fibrils/ml,
with the highest values being for pulping. Verwijst also noted
relatively high exposure (0.9 fibrils/ml) during water jet cutting
of composites.
Since initiation of Kevlar para-aramid fibre production (in
about 1971), plant-air and employee exposures have been measured by
the same phase contrast microscopy techniques used for asbestos
(PCAM 239 before about 1982 and NIOSH 7400 "A" rules) (Merriman,
1992). These data are summarized in Table 5. For continuous
filament yarn handling, plant exposures are extremely low
(0.02 fibres/ml maximum) (Reinhardt, 1980). Cutting of staple and
floc fibre produced levels of 0.2 fibres/ml or less with a single
peak measurement of 0.4 fibres/ml. Pulp drying and packaging
operations led to maximum concentrations of 0.09 fibres/ml.
Airborne respirable concentrations of Nomex/Conex have been
reported to be less than the limit of detection i.e. 0.01 fibres/ml
(Reinhardt, 1980; ILO, 1989).
4.1.2.2 End-use processing and processing of composites
Para-aramid end-use plant monitoring data using PCOM are
summarized in Table 5 for brake pad production, gasket composite
fabrication, and staple yarn spinning processes (Merriman, 1992).
No exposures exceeded 0.19 fibres/ml for brake pad manufacturing,
where dry para-aramid pulp was mixed with powdered fillers and
resin, pressed, cured, ground and drilled. Average fibre exposures
were less than 0.1 fibres/ml.
In gasket sheet and gasket manufacturing, para-aramid pulp is
mixed with fillers and solvated rubber cement, rolled into sheets
and die-cut into smaller pieces that may be finished by sanding the
edges. A total of 64 samples in four plants gave no personal
exposures greater than 0.15 fibres/ml and no area concentrations
greater than 0.25 fibres/ml. Mean exposures were less than
0.1 fibres/ml for all operations. In a single test where heat-aged
gaskets were scraped from flanges, short-term exposure
concentrations were less than 0.2 fibres/ml.
Machining of para-aramid fabric-reinforced organic matrix
composites produced very low exposures; most were less than
0.1 fibres/ml although one exposure reached 0.25 fibres/ml during
grinding. Although operator exposure during water-jet cutting was
only 0.03 fibres/ml, the cutting sludge in a single sample was
highly enriched with respirable fibrils and produced much higher
levels (2.9 fibres/ml).
Table 5. Airborne fibre concentrations in workplaces handling
para-aramid fibre pulpa
Manufacturing Operations Samples Mean Maximum
industry (fibres/ml) (fibres/ml)
Brake pads mixing 20 0.07 0.15
preforming 17 0.08 0.19
grinding/drilling 8 0.04 0.08
finishing/inspecting 3 0.05 0.10
Gaskets mixing 30 0.05 0.15
calendering 1 0.02 0.02
grinding 5 0.08 0.25
cutting 15 0.02 0.07
Composite sanding/trimming 5 0.08 0.25
water jet cutting 1 0.03 2.91
Staple yarn grinding 5 0.18 0.28
carding 16 0.39 0.79
drawing 4 0.32 0.87
roving 6 0.33 0.72
spinning 15 0.18 0.57
twisting/winding 13 0.55 2.03
finishing 2 0.30 0.48
weaving 6 0.35 0.58
From: Merriman (1992)
The end-use most likely to produce significant para-aramid
fibril exposure levels has been found to be staple fibre carding and
subsequent processing into yarn. Carding is highly abrasive and the
fibrils produced are immediately entrained in the high air flows
created by the spinning cylinders. Monitoring of operators in six
yarn-spinning mills (67 personal samples) gave average exposures
ranging from 0.18 to 0.55 fibres/ml, with one operation reaching a
maximum of 2.03 fibres/ml.
Kauffer et al. (undated report) characterized airborne fibre
concentrations and size distributions during the monitoring of
machining of carbon fibre- and aramid-based composites in industry
and the laboratory. Concentrations encountered were typically well
below 1 fibril/ml, as determined by optical microscopy. SEM showed
mean lengths from 1.9 to 4.3 µm, and mean length/diameter ratios
values varied from 4.4 to 8.8. The authors concluded that most of
the respirable material consisted of resin debris.
Particle and gaseous emissions during laser cutting of aramid
fibre-reinforced epoxy plastics have been studied (Busch et al.,
1989). The mass-median aerodynamic diameter of particles generated
was 0.21 µm, but the concentration of dust and the fibre content of
the dust were not reported. GC/MS analyses of samples on charcoal
and silica tubes demonstrated the following release of gases per
gram of material pyrolized during cutting: 5.4 mg benzene, 2.7 mg
toluene, 0.45 mg phenylacetylene, 1.4 mg benzo nitrile, 1.0 mg
styrene, 0.55 mg ethylbenzene, 0.15 mg m-xylene and p-xylene,
0.04 mg o-xylene, 0.28 mg indene, 0.16 mg benzofurane, 0.15 mg
naphthalene, and 0.73 mg phenol.
Limited personal exposure monitoring was conducted during
laser cutting of Kevlar-reinforced epoxy matrix (Moss & Seitz,
1990). An air sample collected within a few feet of the cutting
operation revealed few fibres (0.15-0.25 µm in diameter and < 10 µm
in length) in TEM analyses. In addition to fibre measurements,
hydrogen cyanide concentrations in the cutting area ranged from
0.03 to 0.08 mg/m3 with a TWA of 0.05 mg/m3. Carbon monoxide
concentrations ranged from 10 to 35 ppm and nitrogen dioxide
concentrations were < 0.5 to 5 ppm.
4.1.3 Polyolefin
4.1.3.1 Production
Limited air monitoring data in a facility producing
polypropylene fibres have been reported (Hesterberg et al., 1991).
Samples for PCOM analyses were collected in the breathing zone of
workers, and fibres were counted using NIOSH method 7400 ("B"
rules). Gravimetric measurements of total dust exposures were also
made. Airborne levels of fibres longer than 5 µm ranged from none
detected to a peak of 0.5 fibres/ml, with most values being less
than 0.1 fibres/ml. The total dust exposures ranged from below
detection limits to 0.7 mg/m3, most values being less than
0.25 mg/m3. SEM analyses showed airborne fibre diameters to range
from 0.25 to 3.5 µm (mean = 1.97) and lengths to range from 1.68 to
69 µm (mean = 29.4 µm).
4.2 General environment
Henry et al. (1982) reported the results of limited sampling
and analysis by light microscopy of the concentration of carbon
fibres in ambient air in the vicinity of carbon fibre facilities.
The concentration of fibres in ambient air downstream of the
baghouse for rapier weaving was reported to be 0.0003 fibres/ml
(Henry et al., 1982). The average length of fibres was 706 µm and
the average width 3.9 µm.
The release of carbon fibres at the crash and burn site of two
military aircrafts has been described (Formisano, 1989; Mahar,
1990). Both aircrafts were manufactured using carbon fibre
composites and the air sampling was conducted several days after
the crash. At the first site 21 area samples and 11 personal
samples were taken over four days. The mean durations of the area
and personal samples, respectively, were 168 ± 86 min and 48 ± 46
min at 2 litres/min. Twelve of the 21 area samples contained low
fibre concentrations and were considered to be estimates. The mean
fibre concentration of the remaining air samples was 0.29 ± 0.41
(range 0.015-1.060) fibres/ml. In the case of the 11 personal
samples, 4 were estimates and the mean concentration of the others
was 3.40 ± 2.52 (range 0.063-6.998) fibres/ml.
A mixture of floor-wax and water was poured over the aircraft
wreckage in both cases to suppress the dust. The personal air
samples were collected while a variety of tasks commonly performed
at a crash site were performed. The author reported that fibres may
have been lost by electrostatic binding to the sampling cassette.
While a few interfering fibres were present, the straight fibres
with clean edges were recognized by the analyst as man-made fibres.
Air sampling at the second crash site produced barely detectable
levels, and the highest concentration determined by PCOM was
0.58 fibres/ml.
5. DEPOSITION, CLEARANCE, RETENTION, DURABILITY AND TRANSLOCATION
5.1 Introduction
It is considered that the potential respiratory health effects
related to exposure to fibre aerosols are a function of the internal
dose to the target tissue, which is determined by airborne
concentrations, patterns of exposure, fibre shape, diameter and
length (which affect lung deposition and clearance) and
biopersistence. The potential responses to fibres, once they are
deposited in the lungs, are a function of their individual
characteristics. In inhalation studies in rodents, fibres with
dimensions similar to those that humans can inhale should be used,
provided that a high proportion is within the range respirable for
the rat. In addition, complete characterization (e.g., dimensions,
number, mass and aerodynamic diameter) of the rat-respirable fibre
aerosol and retained dose is essential. Methods for aerosol
generation should insure that fibre lengths are preserved.
The following general principles have been derived principally
on the basis of results of studies with particulates, man-made
mineral fibres and asbestos. It should be recognized, however,
that these parameters have not been evaluated in detail specifically
for the synthetic organic fibres.
Because of the tendency of fibres to align parallel to the
direction of airflow, the deposition of fibrous particles in the
respiratory tract is largely a function of fibre diameter, with
length and aspect ratio being of secondary importance. In
addition, the shape of the fibres as well as their electrostatic
charge may have an effect on deposition (Davis et al., 1988).
Since most of the data on deposition have been obtained in
studies on rodents, it is important to consider comparative
differences between rats and humans in this respect; these
differences are best evaluated on the basis of the aerodynamic
diameter. The ratio of fibre diameter to aerodynamic diameter is
approximately 1:3. Thus, a fibre measured microscopically to have
a diameter of 1 µm would have a corresponding aerodynamic diameter
of approximately 3 µm. A comparative review of the regional
deposition of particles in humans and rodents (rats and hamsters)
has been presented by US EPA (1988). The relative distribution
between the tracheobronchial, and pulmonary regions of the lung in
rodents follows a pattern similar to human regional deposition
during nose breathing for insoluble particles with a mass median
aerodynamic diameter of less than 3 µm. Figures 2 and 3 illustrate
these comparative differences. As can be seen, particularly for
pulmonary deposition of particles, the percentage deposition in the
rodent is considerably less, even within the overlapping region of
respiratory tract deposition, than in humans. These data indicate
that, although particles with an aerodynamic diameter of 5 µm or
more may have significant deposition efficiencies in man, the same
particles will have extremely small deposition efficiencies in the
rodent.
Fibres of various shapes are more likely than spherical
particles to be deposited by interception, mainly at bifurcations.
Available data also indicate that pulmonary penetration of curly
chrysotile fibres is less than that for straight amphibole fibres.
In the nasopharyngeal and tracheobronchial regions, fibres are
generally cleared fairly rapidly via mucociliary clearance, whereas
fibres deposited in the alveolar space appear to be cleared more
slowly, primarily by phagocytosis, to a lesser extent via
translocation, and possibly by dissolution. Translocation refers to
the movement of the intact fibre after initial deposition at foci in
the alveolar ducts and on the ciliated epithelium at the terminal
bronchioles. These fibres may be translocated via ciliated mucous
movement up the bronchial tree and removed from the lung, or may be
moved through the epithelium with subsequent migration to
interstitial storage sites or along lymphatic drainage pathways or
transport to pleural regions. Fibres short enough to be fully
ingested are thought to be removed mainly through phagocytosis by
macrophages, whereas longer fibres may be partially cleared at a
slower rate either by translocation to interstitial sites, breakage
or by dissolution. A higher proportion of longer fibres is,
therefore, retained in the lung.
5.2 Studies in experimental animals
Fibres administered in studies on animals are only a subset of
those normally present in the occupational environment. Due to the
above-mentioned limitations in the anatomy of the rat as a model for
man, wherever possible the fraction of rat-respirable fibres in the
generated aerosol is specified.
5.2.1 Carbon/graphite fibres
It should be noted that in some of these studies, animals were
exposed to fibres that were not respirable for the rodent; in
others, animals were exposed principally to dusts, the fibrous
content of which was unspecified. For carbon fibres, only one of
the studies described here involved exposure to fibres that were
respirable for the rat (Warheit et al., in press/a).
In a study designed to investigate the histopathological
effects of carbon fibres on the lungs of guinea-pigs, production of
a particulate aerosol from a chopped PAN-based carbon fibre ("RAE
type 2") proved difficult, although with similar apparatus
concentrations of 6000 respirable fibres/ml of asbestos (type
unspecified) had been maintained for long periods (Holt & Horne,
1978). The maximum concentration of carbon particles "smaller than
5 µm" produced by the apparatus was 370/ml, with 99% being
nonfibrous. Of these, 2.9 fibres/ml ("black fibres") were of
respirable size in the dust cloud. These were mainly less than
10 µm in length and about 1 µm in diameter. A total of
15 specific-pathogen-free (SPF) guinea-pigs were exposed to the
aerosol described above. At various time intervals up to 104 h
post-exposure, groups of animals were sacrificed. Histopathological
examination of the lungs of exposed animals revealed macrophages
filled with nonfibrous carbon particles, together with a few carbon
fibres which were generally extracellular. There were no
ferruginous bodies with a carbon core and no pathological effects
(Holt & Horne, 1978).
In a follow-up study in which the test atmosphere was similar
to that in the investigation described above, groups of SPF
guinea-pigs (2-9 per group) inhaled carbon dust for 100 h and were
killed at various intervals up to 2 years post-exposure. The
respirable fraction of the inhaled dust was mainly nonfibrous and
there were very few fibrous particles in sections of the lung.
There was some indication, however, that extracellular submicron
particles present in the tissue were washed out during histological
processing. An occasional ferruginous body was observed. Submicron
carbon dust reached the alveoli; phagocytosis of particles began
immediately but proceeded slowly over many months and dust-filled
macrophages were still evident after 2 years (Holt, 1982).
As described in section 6.1.2.1, a study was conducted in which
male Crl:CDBR rats were exposed (nose only) to aerosols of pitch- or
PAN-based rat-respirable carbon fibres at target concentrations of
50 or 100 mg/m3 (47 and 62 fibrils/ml) for periods ranging from 1
to 5 days (6 h/day) and evaluated at 0, 24 and 72 h, 10 days, and 1
and 3 months post-exposure. Pigment-laden alveolar macrophages were
observed primarily at the junctions of the terminal bronchioles
(Warheit et al., in press/a).
5.2.2 Aramid fibres
Smaller fibrils can peel from the surface of para-aramid fibres
as ribbons and complex branching. Because these fine fibrils have
unusual shapes and tend to be statically charged, they frequently
agglomerate. It is necessary to use specially prepared samples,
enriched in respirable fibrils with high-pressure air mills, to
generate significant airborne para-aramid fibril concentrations
(i.e. > 100 fibres/ml) in inhalation studies on experimental
animals.
In a short-term (2 week) inhalation study on rats (as outlined
in section 6.1.3.1), Lee et al. (1983) reported that, after
6 months, inhaled Kevlar fibrils accumulated mainly at the
bifurcations of the alveolar ducts and adjoining alveoli, with only
a few fibrils being deposited in peripheral alveoli of the acinus.
The pattern of deposition and persistence of aramid fibrils in
a two-year study was similar to that stated above (Lee et al.,
1988). Kevlar fibrils, which are much more curled than chrysotile
fibres, were retained mostly in the respiratory bronchioles and
alveolar duct region, especially in the ridges of alveolar duct
bifurcations. One year after termination of a 12-month exposure to
400 fibrils/ml, the lengths of fibres in lung tissue appeared to be
reduced. At the three highest dose levels in the study (25, 100 and
400 fibrils/ml), there was a minute amount of dust accumulation in
the alveolar macrophages and in tracheobronchial lymph nodes
resulting from the transmigration of intrapulmonary Kevlar fibrils.
Most particles in the alveolar macrophages were less than 1 µm long.
In a study by Warheit et al. (1992), Crl:CD rats were exposed
(nose only) to ultrafine Kevlar fibrils (6 h/day) for 3 or 5 days at
concentrations ranging from 600 to 1300 fibrils/ml (gravimetric
concentrations ranging from 2 to 13 mg/m3) and evaluated at 0,
24, 72 and 96 h, 1 week, and 1, 3 and/or 6 months. Kevlar fibres
were found to be deposited at alveolar duct bifurcations located
nearest the bronchiolar-alveolar junctions. The median lengths and
diameters of ultrafine Kevlar samples in the air and in the lungs
were virtually identical immediately following exposure. There was
no morphological evidence that fibrils had translocated to
epithelial or interstitial compartments, in contrast to patterns
observed with chrysotile asbestos. Fibre clearance studies
demonstrated a transient increase in the numbers of retained
fibrils at 1 week post-exposure, with rapid clearance of fibres
thereafter. The transient increase in the number of fibres could
have been due to transverse cleaving of the fibres, since the
average lengths of retained fibres continued to decrease over time.
In this respect, a progressive decrease in the mean length (12.5 to
7.5 µm) and diameter (0.33 to 0.24 µm) of inhaled fibres was
measured over a 6-month post-exposure period. The percentage of
fibres longer than 15 µm decreased from 30% at time 0 to < 5% at
6 months post-exposure (Warheit et al., in press/b).
Following intratracheal instillation in rats of 25 mg "Kevlar
polymer dust" containing a low but undetermined proportion of
fibres considered to be in the respirable range (< 1.5 µm in
diameter and between 5 and 60 µm in length) in physiological saline
for 21 months, large particles were found in the terminal
bronchioles, and smaller particles with dimensions of < 5 µm were
present in alveolar ducts (Reinhardt, 1980).
In a study in which 5 mg of Kevlar fibres (fibre size
distribution and sample preparation methods unspecified) was
injected intraperitoneally into Wistar rats, it was reported that
fragments were transported through lymphatic pathways and stored in
lymph nodes where they caused inflammatory reactions (Brinkman &
Müller, 1989).
5.2.3 Polyolefins
Groups of 22 male Fischer-344 rats were exposed nose-only
(6 h/day, 5 days/week) for 90 days to filtered air or to 15, 30 or
60 mg/m3 polypropylene fibre (99.9% purity; 12.1, 20.1 and
45.8 fibres/ml, respectively, size-selected to have an average
diameter of 1.6 µm (46% < 1 µm) and an average length of 20.4 µm).
There was a strong association between the administered
concentration, the time of exposure and the lung fibre burden.
Although the length and diameter did not change during the study,
the authors hypothesized that the segmentation of these fibres on
SEM stubs may have resulted from chemical alteration or partial
dissolution of sections of the fibres within the lungs, which made
these sections dissolve further during final processing for SEM
analysis. This segmentation increased with the administered
concentration and period of exposure, as well as with the period of
recovery after termination of exposure at 90 days (Hesterberg
et al., 1992).
5.3 In vitro solubility studies
In an investigation in which the solubility of various natural
and synthetic fibres in physiological Gamble's solution (at 37 °C
or more for 1 h to 20 weeks and 1 h to 2 weeks for closed and open
system conditions, respectively; pH not specified) was determined by
atomic absorption spectrometry, carbon and aramid fibres (source and
fibre-size distribution unspecified) were found to be "practically
insoluble". There was also no evidence of alteration of the surface
during examination by SEM with energy dispersive spectrometry
(Larsen, 1989).
In a study by Law et al. (1990), test materials including
three polymeric organic fibre compositions (polypropylene,
polyethylene and polycarbonate) were compared for solubility in
physiological Gamble's solution (pH 7.6). The test materials were
subjected to leaching for 180 days in a system that provided a
continuous flow through sample holders containing the test fibres.
After this period, the fibres were examined by electron microscopy
for changes in surface area, total specimen weight and surface
characteristics. There was virtually no dissolution and no
significant change in surface area. There were only slight weight
gains, ranging from 0.08 to 0.5%, and no visible surface changes,
in contrast to results obtained for several man-made mineral fibres
(MMMFs).
6. EFFECTS ON EXPERIMENTAL ANIMALS AND IN VITRO TEST SYSTEMS
6.1 Experimental animals
6.1.1 Introduction
There are several factors that should be considered when
evaluating experimental data on the biological effects of synthetic
organic fibres. (Several of these factors were discussed in relation
to man-made mineral fibres by WHO, 1988). Most importantly,
synthetic organic fibres should not be considered as a single
entity, except in a very general way. There are substantial
differences in the physical and chemical properties (e.g., fibre
lengths and chemical composition) of the fibres, and it is expected
that these would be reflected in their biological responses.
Finally, the fibres used for specific research protocols may be
altered to determine the biological effects in experimental animals.
In this case, they may not represent the hazard potential in humans
exposed to the commercial or degradation products during the
manufacture, processing, use and disposal.
Notwithstanding the above comments, there are certain
characteristics of synthetic organic fibres that are important
determinants of effects on biological systems. The most important of
these appear to be fibre size (length, diameter, aspect ratio,
shape), biopersistencea and durabilitya, chemical composition,
surface area and chemistry, electrostatic charge and number or mass
of fibres (dose).
a The term biopersistence refers to the ability of a fibre to
stay in the biological environment where it was introduced. The
term is of particular use in inhalation and intratracheal
instillation studies, because a large percentage of the fibres
that reach the lung are removed by pulmonary clearance and
relatively few are retained (persist). The length of time
that fibres persist in the tissue is also a function of their
durability, which is directly related to their chemical
composition and physical characteristics. The term solubility,
as used here, relates to the behaviour of fibres in various
fluids. In general, the term solubility is more appropriate
for use in in vitro than in vivo studies, because the
degradation of fibres in tissues is not only a function of
their solubility. While the concept of biopersistence is
important, quantitative procedures for evaluation of this
parameter have not been established.
Other considerations relate to the extrapolation of
experimental findings for hazard assessment in man. It appears that
if a given fibre comes into contact with a given tissue in animals
producing a response, a similar biological response (qualitative)
might be expected in humans under the same conditions of exposure.
There is no evidence that the biological reaction to fibres differs
between experimental animals and humans. However, there may be
quantitative differences between species, some being more sensitive
than others.
There has been a great deal of debate concerning the relevance
of various routes of exposure in experimental animal studies to
hazard assessment in man (McClellan et al., 1992). The advantages
and disadvantages of each of these routes are discussed in the
following sections. It was the consensus of the Task Group that
the results of studies by all routes of administration should be
considered in evaluating the weight of evidence in hazard
evaluation.
Each route cannot be discussed in detail here, but some general
observations can be made. Positive results in an inhalation study
on animals have important implications for hazard assessment in
man. Strong scientifically based arguments would need to be made
against the relevance of such a finding to man. Conversely, the
lack of a response in an inhalation study does not necessarily mean
that the material is not hazardous for humans. Rats, being
obligate nose-breathers, have a greater filtering capacity than
humans. However, if it were demonstrated that the "target tissue"
was adequately exposed and that a biologically important response
was not noted, then such a result would be of value for hazard
assessment in humans.
As discussed in the Environmental Health Criteria for Man-made
mineral fibres (WHO, 1988), a negative result in studies using
non-physiological exposure conditions (e.g., intratracheal
instillation, intrapleural injection or implantation,
intraperitoneal injection) would suggest that a specific fibre may
not be hazardous for parenchymal lung tissue and/or the mesothelium.
In contrast, a positive result in such studies should be confirmed
by further investigation in inhalation studies for a complete
assessment of the hazard for humans.
However, the Task Group believes that the use of
instillation/injection studies may not necessarily be appropriate
for certain synthetic organic fibres. This recommendation is based
on the lack of, or weak response to, para-aramid fibres seen in
three intraperitoneal studies (Pott et al., 1987, 1989; Davis,
1987), whereas a definite neoplastic response was seen in a chronic
inhalation study (Lee et al., 1988) (see also Appendix 1). The
authors of these intraperitoneal studies speculated that the lack of
response to para-aramid fibres may be due to the ability of this
type of fibre to agglomerate and thereby reduce the actual number
of single fibres. These results suggested to the Task Group that a
negative result in an instillation/injection study with selected
synthetic organic fibres should not always be considered to
indicate an absence of hazard in humans by inhalation exposure.
In vivo dissolution and translocation studies play an
important part in our understanding of the behaviour of fibres in
the lung. Such studies (Bernstein et al., in press) of
comparative biopersistence can play a part in hazard identification.
In several studies, the effects of prostheses composed of
various synthetic organic fibres on tissues have been examined
(Neugebauer et al., 1981; Tayton et al., 1982; Makisato et al.,
1984; Parsons et al., 1985; Henderson et al., 1987). These
studies have not been reviewed here since they are not relevant to
an assessment of the effects of inhaled synthetic organic fibres.
6.1.2 Carbon/graphite fibres
There are no adequate studies available in which the fibrogenic
or carcinogenic potential of carbon/graphite fibres have been
examined.
6.1.2.1 Inhalation
In a subchronic inhalation study, a group of 60 male
Sprague-Dawley rats was exposed to pulverized carbon fibres
(20 mg/m3; bulk product PAN-based Celion fibres with mean diameter
of 7 µm and 20-60 µm in length; aerosol not characterized), 6 h/day,
5 days/week for up to 60 weeks, and a similarly sized control group
was exposed to air alone (Owen et al., 1986). There was a slight
decrease in the rate of body weight gain in the first 4 weeks of
exposure. In the post-exposure period, however, the average body
weight of the exposed rats was slightly but not significantly less
than that of the control animals. Although there were variable
changes in the average airway resistance for inspiration, and the
respiratory rate and minute volume of the exposed animals were
significantly less than those of the controls, these changes were
not considered by the authors to be related to exposure to carbon
fibres.
In animals exposed to carbon fibres, there was a low-grade,
diffuse increase in alveolar macrophages containing fibrous
particles in the lungs but no pulmonary fibrosis or inflammatory
reaction. By 32 weeks post-exposure, there were only occasional
alveolar macrophages containing fibres or particles scattered
throughout the lungs (Owen et al., 1986). The Task Group noted
that the lack of observed effects in the lungs may be attributable
to the fact that the administered fibres were not in the respirable
size range for the rat.
Groups of unspecified numbers of male Crl:CDBR rats were
exposed (nose only) to aerosols of pitch- or PAN-based respirable
carbon fibres at target concentrations of 50 or 100 mg/m3 for
periods ranging from 1 to 5 days (6 h/day) and evaluated at 0, 24
and 72 h, 10 days, and 1 and 3 months (Warheit et al., in
press/a). A 5-day exposure to respirable pitch-based carbon fibres
(47 or 106 mg/m3; 47 and 62 fibres/ml, probably > 5 µm in
length; MMADs of 1.3 µm and 1.6 µm, respectively) produced
dose-dependent, transient inflammatory responses in the lungs of
exposed rats, manifested by increased levels of neutrophils and
concomitant significant increases in lactate dehydrogenase, protein
or alkaline phosphatase in bronchoalveolar fluids at early
post-exposure time periods. These changes were reversible within
10 days after exposure. There were no significant differences in
the morphology or in vitro phagocytic capacities of macrophages
recovered from rats between the sham-exposed control group and
those exposed to pitch-based carbon fibres. Results from cell
labelling studies in rats exposed to pitch-based carbon fibres for
5 days demonstrated an increased turnover of lung parenchymal cells
at 10 days or 1 month after exposure, which did not correlate with
the measures of inflammation in bronchoalveolar fluids. No
increases in turnover of terminal bronchiolar cells were measured
at any time post-exposure. Pigment-laden alveolar macrophages and
minimal type II epithelial cell hyperplasia were observed primarily
at the junctions of the terminal bronchioles and alveolar ducts. In
an additional group of rats used as negative controls, exposed for
6 h to PAN-based carbon fibres (diameter outside the respirable
range; MMAD > 4.4 µm), there were no cellular, cytotoxic or
alveolar/capillary membrane permeability changes at any time
post-exposure.
6.1.2.2 Intratracheal administration
Following characterization of dusts from machined composites
containing five graphite fibres and one fibrous glass, as described
in section 4.1.1.2 (Boatman et al., 1988), groups of five male
pathogen-free Charles River rats received a single intratracheal
injection of 5 mg of human respirable fractions of the dusts in
sterile, pyrogen-free phosphate-buffered saline (Luchtel et al.,
1989). The graphite composites included in the study were a
proprietary material designated as graphite-p, two graphite
materials manufactured from PAN, one pitch-based graphite composite
and graphite-PAN/Kevlar. The machining operations included spindle
shaping (3450 or 10 000 rpm), hand routing (23 000 rpm) and use of a
saber saw. The mean particle diameter of tested fractions was
2.7 µm, with 74% being less than 3.0 µm in diameter (99.9% < 10 µm
in aerodynamic diameter). For comparison, similarly sized groups of
animals were exposed to phosphate-buffered saline, aluminium oxide
(negative control) and quartz (positive control). No information on
the fibre content of the dust samples was presented.
In a study by Martin et al. (1989), one of the lungs of each
rat was examined histopathologically one month following injection
and the other was lavaged to recover airway cells and fluid. For
the six composite-epoxy materials, there was a continuum of lung
response (from an increase in alveolar macrophages to fibrosis)
that fell between that observed in the positive (quartz) and
negative (non-fibrous aluminium oxide) control groups. None of the
composite dusts induced effects that were as severe as those
observed with quartz. However, four of the dusts (fibrous glass
composite, the two graphite materials manufactured from PAN and the
pitch-based graphite composite) produced responses (discussed below)
that were more severe than those to aluminium oxide. Among the
composite materials, the reactions for one of the PAN-based graphite
and fibrous glass composites were the most severe. The pitch-based
graphite and one of the PAN-based composites caused the greatest
increase in total cells in lung lavage fluid. However, total cells
recovered in animals treated with any of the composite samples did
not differ significantly from those for animals treated with either
NaCl or aluminium oxide. In contrast, the quartz-treated animals
had ten times more lavaged cells than the NaCl-treated animals and
five times more than any of the animals treated with composite
samples. There were also highly significant differences in the
types of cells present in the lavage fluid among the various groups,
with the same pitch-based graphite and one of the PAN-based
composites producing the greatest increase in both percentage and
total number of lavaged neutrophils among the composites tested.
The percentage and the absolute number of neutrophils were greater
in the quartz-treated animals than in any other group.
The authors noted that bolus administration by intratracheal
instillation may overwhelm lung defence mechanisms, but felt that
these results raised the possibility that some types of composite
dusts may be fibrogenic in humans (Luchtel et al., 1989).
6.1.2.3 Intraperitoneal administration
Groups of twelve albino Wistar rats (six male and six female)
were injected intraperitoneally with "carbon dust" (50 mg/kg body
weight) suspended in physiological saline at a concentration of
10 mg/ml. Particle diameters determined by electron microscopy
ranged from 0.2 to 15 µm. The particle concentration in the
injected solution measured by haemocytometer (resolution power,
1 µm) was 3.75 x 106 (reported as x 10-6) per ml suspension
containing 0.5 mg of dust. At 1 and 3 months after administration,
there were no treatment-related lesions in tissues examined
histopathologically (i.e. omentum, spleen, liver and pancreas),
whereas in U.I.C.C. Rhodesian chrysotile-exposed animals
(12.5 mg/kg body weight at 2.5 mg/ml in physiological saline) there
were characteristic fibrotic nodules in the peritoneum (Styles &
Wilson, 1973). The Task Group noted that in the published account
of this study it was not clear whether the material was fibrous,
particulate or both.
6.1.2.4 Dermal administration
In a study reported by Depass (1982), four types of carbon
fibres (continuous pitch-based filament, pitch-based carbon fibre
mat, polyacrylonitrile continuous fibres and oxidized PAN-based
fibres) were ground and suspended in benzene (25 µl of a 10% (w/v)
suspension) and applied to the clipped skin of the back of 40 male
C3H/HeJ mice three times weekly until death. No statistically
significant increases in skin tumours were observed in any of the
exposed groups compared to controls receiving vehicle alone. The
Task Group considered this study inadequate for an evaluation due to
the lack of reporting data regarding the nature of the materials
including the particle size and morphology of the test materials.
6.1.3 Aramid fibres
6.1.3.1 Inhalation
A summary of the design of the inhalation studies with aramid
fibrils is presented in Table 6.
Reinhardt (1980) reported a summary of the results of an acute
and a 2-week inhalation study in rats exposed to a mixture of
paraaramid dust containing 2% elongated particles with an aspect
ratio of at least 3:1 (fibre). Details provided in the published
account of these studies were insufficient for evaluation. In the
short-term study, rats (number and strain unspecified) were exposed
to Kevlar dust (130 mg/m3) 4 h/day, 5 days/week, for 2 weeks.
Control rats were exposed to filtered air concurrently. Following
the final exposure, half of both the test and control group animals
were sacrificed and 21 tissues examined histopatho logically. The
remaining rats were sacrificed and examined similarly following a
14-day recovery period. During exposure, test animals were slightly
less active and gained less weight than controls, effects which were
not observed during the recovery period. In rats examined after the
tenth exposure, there were numerous macrophages in the lung tissue.
At the end of the recovery period, these macrophages decreased in
number and formed discrete clusters, which the author interpreted as
a "non-specific response to foreign particles in the lung"
(Reinhardt, 1980). The Task Group noted that available information
presented in the published account of this study was insufficient to
determine the fraction of the aerosol which was respirable for the
rat.
Table 6. Inhalation studies on aramid fibres
Fibre type Concentrations of fibrils Size Exposure type Histopathological Number and Reference
distributionc,d and duration evaluation strain of
(sacrifices) animal
Mass Number
(mg/m3) (per ml)
Para-aramid dust 150 low proportion whole body; not specified not specified Reinhardt
with 2% elongated < 1.5 µm acute, 4 h (14 day) (rat) (1980)
(fibres) 130 not specified not specified 4 h/day, 5 days/ 0 and 14 days not specified
week, 2 weeks post-exposure (rat)
Kevlar fibrils 0 0 (length: 53% whole body; 0, 5 male rats/ Lee et al.
> 20 µm) 6 h/day, 5 days/ 2 weeks group Crl:CD (1983)
0.1 ± 0.06 1.3 85% < 5 µm MMAD week, 2 weeks 3 months Sprague-Dawley
0.52 ± 0.14 26 87% < 5 µm MMAD 6 months (Charles River)
3.0 ± 0.4 280 94% < 5 µm MMAD (post-exposure)
18.2 ± 2.8 not determined 49% < 5 µm MMAD
17.6a ± 4.4 not determined 13% < 5 µm MMAD
Table 6 (contd).
Fibre type Concentrations of fibrils Size Exposure type Histopathological Number and Reference
distributionc,d and duration evaluation strain of
(sacrifices) animal
Mass Number
(mg/m3) (per ml)
Kevlar fibrils 0 0 more than 70% whole body; 3 months 100 male and 100 Lee et al.
0.08 ± 0.04 2.4 ± 0.80 by mass, < 5 µm 6 h/day, 5 days/ 6 months female per group (1988)
0.32 ± 0.08 25.5 ± 9.9 MMAD week, 2 years 12 months; Crl:CD(SD)BR
0.63 ± 0.14 100 ± 37 final sacrifice (Charles River)
2.23 ± 0.46 411b ± 109 at 24 months (no
recovery period
after 24 months)
Kevlar fibrils 2.0-13.3 600-1300 3.2-4.7 µm MMAD nose only; 0 4 males per Warheit
CMC = 10 µm 3 or 5 days 24 h group Crl:CDBR et al.
CMD = 0.3 µm 72 h (Charles River) (1992)
96 h
1 week
1, 3, 6 months
(post-exposure)
a Commercial Kevlar sample
b Exposure was for 1 year only due to toxic response. Surviving animals were held for an additional 1 year without exposure.
c It should be noted that the rat can inhale into the distal airways fibres with diameters less than 1 to 1.5 µm.
d MMAD = mass median aerodynamic diameter
In a short-term inhalation study, groups of male Sprague-Dawley
rats (number in each group unspecified) were exposed to
0, 0.1 mg/m3 (1.3 fibrils/ml), 0.5 mg/m3 (26 fibrils/ml),
3.0 mg/m3 (280 fibrils/ml) or 18 mg/m3 (number concentration not
deter mined) of ultrafine Kevlar pulp fibrils prepared specially by
a high-pressure air impingement device (60-70% < 1 µm in diameter
and between 10 and 30 µm long) for 6 h/day, 5 days/week, for 2
weeks. Another group was exposed to 18 mg/m3 commercial Kevlar
fibres (2.5 mg/m3 of respirable dust) for the same period. Five
rats in each group were killed at the end of exposure and at several
periods up to 6 months after exposure. In animals exposed to 0.1 or
0.5 mg/m3 of the ultrafine fibres there was a macrophage response
in the alveolar ducts and adjoining alveoli, which was almost
completely reversible within 6 months after exposure. In rats
exposed to 3 mg/m3 ultrafine fibres or 18 mg/m3 of the
commercial product, there was occasional patchy thickening of
alveolar ducts with dust and inflammatory cells (but no collagen)
6 months after exposure. In the group exposed to 18 mg/m3
ultrafine Kevlar, there were granulomatous lesions with dust cells
in the respiratory bronchioles, alveolar ducts and adjoining alveoli
after two weeks of exposure. One month following exposure, there was
patchy fibrotic thickening in the alveolar duct regions and adjacent
alveoli, as well as dust cells. The fibrotic lesions were markedly
reduced in cellularity, size and numbers from 3 to 6 months after
exposure but contained networks of reticulum fibres with some
collagen fibres (Lee et al., 1983). (It should be noted that there
is some discrepancy between the results presented in the published
account of this study and a summary of this study included in the
report of the longer-term bioassay (Lee et al., 1988 described
below).
Groups of 24 male Crl:CDBR rats were exposed (nose only) to
ultrafine Kevlar fibres (fibrils) 6 h/day for 3 or 5 days at
concentrations ranging from 600 to 1300 fibres/ml (gravimetric
concentrations ranging from 2 to 13 mg/m3) and subgroups of four
rats were subsequently evaluated at 0, 24, 72 and 96 h, 1 week, and
1, 3 and/or 6 months post-exposure (Warheit et al., 1992; in
press/b). The authors suggested that at higher gravimetric
concentrations, there was probably an agglomeration of fibres in the
aerosols. Five-day exposures elicited a transient granulocytic
inflammatory response with an influx of neutrophils into alveolar
regions and concomitant increases in bronchio-alveolar lavage fluid
levels of alkaline phosphatase, lactate dehydrogenase and protein,
which returned to control levels at time intervals of between 1 week
and 1 month post-exposure. Macrophage function (as determined by
surface morphology and in vitro phagocytic and chemotactic
capacities) in Kevlar-exposed alveolar macrophages was not
significantly different from that of sham controls at any time
interval. Increased pulmonary cell labelling was noted in terminal
bronchiolar cells immediately after exposure but values returned to
control levels one week later. Histopathological examination of the
lungs of Kevlar-exposed animals revealed only minor effects,
characterized by the presence of fibre-containing alveolar
macrophages situated primarily at the junctions of terminal
bronchioles and alveolar ducts.
In the only chronic inhalation study conducted for any of the
organic fibres considered in this document, groups of 100 male and
female Crl:CD(SD)BR weanling rats were exposed to ultrafine Kevlar
fibrils at concentrations of 0, 2.4, 25.5, and 100 fibrils/ml (0,
0.08, 0.31 and 0.63 mg/m3, respectively), 5 days/week for two
years (Lee et al., 1988). An additional group of 100 animals was
exposed according to the same schedule to 400 fibrils/ml
(2.23 mg/m3), and, owing to toxicity, exposure was terminated at
12 months and the animals were followed for an additional year.
There were interim sacrifices of 10 males and 10 females per group
at 3, 6 or 12 months. Fibrils were separated from the Kevlar pulp
matrix by high-pressure air impingement as described above for the
study conducted by Lee et al. (1983). A summary of the effects on
the lung is presented in Table 7. At a concentration of
2.5 fibrils/ml, the alveolar architecture of the lungs was normal
with a few dust-laden macrophages in the alveolar airspaces
(alveolar macrophage response was considered by the authors to be
the no-observed-adverse-effect level). At 25 and 100 fibrils/ml,
there was a dose-related increase in lung weight, a dust cell
response, slight type II pneumocyte hyperplasia, alveolar
bronchiolarization and a "negligible" amount of collagenized
fibrosis in the alveolar duct region. In addition, at 100
fibrils/ml, "cystic keratinizing squamous cell carcinomas" (CKSCC;
tumours not observed spontaneously in this strain or in man) were
found in four female rats (6%) but not in any male animals. Female
rats also had more prominent foamy alveolar macrophages, cholesterol
granulomas and alveolar bronchiolar ization, and this was related to
the development of CKSCC. At 400 fibrils/ml, 29 male and 14 female
rats died due to obliterative bronchiolitis resulting from dense
accumulation of inhaled Kevlar fibrils in the ridges of alveolar
duct bifurcations during exposure for one year to 400 fibrils/ml. In
the case of animals surviving one year post-exposure, the lung dust
content, average fibre length and the pulmonary lesions in surviving
rats were markedly reduced, but there were slight centriacinar
emphysema and minimal fibrosis in the alveolar duct region. One male
(1/36; 3%) and six female (6/56; 11%) rats in this experimental
group developed CKSCCs.
The CKSCCs developed between 18 and 24 months of age. The
investigators reported that microscopically, it was extremely
difficult to distinguish between squamous metaplasia and CKSCC since
the lung tumours were well differentiated and there was no evidence
of either tumour metastasis or invasion to adjacent tissue. The
tumours were, therefore, considered by the authors to be benign
neoplastic lesions which were classified as CKSCC due to the fact
that there is no benign type of squamous cell lung tumour widely
accepted in humans. The investigators also suggested that these
changes should perhaps be considered as either metaplastic or
dysplastic rather than as neoplastic lesions. In instillation
studies in rats with silica, however, CKSCCs have been described as
precursors of squamous cell carcinomas, which develop if the life
span is sufficiently long (Pott et al., in press). The Task Group
members noted that there is considerable debate concerning the
biological potential of these lesions (CKSCC) and their relevance to
humansa. It was the view of the Task Group that the CKSCCs were
related to exposure to aramid and that these lesions are part of the
neoplastic spectrum. However, the Task Group also felt that the high
dose (400 fibres/ml) exceeded the maximum tolerated dose. Finally,
the Task Group noted that the study was terminated at 24 months; a
longer observation time might have yielded a higher incidence of
these tumours.
6.1.3.2 Intratracheal administration
In the same brief account referred to in section 6.1.3.1,
Reinhardt (1980) reported the protocol and results of a study in
which 2.5 mg of shredded Nomex paper in physiological saline was
instilled into the trachea of rats (number, strain, instillation
schedule and nature of control group not specified). Fibre sizes
were reported to vary from 2 to 100 µm in length and 2-30 µm in
diameter. The lungs of groups of rats were examined histologically
at 2 and 7 days, 3 and 6 months, and at 1 and 2 years. There were
no adverse effects other than initial transitory acute inflammation
followed by foreign body granuloma formation. These mild tissue
reactions became less obvious post-exposure and the lungs were
essentially normal without formation of collagenized fibrosis two
years after exposure.
a After the meeting of the IPCS Task Group an international panel
of 13 pathologists evaluated these cystic lesions. The summary
of their evaluation and the names of the participants are in
Appendix 1.
Table 7. Summary of effects on the lungs of chronic inhalation of aramid fibrilsa
Exposure Mortality Lung fibrosis Keratinized squamous Cystic keratinizing Adenomas
concentration cell metaplasia squamous cell
(fibrils/ml) carcinoma (CKSCC)b
0 0 - - - 1/69 (1%) male
0/68 (0%) female
2.5 0 - - - 1/69 (1%) male
0/64 (0%) female
25 0 dose-related increase - - 1/67 (2%) male
in severity and 0/65 (0%) female
incidence
100 0 - 0/68 male 1/68 (2%) male
4/69 (6%) female 3/69 (4%) female
400 29/65 male 0/36 male 1/36 (3%) male 2/36 (6%) male
14/70 female 6/69 (9%) female 6/56 (11%) female 2/56 (4%) female
a From: Lee et al. (1988)
b See also Appendix 1
In a similar briefly documented study, 25 mg Kevlar in
physiological saline was instilled intratracheally into rats (number
and strain not specified) and control rats were administered saline
alone. Animals were sacrificed 2, 7, and 49 days and 3, 6, 12 and
21 months after treatment and the respiratory tract was examined
histopathologically. Following instillation, particles could be
detected in lung tissue. An initial, non-specific inflammatory
response subsided within about a week and foreign body granulomas
with minimal collagen were seen at later sacrifices. Tissue
responses decreased with increasing time post-exposure (Reinhardt,
1980).
6.1.3.3 Intraperitoneal administration
In studies by Pott et al. (1987), 5-week-old female Wistar
rats were administered 10 mg Kevlar fibres prepared by ultrasonic
treatment in three weekly intraperitoneal injections of 2, 4 and
4 mg. At the end of the study (surviving animals sacrificed
2.5 years after treatment), 4 out of 31 animals (12.9%) had tumours
(sarcoma, mesothelioma or carcinoma of the abdominal cavity). In an
additional study in which there was an attempt to obtain finer
fibres and better suspension by drying, milling and ultrasonic
treatment, 20 mg Kevlar (50% < 3.4 µm in length and 50% < 0.47 µm
in width) in saline was injected intraperitoneally into 8-week-old
Wistar rats (5 injections of 4 mg weekly). At 28 months after
injection, the percentage of tumour-bearing animals was 5.8
(preliminary results; 34 animals sacrificed and 18 survivors). The
authors commented that it was difficult to produce a homogeneous
suspension of Kevlar fibres and that, as a result, these fibres were
more likely to be present in clumps in the peritoneal cavity than
were other dusts. In these studies, tumour incidences after
injection of 0.25 to 0.5 mg actinolite, chrysotile, crocidolite or
erionite were between 50 and 80%, and, of 204 rats injected
intraperitoneally with saline alone, 5 (2.5%) had malignant tumours
in the abdominal cavity. The Task Group noted that there were some
discrepancies between the reported results of this investigation and
the later study by Pott et al. (1989). Moreover, it was unclear
whether these references refer to the same or different studies.
In a subsequent report by Pott et al. (1989), in which both
the fibre size distribution (90% of the fibre diameters in the
administered material were < 0.76 µm, and 50% of fibre lengths
were > 4.9 µm and 90% were < 12 µm) and number of fibres were
characterized, there was no significant increase in peritoneal
tumours. There were tumours in 3 out of 53 (5.7%) female Wistar
rats compared to 2 out of 102 (2%) in the controls at 130 weeks
following intraperitoneal administration of 4 weekly doses of 5 mg
of milled bulk Kevlar (total dose, 20 mg). The number of Kevlar
fibres administered was 1260 x 106. In contrast, following
administration of a much smaller total dose of UICC chrysotile
(0.25 mg, 202 x 106 fibres), tumour incidence was 68% (Pott et
al., 1989).
Based on an examination of two animals from the study of Pott
et al. (1989), Brinkman & Müller (1989) described the following
stages of events following intraperitoneal injection of 5 mg of
Kevlar fibres (fibre size distribution or sample preparation methods
not specified) suspended in 1 ml physiological saline injected into
8-week-old Wistar rats at weekly intervals for 4 weeks. At 28 months
after the first injection, the rats were sacrificed and the greater
omentum with pancreas and adhering lymph nodes were removed and
examined histologically by light and scanning electron microscopy.
In an initial stage, there were multinucleated giant cells with
phagocytosis of the Kevlar fibres and an inflammatory reaction. In a
second stage, granulomas with central necrosis developed indicating
the cytotoxic nature of the fibres. A third stage was characterized
by "mesenchymal activation with capsular structures of collagenous
fibres as well as a slight submesothelial fibrosis". Finally, the
reactive granulomatous changes in the greater omentum of the rats
were accompanied by proliferative mesothelial changes which, in one
of the two animals examined, led to mesothelioma. The authors
commented that the reaction to Kevlar in the intraperitoneal test
resembled the well-studied reaction to similar injections of glass
or asbestos fibres. It was also noted that, as in the case of
mineral fibres, fragments of Kevlar fibres were transported through
lymphatic pathways and stored in lymph nodes where they caused
inflammatory reactions.
In a study in which 25 mg of Kevlar (fibre size distribution
unspecified) were administered intraperitoneally to Sprague-Dawley
rats (20 of each sex), there were no peritoneal mesotheliomas at
termination (104 weeks) (Maltoni & Minardi, 1989). In an additional
study by the same investigators, there were no peritoneal
mesotheliomas at 76 weeks in similarly sized groups of rats of the
same strain following intraperitoneal administration of 1, 5 or 10
mg Kevlar fibres (fibre size distribution unspecified) (Maltoni &
Minardi, 1989).
Doses of either 0.25, 2.5 or 25 mg of Kevlar pulp vigorously
disaggregated by a turreted tissue homogenizer (96% with diameters
< 1 µm; 56% with diameters < 0.25 µm) were administered by single
intraperitoneal injection in phosphate buffered saline to three
groups of 3-month-old male AF/Han strain rats comprising 48, 32 and
32 animals, respectively (Davis, 1987). An additional group of 12
animals was injected with 25 mg of disaggregated Kevlar pulp and
killed at intervals of between 1 week and 9 months after injection
to examine the early histopathological reaction. A group of 48
untreated rats was maintained as a control. The authors commented
that it was not possible to report the fibre length distribution or
fibre number concentration since it was often not possible to
determine whether disaggregated fibrils were still attached or
simply tangled with the larger fibres in the material prepared for
injection. Although the number of separate free fibrils greatly
exceeded that of the larger fibres present, the latter most probably
made up the bulk (by mass) of the injected material. Consequently,
the number of fibrils per unit mass injected that were within the
size range normally considered to be most potent in the induction of
mesothelioma was much lower than for most asbestos or man-made
mineral fibre preparations examined to date. (The dust generation
technique used in the studies of Lee et al. (1983, 1988) produced
a much finer preparation than was used in the current study).
There were no significant differences in survival between the
exposed and control groups. The cellular reaction to injected
Kevlar was considered to be vigorous with development of large
cellular granulomas. Although not a significant increase, 2 out of
32 animals in the highest dose group (25 mg) developed peritoneal
mesotheliomas and it was concluded that the Kevlar preparation
possessed a low but definite carcinogenic potential (Davis, 1987).
6.1.4 Polyolefin fibres
There are no adequate studies in which the fibrogenic or
carcinogenic potential of polyolefin fibres have been examined.
6.1.4.1 Inhalation
In a study by Hesterberg et al. (1992), groups of 22 male
Fischer-344 rats were exposed nose-only (6 h/day, 5 days/week for 90
days) to filtered air or to 15, 30 or 60 mg/m3 polypropylene
fibre (99.9% purity; 12.1, 20.1 and 45.8 fibres/ml, respectively,
size-selected to have an average diameter of 1.6 µm (46% < 1 µm)
and an average length of 20.4 µm). No abnormal clinical signs were
observed in any exposure group. There were no statistically
significant changes in body or lung weight or excess mortality as
compared to the control. Necropsy and histopathological
investigations were performed on subgroups of 6 to 10 rats randomly
selected from each group immediately after 30 and 90 days of
exposure and 30 days after the 90-day exposure was terminated. At
all time points in the study there were dose- and duration-dependent
changes in the lungs characterized by increased cellularity and
early bronchiolitis but no deposition of collagen. These cellular
changes appeared to be reversible at the lower dose levels 30 days
post-exposure. There was a strong association between the
administered concentration, the time of exposure and the lung fibre
burden.
6.1.4.2 Intratracheal administration
In a study (summary report) by M.B. Research Laboratories
(1980), single doses (unspecified) of ozonized (i.e. charge
neutralized) "polyethylene SHFF", ozonized "polypropylene SHFF" or
"HHF polypropylene" (source and fibre size distribution
unspecified) were administered by intratracheal insufflation in
Tween 60 to groups of 40 male Long-Evans rats. No effects on the
lung were reported, but the Task Group considered that the data
presented in this report were insufficient for evaluation.
6.1.4.3 Intraperitoneal administration
Groups of 12 Wistar rats (Alderley Park strain; 6 male and 6
female) were injected intraperitoneally with a single dose of
50 mg/kg (5 ml; 10 mg/ml) of either polyethylene ("Alkathene") or
polypropylene dusts in physiological saline (Styles & Wilson, 1973).
Particle diameters determined by electron microscopy were 3 to
75 µm (polyethylene) and 4 to 50 µm (polypropylene). Particle
concentrations in the injected solution measured by haemocyto meter
(resolution power, 1 µm) in 1 ml suspensions containing 0.5 mg of
dust were 2.38 x 106 (reported as x 10-6) for polyethylene and
1.94 x 106 (also reported as x 10-6) for polypropylene. At 1
and 3 months, animals were sacrificed and the omentum, spleen, liver
and pancreas examined histopathologically. No treatment-related
lesions were observed, whereas in U.I.C.C. Rhodesian
chrysotile-exposed animals (12.5 mg/kg body weight at 2.5 mg/ml in
physiological saline) there were characteristic fibrotic nodules
(Styles & Wilson, 1973). It was not clear to the Task Group
whether this material was fibrous, particulate or both.
No significant increase in peritoneal tumours was observed in
one intraperitoneal study, reported by Pott et al. (1987, 1989),
where 10 mg of polypropylene fibres (50% < 7.4 µm in length and
50% < 1.1 µm in diameter) in saline was injected intraperitoneally
into 8-week-old Wistar rats once a week for 5 weeks (total, 50 mg).
At 28 months after injection, 4% (2 out of 51) of the animals had
tumours (sarcoma, mesothelioma or carcinoma of the abdominal
cavity) (Pott et al., 1989). In other studies, tumour incidences
after injection of 0.25 to 0.5 mg actinolite, chrysotile,
crocidolite or erionite were between 50 and 80%; 2 out of 102 rats
injected intraperitoneally with saline alone had malignant tumours
in the abdominal cavity (Pott et al., 1987, 1989). The Task Group
noted that there were some discrepancies between the reported
results of this investigation and the later study by Pott et al.
(1989). Moreover, it was not clear whether these references
referred to the same or different studies.
6.2 In vitro studies
In vitro short-term studies, e.g., cytotoxicity,
cytogenicity, and cell transformation studies, contribute to an
understanding of the mechanisms of action of fibres. The results
of such studies are useful in the overall assessment of fibre
toxicity, but should not be used alone for hazard assessment.
However, it should be noted that there are no known negative
controls for in vitro studies with fibrous materials.
6.2.1 Carbon fibres
Martin et al. (1989) evaluated the in vitro effects of a
series of five graphite fibre composite materials machined by
various operations (as characterized by Boatman et al. (1988) and
described in section 4.1.1.2) in rabbit alveolar macrophages by
trypan blue exclusion, release of 51Cr from prelabelled
macrophages and phagocytosis as measured by light microscopy. The
Task Group noted that it was not clear in the report whether this
material was fibrous, particulate or both. Approximately 74% and
99.9% of the particles in each sample were less than 3.0 µm and
10 µm in aerodynamic diameter, respectively. For comparison, a
fibreglass composite material, aluminium oxide (negative control)
and alpha quartz (positive control) were also tested. Following
administration of 500 µg/ml, two of the samples ("graphite-PAN with
epoxy and aromatic amine curing agent with geometric mean particle
diameter of 1.8 µm" and "graphite-pitch with epoxy and aromatic
amine curing agent with geometric mean particle diameter of 1.6 µm")
were consistently the most cytotoxic producing the greatest release
of 51Cr from labelled alveolar macrophages and the greatest
reduction in viability based on trypan blue exclusion. The
cytotoxicity of "fibreglass with epoxy and amine curing agent with
geometric mean particle diameter of 2.6 µm" and "graphite-PAN with
epoxy and amine curing agent with geometric mean particle diameter
of 1.1 µm" was similar to that of the negative control (aluminium
oxide). The cytotoxicity of "graphite-p with nonepoxy
polyetherketone thermoplastic with geometric mean particle diameter
of 1.6 µm" and "graphite-PAN/Kevlar with epoxy and amine curing
agent with geometric mean particle diameter of 1.9 µm" was
intermediate.
In studies conducted by Styles & Wilson (1973), "carbon dust"
was not considered cytotoxic in rat alveolar and peritoneal
macrophages. Less than 2% of peritoneal macrophages and 5% of
alveolar macrophages were killed following phagocytosis of carbon
dust with particle diameters determined by electron microscopy to
range from 2 to 15 µm. Particle concentrations measured by
haemocytometer (resolution power, 1 µm) in 1-ml suspensions
containing 0.5 mg of dust were 3.75 x 106 (reported as x 10-6).
Cell cultures were incubated for 2 h with an unspecified volume of a
stock suspension containing 150 mg of dust per ml in (BSS); samples
were taken at 0, 1, and 2 h after the addition of the dust (Styles &
Wilson, 1973).
"Acetone reconstituted benzene extracts" of two carbon fibre
types (pitch-based and PAN-based carbon fibres) were tested in a
series of genotoxicity assays (US EPA, 1988). The test materials
were not mutagenic but were weakly clastogenic. The Task Group
considered that these studies are of little or no value for an
evaluation since no information was presented concerning the nature
of the test substances.
6.2.2 Aramid fibres
Aqueous solutions containing 25, 50, 100 and 250 µg/ml Kevlar,
extracted from commercial grade Kevlar by dispersion and settling
in distilled water (90% < 5 µm in length and 0.25 µm in diameter;
average length and diameter, 2.72 and 0.138 µm, respectively), were
cytotoxic to pulmonary alveolar macrophages obtained from adult male
Long-Evans black hooded rats, based on determination of leakage of
cytoplasmic lactic dehydrogenase, lysosomal enzymes,
beta-galactosidase, and ATP content (incubation time, 18 h). The
cytotoxic response in freshly harvested and cultured cells was
considered to be similar to or greater than that for UICC B Canadian
chrysotile (Dunnigan et al., 1984). The Task Group noted that due
to their short length, these fibres would not be included in fibre
counts in the occupational setting, according to WHO criteria (WHO,
1985).
6.2.3 Polyolefin fibres
In studies conducted by Styles & Wilson (1973), polyethylene
("Alkathene") and polypropylene were considered, on the basis of
cytotoxicity in rat alveolar and peritoneal macrophages, to be
among the least toxic of various dusts. Less than 2% of peritoneal
macrophages and 5% of alveolar macrophages were killed following
phagocytosis of polyethylene and polypropylene with particle
diameters determined by electron microscopy to range from 3 to
75 µm and 4 to 50 µm, respectively. The Task Group noted that it
was not clear in the report whether this material was fibrous,
particulate or both. Particle concentrations measured by
haemocytometer (resolution limit, 1 µm) in 1 ml suspensions
containing 0.5 mg of dust were 2.38 x 106 (reported as x 10-6)
and 1.94 x 106 (reported as x 10-6). Cell cultures were
incubated for 2 h with an unspecified volume of a stock suspension
containing 150 mg of dust per ml in BSS; samples were taken at 0, 1,
and 2 h after the addition of the dust (Styles & Wilson, 1973).
Extracts from three different types of polyethylene granules
(each with and without additives) and three polyethylene films
(high content of additives) were not mutagenic in Salmonella
typhimurium strains TA98, TA100 and TA1537 (Fevolden & Moller,
1978). Information included in the published account of this study
(limited to an abstract) was insufficient for evaluation.
7. EFFECTS ON HUMANS
The data available on the health effects of synthetic organic
fibres in humans are extremely limited. Information is currently
limited to case reports and small cross-sectional morbidity studies
of workers, without standardized methods and appropriate control
groups. The negative results of some of these studies are most
likely a function of their limited power to detect an effect, since
only relatively small groups of workers with relatively low and
short exposures have been examined to date. On the other hand,
positive results reported from studies with possibly higher
exposures are poorly documented and observed effects may be due, in
part, to other substances present in the occupational environment.
7.1 Carbon/graphite fibres
In a cross-sectional study of 88 out of 110 workers in a
PAN-based continuous filament carbon fibre production facility,
there were no adverse respiratory effects, as assessed by
radiographic and spirometric examination (for determination of FEV1
and FVC) and the replies to questionnaires on respiratory symptoms.
Total dust concentrations were 0.08 to 0.39 mg/m3, with 40% being
in the respirable range. Only 31 of the workers examined, however,
had been employed for more than 5 years in the facility in which
carbon fibre production began in 1972 (Jones et al., 1982). The
Task Group considered that the short duration of exposure in this
study was insufficient for a reliable assessment of the potential
health effects of these fibres.
Troitskaya (1988) reported that in a population of 327
examined workers in a PAN-based carbon fibre production facility,
67.9% had pharyngitis or rhinopharyngitis, 34% reported bronchitis
and 39.6% had dermal effects. In addition to carbon fibres, workers
were exposed to other substances such as ammonia, acrylonitrile and
"carbon oxides". It was not possible for the Task Group to assess
the validity of these results based on data provided in the
published account of the study.
Cases of dermatitis in two workers were reported during a
clean-up operation at the site of an aircraft crash where carbon
fibres were detected at concentrations of up to 7 fibres/ml.
Additional details on the monitoring methods used in this study are
presented in section 4.2 (Formisano, 1989).
7.2 Aramid fibres
There was no change in diffusing capacity in workers (n = 167)
involved in polyester fibre processing, for which exposures to
para-aramid fibres and sulfur dioxide were "low", when compared to
those in a non-exposed control group (n = 142) (Pal et al.,
1990). The Task Group noted that no firm conclusions could be drawn
on the basis of this study due to the lack of an appropriate
control group.
Reinhardt (1980) briefly reported the results of patch testing
of panels of human volunteers to assess skin irritancy and
sensitization. In these studies, which involved more than 100
individuals, there was no skin sensitization but some minimal skin
irritation following dermal contact with Kevlar or Nomex fabrics
(Reinhardt, 1980). It was reported that because these fibres,
especially Kevlar, are stiff, there is a potential for causing
abrasive skin irritation under restrictive contact.
8. EVALUATION OF HUMAN HEALTH RISKS
8.1 Exposure
Many factors determine the exposure levels and airborne fibre
characteristics for the synthetic organic fibres considered in this
document. Most important among these factors are: (1) the nominal
diameter of the parent fibre and distribution about this nominal
diameter; and (2) the tendency of some fibre types to form smaller
diameter fibres or to liberate smaller diameter fibrils during
processing. Fibres of concern for deposition in the bronchoalveolar
region in humans are those less than approximately 3 µm in
diameter. Limited data are available concerning the fraction of
fibres smaller than 3 µm in diameter for most synthetic organic
fibrous products, although nominal fibre diameters are reported to
be generally greater than 5 µm. Para-aramid fibres have fine-curled
fibrils of less than 1 µm in diameter which can break off during
processing. Some polyolefin fibres are produced which also have
nominal diameters of 0.1-2 µm. Although carbon and graphite fibres
normally have nominal diameters greater than 5 µm, some data suggest
that diameters are reduced during incineration or other burning such
as might occur after an aircraft crash.
Only limited data are available concerning occupational
exposures to synthetic organic fibres and virtually no data are
available with respect to environmental fate, distribution and
general population exposures. Sampling and analytical methods used
for measuring synthetic organic fibre exposures are those normally
used for asbestos and man-made mineral fibres. Although these
methods may be suitable, little method validation has taken place
for synthetic organic fibres.
Occupational exposure data summarized in section 4.1 generally
demonstrate low-level fibre exposures in fibre production
facilities. Exposure levels in carbon fibre production are reported
to be generally less than 0.1 fibres/ml, although levels of
approximately 0.3 fibres/ml have been recorded. Exposure levels in
facilities producing para-aramid fibres typically are less than
0.1 fibres/ml, although levels of over 2 fibres/ml have been
measured during subsequent processing. Spinning and weaving of
para-aramid staple yarns produces higher exposures (average 0.18 to
0.55 fibres/ml). Exposure levels in polypropylene fibre production
and use are generally less than 0.1 fibres/ml, although levels of
0.5 fibres/ml have been reported. Virtually no data exist for
secondary fibre uses and applications. Several exposure studies
have reported levels in mg/m3. While these data may be useful in
overall evaluation of total dust exposures in the workplace,
gravimetric determinations of airborne dust levels are of very
limited value with respect to assessment of organic fibre
exposures.
While airborne synthetic organic fibre concentrations for fibre
types considered in this document are generally lower than
0.5 fibres/ml in the occupational environment, primarily in
production, the possibility for higher exposures in different
applications and uses exists, particularly for those operations that
vigorously disturb fibres and the fibre matrix. Additionally,
applications may involve exposures to other hazardous substances in
the workplace.
8.2 Health effects
The potential adverse effects for humans from inhalation
exposure to synthetic organic fibres are the development of
malignant and non-malignant respiratory diseases. Concern for
these effects is based on the health evidence derived from exposure
to other respirable and durable fibrous materials. Other effects of
concern include contact dermatitis and skin irritation from dermal
exposures.
There is limited information on the health effects of synthetic
organic fibres in humans. The information that does exist is
considered inadequate for assessment because of numerous
shortcomings including the inability to evaluate chronic effects
due to the relative short period (20 years) that the fibres under
review have been in production and use. However, there is some
suggestive evidence that exposure to carbon and aramid fibres can
cause contact dermatitis and skin irritation.
Toxicological data on carbon fibres and polyolefins fibres are
also limited. The available animal data on acute and short-term
inhalation exposures to carbon and polypropylene fibres indicate
minimal respiratory system toxicity. Information on the chronic
and carcinogenic effects of these fibres via inhalation is
unavailable. However, the lack of appropriate animal data does not
reduce the concern for potential health effects associated with
long-term exposures to these fibres.
The available animal studies on exposure to respirable
para-aramid fibrils indicate that acute and short-term inhalation
exposures at concentrations as high as 1300 fibres/ml induce
minimal pulmonary toxicity in rats. However, the results of the
only chronic inhalation study indicate that respirable para-aramid
fibres caused lung fibrosis (> 25 fibres/ml) and lung neoplasms
(> 100 fibres/ml) in the rat. On the basis of limited available
data, a potential for fibrogenic and carcinogenic effects may exist
from exposure to these synthetic organic fibres in the occupational
environment. The potential health risk associated with exposure to
these synthetic organic fibres in the general environment is unknown
at this time, but is likely to be very low.
9. CONCLUSIONS AND RECOMMENDATIONS FOR PROTECTION OF
HUMAN HEALTH
The data reviewed in this report support the conclusion that
respirable, durable organic fibres are of potential health concern.
The following actions are suggested for protection of human health.
1. To the maximum extent possible, the organic fibres that are
produced should be non-inspirable or at least
non-respirable. Respirable fibres should not be produced by
splitting or abrading during subsequent processing, use and
disposal.
2. If small-diameter respirable fibres are necessary for specific
products or applications, these fibres should not be
biopersistent or exhibit other toxic effects.
3. All fibres that are respirable and biopersistent must
undergo testing for toxicity and carcinogenicity. Exposures to
these fibres should be controlled to the same degree as that
required for asbestos until data supporting a lesser degree of
control become available. The available data suggest that
para-aramid fibres fall within this category. Furthermore,
other respirable organic fibres should be considered to fall
within this category until data indicating a lesser degree of
hazard become available.
4. Populations potentially exposed to respirable organic fibres
should have their exposure monitored in order to evaluate
exposure levels and the possible need for additional control
measures.
5. Populations identified as being those most exposed to
respirable organic fibres should be enrolled in preventive
medicine programmes that focus on the respiratory system. These
data should be reviewed periodically for any early signs of
adverse health effects.
10. FURTHER RESEARCH
10.1 Sampling and analytical methods
Sampling and analytical methods for synthetic organic fibres
have generally been adapted from methods that have been used for
asbestos. These include the use of membrane filter samples with
analyses by phase contrast optical microscopy or limited use of
scanning and transmission electron microscopy. Further validation
of these methods for synthetic organic fibres is necessary, with
particular attention to: (1) possible effects of high electrostatic
charges of organic fibres on sampling and analysis; (2) effects of
sample preparation on the integrity of fibres; and (3) combined
effects of fibre size and refractive index on visibility of organic
fibres under phase contrast microscopy. These parameters could
introduce a negative bias in air sample results.
In addition, methods for sampling and analysis of synthetic
organic fibres in biological tissues need further development and
evaluation.
10.2 Exposure measurement and characterization
Much more information is needed relative to levels and
characteristics of exposure in plants producing and using synthetic
organic fibres. Complete size distributions, with special attention
to those fibres less than approximately 3 µm in diameter, are
needed for synthetic organic fibres. While some data are available
for the fibre-producing industries, very little information
concerning respirable fibre exposure is available in industries
using or applying these fibres.
Information concerning environmental releases of synthetic
organic fibres or fibre concentrations in environmental media is
also very limited. Data need to be collected concerning the
environmental fate and distribution of these materials and
resultant non-occupational exposures.
10.3 Human epidemiology
No reliable data exist concerning chronic effects of synthetic
organic fibre exposures. Multi-centre studies are needed in order
to develop adequately sized cohorts for epidemiological studies.
Both cross-sectional and longitudinal studies of respiratory
morbidity, cancer mortality, and cancer incidence are needed.
10.4 Toxicology studies
With the exception of para-aramid fibres and fibrils, no
adequate toxicity data are available for other synthetic organic
fibres. These data are badly needed. Emphasis must be placed on
chronic inhalation studies using respirable fibres of these
materials. These studies must use fibre sizes which are respirable
in the animal species being used and at levels which are at or near
the maximum tolerated doses. They should include studies of tissue
burden in order to confirm the expected tissue doses. A much better
understanding of those characteristics of synthetic organic fibres
(e.g., particle charge, agglomeration) that could effect deposition
is needed.
More data are needed concerning the biopersistence of
synthetic organic fibres. The critical period of lung residence
necessary for development of adverse health effects remains to be
determined. Better test materials for measuring biopersistence are
needed.
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APPENDIX 1. SUMMARY OF PATHOLOGY WORKSHOP ON THE LUNG
EFFECTS OF PARA-ARAMID FIBRILS AND TITANIUM
DIOXIDE
(5-6 October 1992)
An international panel of 13 pathologists met to evaluate the
cystic lesion observed in the lungs of rats in chronic inhalation
studies with Kevlar1 aramid fibrils (Lee et al., 1988) and
titanium dioxide particles (Lee et al., 1985). Slides
representative of the entire spectrum of cystic keratinizing lesions
observed in these studies were sent to each of these pathologists
prior to the meeting.
The pathologists agreed that the most appropriate morphologic
diagnosis is Proliferative Keratin Cyst (PKC). The lesion is a cyst
lined by a well-differentiated stratified squamous epithelium and
with a central keratin mass. Growth appears to have occurred by
keratin accumulation and by peripheral extension into the alveolar
spaces. The lesion is sharply demarcated except in those areas in
which there has been extension into adjacent alveoli. The squamous
epithelium has few mitotic figures and dysplasia is absent.
All participants agreed that the lesion is not a malignant
neoplasm. The majority was of the opinion that the lesion is not
neoplastic. A minority considered that the lesion is probably a
benign tumour. The participants had not seen a similar lung lesion
in humans.
Morphologic features of the cystic lesion that were used to
exclude malignancy were the lack of invasion of the pleura, blood
vessels or the mediastinum as well as the absence of dysplasia and
the paucity of mitotic figures.
1 Kevlar is registered trademark of the DuPont Company for its
para-aramid fibre.
PARTICIPANTS IN PATHOLOGY WORKSHOP ON THE LUNG EFFECTS
OF PARA-ARAMID FIBRILS AND TITANIUM DIOXIDE
(5-6 October 1992)
Participants
Dr M. Brockmann, Institut für Pathologie, Universitätsklinik
"Bergmann's Heil", Bochum, Germany
Dr W.W. Carlton, Department of Veterinary Pathobiology, School of
Veterinary Medicine, Purdue University, West Lafayette, Indiana,
USA
Dr J.M.G. Davis, Institute of Occupational Medicine, Edinburgh,
United Kingdom
Dr V.J. Feron, T.N.O. Toxicology and Nutrition Institute, Zeist,
Netherlands
Dr M. Kuschner, Pathology Department, SUNY at Stonybrook Health
Science Center, Stonybrook, New York, USA
Dr K.P. Lee, Haskell Laboratory, E.I. du Pont de Nemours & Company,
Haskell Laboratory for Toxicology and Industrial Medicine,
Newark, Delaware, USA
Dr L.S. Levy, Institute of Occupational Health, University of
Birmingham, Birmingham, United Kingdom (Chairman)
Dr E. McCaughey, Canadian Reference Center for Cancer Pathology,
Ottawa Civic Hospital, Ottawa, Ontario, Canada
Dr P. Nettesheim, Laboratory of Pulmonary Pathobiology, National
Institute of Environmental Health Sciences, Research Triangle
Park, North Carolina, USA
Dr K. Nikula, Inhalation Toxicology Research Institute,
Albuquerque, New Mexico, USA
Dr R. Renne, Batelle Pacific Northwest, Richland, Washington, USA
Dr M. Schultz, Institut für Pathologie, Bezirkskrankenhaus
Magdeburg, Magdeburg, Germany
Dr V.S. Turusov, Cancer Research Center, Russian Academy of Medical
Sciences, Moscow, Russia
Dr J.C. Wagner, Preston, Weymouth, Dorset, United Kingdom
Observers
Dr C.L.J. Braun, Akzo NV, Arnhem, Netherlands
Dr R.C. Brown, MRC Toxicology Unit, Medical Research Council
Laboratories, Carshalton, Surrey, United Kingdom
Dr S.R. Frame, E.I. du Pont de Nemours & Company, Haskell
Laboratory for Toxicology and Industrial Medicine, Newark,
Delaware, USA
Dr N.F. Johnson, Inhalation Toxicology Research Institute,
Albuquerque, New Mexico, USA
Dr G.L. Kennedy, Jr, E.I. du Pont de Nemours & Company, Haskell
Laboratory for Toxicology and Industrial Medicine, Newark,
Delaware, USA
Dr E.A. Merriman, E.I. du Pont de Nemours & Company, Wilmington,
Delaware, USA
Dr C.F. Reinhardt, E.I. du Pont de Nemours & Company, Haskell
Laboratory for Toxicology and Industrial Medicine, Newark,
Delaware, USA
Dr J.W. Rothuizen, Rothuizen Consulting, En Thiéré, Genolier,
Switzerland
Dr O. von Susani, Du Pont International SA, Grand-Saconnex, Geneva,
Switzerland
Dr D.B. Warheit, E.I. du Pont de Nemours & Company, Haskell
Laboratory for Toxicology and Industrial Medicine, Newark,
Delaware, USA
RESUME
1. Identité, propriétés physiques et chimiques
Les fibres de carbone/graphite sont des filaments de carbone
produits par traitement à haute température de l'une ou l'autre des
trois matières premières suivantes: rayonne (cellulose régénérée),
brais de goudron, de houille ou de pétrole, or encore polyacrylo
nitrile (PAN). Le diamètre nominal des fibres de carbone varie de 5
à 15 µm. Les fibres de carbone sont flexibles; elles conduisent
l'électricité et la chaleur et les variétés à haute performance sont
dotées d'un module de Young élevé (coefficient d'élasticité qui
mesure la souplesse ou la rigidité d'un matériau) et d'une forte
résistance à la traction. Elles sont résistantes à la corrosion,
légères, réfringentes et chimiquement inertes (sauf à l'oxydation).
En outre, elles sont très stables à la traction, possèdent un faible
coefficient de dilatation thermique et une faible densité et sont
très résistantes à l'abrasion et à l'usure.
Les fibres aramides sont préparées par réaction de diamines
aromatiques sur des dichlorures d'acides aromatiques. On les produit
sous la forme de filaments continus, de fibres discontinues et de
pulpe. Il existe deux types principaux de fibres aramides, le para-
et le méta-aramide, qui ont toutes deux un diamètre nominal de 12 à
15 µm. Les fibres para-aramides sont parfois munies de fibrilles
finement recourbées et enchevêtrées qui se situent dans la gamme des
particules respirables (< 1 µm de diamètre) sur la surface de la
fibre centrale. Ces fibrilles peuvent se détacher par abrasion lors
de la fabrication ou de l'utilisation des fibres et être suspendus
dans l'atmosphère. En général, les fibres aramides présentent une
résistance à la traction moyenne à très élevée, une résistance à
l'allongement moyenne et un module de Young moyen à très élevé.
Elles résistent à la chaleur, aux produits chimiques et à
l'abrasion.
Les fibres polyoléfiniques sont constituées de polymères à
longue chaîne composés d'au moins 85% en poids d'unités d'éthylène,
de propylène et d'autres oléfines; le polyéthylène et le
polypropylène sont utilisés dans le commerce. A part pour certains
types comme les microfibres, le diamètre nominal de la plupart des
fibres de polyoléfine est suffisamment important pour que très peu
d'entre elles se situent dans la limite des particules respirables.
Les polyoléfines sont très hydrophobes et chimiquement inertes.
Leur résistance à la traction est beaucoup plus faible que celle des
fibres de carbone ou des fibres d'aramide et elles sont relativement
inflammables, avec un point de fusion situé entre 100 et 200 °C.
Les méthodes qui ont été mises au point pour le comptage des
fibres minérales sont utilisées en hygiène industrielle pour la
surveillance des fibres organiques synthétiques. Toutefois la valeur
de ces méthodes n'a pas été vérifiée dans ce cas. Des facteurs
tenant aux propriétés électrostatiques, à la solubilité dans la
préparation des échantillons et à l'indice de réfraction peuvent
poser des problèmes lorsqu'on a recours à ces méthodes.
2. Sources d'exposition humaine et environnementale
On estime que la production mondiale de fibres de carbone et de
graphite a dépassé 4000 tonnes en 1984. En ce qui concerne les
fibres aramides, elle était supérieure à 30 000 tonnes en 1989 et la
production de fibres polyoléfiniques dépassait les 182 000 tonnes
pour les seuls Etats-Unis d'Amérique. Les fibres de carbone et
d'aramide sont utilisées principalement pour la fabrication de
matériaux composites dont les industries aérospatiales, militaires
et autres ont besoin pour améliorer la résistance, la rigidité, la
durabilité et la conductivité électrique ou la tenue à la chaleur de
certains éléments. Les fibres polyoléfiniques sont surtout utilisées
dans l'industrie textile.
Des cas d'exposition aux fibres organiques synthétiques sur les
lieux de travail ont été rapportés. Ces fibres peuvent pénétrer dans
l'environnement lors de la production, de la transformation ou de la
combustion des matériaux composites ou lors de leur mise au rebut.
On ne dispose que de très peu de données sur les rejets effectifs de
ces produits dans l'environnement.
Les données dont on dispose sur le transport, la distribution
et la transformation des fibres organiques dans l'environnement se
limitent à l'identification des produits résultant de
l'incinération, par les services municipaux, des déchets de
matériaux composites à base de fibres de carbone et des produits de
pyrolyse des fibres de carbone et des fibres aramides. Des
expériences au cours desquelles on a simulé la combustion dans des
incinérateurs municipaux ont permis de constater qu'il y avait
réduction du diamètre et de la longueur des fibres de carbone. La
décomposition pyrolytique de fibres de carbone et d'aramide produit
principalement des hydrocarbures aromatiques, du dioxyde et de
monoxyde de carbone et des cyanures.
3. Concentrations dans l'environnement et exposition
de l'homme
Des poussières de fibres organiques synthétiques peuvent être
libérées sur les lieux de travail lors d'opérations telles que la
production, le bobinage, coupe le tissage et la coupe des fibres, de
même qu'au cours de l'usinage, de la réalisation et de la
manipulation des matériaux composites.
Dans le cas des fibres de carbone et de graphite, les
concentrations en fibres respirables sont généralement inférieures
à 0,1 fibres/ml, mais on a mesuré des concentrations pouvant aller
jusqu'à 0,3 fibres/ml à proximité des opérations de coupe ou de
bobinage. Des fibres peuvent également être libérées dans
l'environnement lors de l'usinage (perçage, sciage, etc) des
composites à base de fibres de carbone, encore que la plupart des
matériaux respirables ainsi produits soient non fibreux.
On a fait état, sur les lieux de travail, de concentrations
moyennes en fibrilles de para-aramide aéroportées inférieures à
0,1 fibrille/ml au cours des opérations concernant les fils continus
et à 0,2 fibrilles/ml lors de la coupe en floc et en pulpe. En
filature, on observe des concentrations moyennes en fibrilles
aéroportées qui se caractérisent par une valeur inférieure à
0,5 fibrille/ml, mais on a aussi fait état de concentrations de
l'ordre de 2,0 fibrilles/ml. On note aussi des valeurs
caractéristiques de l'exposition moyenne inférieures à
0,1 fibrille/ml pour d'autres applications avec des maxima
atteignant 0,3 fibrilles/ml. On a mis en évidence un risque
particulier d'exposition lors du découpage des composites au jet
hyperbarique, les concentrations pouvant atteindre 2,91
fibrilles/ml. Au cours de découpage au laser de résines époxy
renforcées par des fibres d'aramide, des particules d'un diamètre
aérodynamique moyen de 0,21 µm on été observées mais l'étude en
question n'indique pas la teneur en fibrilles de la poussière. Au
cours de ce type d'opérations, il y également production d'un
certain nombre de composés organiques volatils (notamment du
benzène, du toluène, du benzonitrile et du styrène) ainsi que des
gaz comme le cyanure d'hydrogène, le monoxyde de carbone et le
dioxyde d'azote.
D'après des données limitées établies durant la surveillance de
l'air dans un atelier produisant des fibres de polypropylène, les
concentrations maximales de fibres aéroportées d'une longueur
dépassant les 5 µm, se situent autour de 0,5 fibre/ml, la plupart
des valeurs étant inférieures à 0,1 fibre/ml. L'examen au
microscope électronique à balayage a montré que le diamètre des
fibres aéroportées allait de 0,25 à 3,5 µm et leur longueur de
1,7 à 69 µm. Dans un échantillon unique d'air ambiant prélevé à
proximité d'une unité de tissage de fibres de carbone, on a relevé
une concentration de 0,0003 fibre/ml. Les dimensions moyennes des
fibres étaient de 706 µm par 3,9 µm. On a également signalé la
présence de fibres de carbone à l'endroit où s'étaient écrasés deux
aéronefs militaires, présence attribuable à la combustion du
composite de fibres de carbone utilisé pour la construction de ces
appareils. Aucune autre donnée utile sur les concentrations dans
l'environnement n'a été fournie.
4. Dépôt, élimination, rétention, persistance et redistribution
On ne dispose que de peu de données sur les fibres organiques
synthétiques. Des données relatives aux fibres de para-aramide
(Kevlar) indiquent que ces fibres se déposent au niveau de la
bifurcation des canaux alvéolaires. On pense également que ces
fibres sont transportées jusqu'aux ganglions lymphatiques
trachéo-bronchiques.
5. Effets sur les animaux de laboratoire et les systèmes in vitro
On manque de données satisfaisantes résultant d'études
expérimentales valables sur les divers types de fibres organiques
synthétiques examinées ici.
Aucune étude valable n'a été consacrée à l'examen du pouvoir
fibrogène ou cancérogène des fibres de carbone et de graphite.
Parmi les effets observés à la suite de l'exposition, pendant
quelques jours, par la voie respiratoire, de rats à des fibres de
dimension respirable produites à partir de différents types de
brais, on a relevé des réactions inflammatoires, un accroissement
du remplacement des cellules parenchymateuses et une hyperplasie
minime des cellules alvéolaires (pneumocytes de Type II). Les
données fournies par une étude au cours de laquelle on a procédé à
des instillations intratrachéennes et à des injections
intrapéritonéales, sont jugées insuffisantes pour permettre une
évaluation, du fait que les matériaux étudiés n'ont pas été
caractérisés et que l'étude ne donne pas suffisamment de
renseignements sur le protocole suivi et les résultats obtenus. Une
étude par badigeonnage cutané sur la souris, portant sur quatre
types de fibres de carbone en suspension dans le benzène, a été
considérée comme insuffisante pour permettre une évaluation de
l'activité cancérogène de ces fibres.
Dans le cas des fibres de para-aramide, l'essentiel des données
résulte de l'expérimentation sur le Kevlar. Des études de brève
durée (deux semaines), au cours desquelles des animaux ont été
exposés à de la poussière de Kevlar par la voie respiratoire, ont
montré qu'il y avait une réaction dont l'intensité diminuait après
cessation de l'exposition au niveau des macrophages pulmonaires.
Des études de brève durée portant sur des fibrilles ultrafines de
Kevlar, ont révélé une réaction analogue au niveau de macrophages
avec des plaques d'épaississement au niveau des canaux alvéolaires.
Ces deux types de lésion ont également régressé après l'exposition,
mais trois à six mois plus tard on observait encore une fibrose
minime résiduelle. L'exposition par la voie respiratoire, pendant
deux ans, de rats à des fibrilles de Kevlar a produit une fibrose
pulmonaire liée à cette exposition (à une concentration > 25
fibres/ml) et a conduit à la formation de tumeurs pulmonaires (11% à
la concentration de 400 fibres/ml et 6% à la concentration de
100 fibres/ml chez les femelles; 3% à la concentration de
400 fibres/ml chez les mâles) d'un type inhabituel (carcinomes
spino-cellulaires kystiques kératinisants). Une surmortalité due à
une toxicité pulmonaire a été observée à la concentration la plus
élevée, ce qui indique que la dose maximale tolérable avait été
dépassée. La portée biologique de ces lésions et leur signification
pour la santé humaine ont été très débattues. Il est possible que
cette étude, qui s'est achevée au bout de 24 mois, n'ait pas révélé
la totalité du pouvoir cancérogène des fibrilles.
L'instillation intertrachéenne d'une dose unique de papier
désagrégé Nomex (2,5 mg) contenant des fibres d'un diamètre allant
de 2 à 30 µm a produit une réaction inflammatoire non spécifique.
Une réaction granulomateuse s'est produite deux ans après
l'exposition. L'instillation intratrachéenne d'une dose unique de 25
mg de Kevlar a provoqué une réaction inflammatoire non spécifique
qui a disparu en l'espace d'environ une semaine. Ultérieurement, on
a observé une réaction granulomateuse et une fibrose minime.
Dans trois études, l'injection intrapéritonéale de fibres de
Kevlar (jusqu'à 25 mg/kg) a induit une réponse granulomateuse sans
toutefois que l'incidence des néoplasmes ne présente d'augmentation
significative. Selon les auteurs de cette étude, l'absence de
réponse tumorale pourrait s'expliquer par l'agglomération des
fibrilles de Kevlar dans la cavité péritonéale.
Il n'existe pas d'études valables au cours desquelles on ait
examiné le pouvoir cancérogène ou fibrogène des fibres de
polyoléfines. Une étude au cours de laquelle on a exposé des rats à
des fibres respirables de polypropylène pendant 40 jours par la voie
respiratoire (46% des fibres < 1 µm) à des concentrations allant
jusqu'à 50 fibres/ml, a permis de constater des modifications liées
à la dose et à la durée de l'exposition et qui se caractérisaient
par un accroissement de la cellularité et une bronchiolite. On ne
dispose d'aucune donnée utile sur l'effet de l'instillation
intratrachéenne. Des études au cours desquelles on a injecté à des
rats des fibres ou de la poussière de polypropylène dans la cavité
intrapéritonéale, n'ont pas révélé d'accroissement sensible du
nombre de tumeurs péritonéales.
On ne dispose pas de données suffisantes pour pouvoir évaluer
la toxicité in vitro et la génotoxicité des fibres organiques
synthétiques. Dans le cas des aramides, les études montrent que le
fibrilles de para-aramide fines et courtes présentent des propriétés
cytotoxiques. Pour ce qui est des fibres polyoléfiniques, il
semblerait que les fibres de polypropylène présentent une certaine
cytotoxicité. Les tests de mutagénicité sur des extraits de granulés
de polyéthylène n'ont donné que des résultats négatifs.
6. Effets sur l'homme
Une étude transversale relative à 88 des 110 employés d'une
unité de production de fibres de carbone continues à base de
polyacrylonitrile, n'a révélé aucun effet nocif sur la fonction
respiratoire comme l'ont montré les examens radiographiques et
spirométriques et les questionnaires sur les symptômes
respiratoires. D'autres études moins bien documentées ont fait état
d'effets indésirables chez des ouvriers travaillant à la production
de fibres de carbone et de polyamide; les données qui figurent dans
ces publications sont toutefois insuffisantes pour qu'on puisse se
prononcer sur la validité des inférences indiquées.
7. Résumé de l'évaluation
On ne dispose que de données limitées sur les niveaux
d'exposition à la plupart des fibres organiques synthétiques. Les
données disponibles indiquent en général que sur les lieux de
travail, l'exposition est faible. Toutefois il subsiste un risque
d'exposition plus importante lors d'applications et d'utilisations
futures. On ne dispose pratiquement d'aucune donnée sur la destinée
et la répartition dans l'environnement de ces fibres ni sur
l'exposition de la population générale.
En se basant sur les données toxicologiques limitées fournies
par l'expérimentation animale, on peut conclure qu'il existe une
possibilité d'effets nocifs sur la santé en cas d'exposition par la
voie respiratoire à ces fibres organiques de synthèse sur le lieu de
travail. On ignore pour l'instant le risque qu'impliquerait
l'exposition à ces fibres dans l'environnement général, mais il est
probablement très faible.
RESUMEN
1. Identidad, propiedades físicas y químicas
Las fibras de carbono/grafito son formas filamentosas de carbón
que se obtienen procesando a alta temperatura alguno de los tres
materiales precursores siguientes: rayón (celulosa regenerada), brea
(residuo de petróleo o alquitrán) o poliacrilonitrilo (PAN). El
diámetro nominal de las fibras de carbono oscila entre 5 y 15 µm.
Las fibras de carbono son flexibles, eléctrica y térmicamente
conductivas, y sus variedades de alto rendimiento poseen un módulo
de Young (coeficiente de elasticidad que refleja la mayor o menor
rigidez del material) alto y una gran resistencia a la tracción. Son
resistentes a la corrosión, ligeras, refractivas y químicamente
inertes (excepto a la oxidación), y presentan una gran estabilidad
frente a las fuerzas de tracción, una baja densidad y expansión
térmica y una alta resistencia a la abrasión y al desgaste.
Las fibras de aramida se forman por reacción entre diaminas
aromáticas y dicloroácidos aromáticos. Son producidas en forma de
filamentos continuos, hebras y pulpa. Hay dos tipos principales de
fibras de aramida: para- y meta-aramida, ambas con un diámetro
nominal de 12-15 µm. Las fibras de para-aramida pueden presentar,
adheridas a la superficie de su parte central, fibrillas muy rizadas
y entrelazadas del tamaño de las partículas respirables
(< 1 µm de diámetro). Estas fibrillas pueden desprenderse y quedar
suspendidas en el aire en caso de abrasión durante su fabricación o
empleo. Por lo general, las fibras de aramida presentan una
resistencia a la tracción entre mediana y muy alta, una elongación
entre mediana y baja, y un módulo de Young entre moderado y muy
alto. Son resistentes al calor, a los productos químicos y a la
abrasión.
Las fibras de poliolefina son polímeros de cadena larga
compuestos por al menos un 85% (respecto al peso) de etileno,
propileno u otras unidades de olefina; el polietileno y el
polipropileno se emplean comercialmente. Salvo algunas excepciones,
como la microfibra, los diámetros nominales de la mayoría de los
distintos tipos de fibras de poliolefina son bastante grandes, y no
abundan los de tamaño respirable.
Las poliolefinas son extremadamente hidrofóbicas e inertes. Su
resistencia a la tracción es notablemente inferior a la del carbono
o de las fibras de aramida. Son relativamente inflamables, y sus
temperaturas de fusión oscilan entre 100 y 200 °C.
Para la vigilancia, a efectos de higiene industrial, de las
fibras orgánicas sintéticas se han utilizado métodos desarrollados
para contar fibras minerales, métodos que, no obstante, no han sido
validados para esa finalidad. Factores tales como las propiedades
electrostáticas, la solubilidad en el líquido de montaje y el índice
refractivo pueden ser fuente de problemas al emplear esos métodos.
2. Fuentes de exposición humana y ambiental
La producción mundial estimada de fibras de carbono y grafito
superó las 4000 toneladas en 1984. Por lo que se refiere a la
aramida, se superaron las 30 000 toneladas en 1989, y la producción
de fibras de poliolefina sobrepasó las 182 000 toneladas (sólo en
los Estados Unidos de América). Las fibras de carbono y aramida se
emplean principalmente para fabricar materiales compuestos avanzados
en las industrias aeroespacial, militar y otras, con el objeto de
mejorar su resistencia, rigidez, durabilidad, conductividad
eléctrica o resistencia térmica. Las fibras de poliolefina se
utilizan normalmente en la industria textil.
Se han descrito casos de exposición a fibras orgánicas
sintéticas en el medio ocupacional. Durante la producción,
elaboración o combustión de materiales compuestos, así como durante
su evacuación, puede producirse una liberación de fibras orgánicas
sintéticas en el medio ambiente. Se dispone de muy pocos datos sobre
la liberación real de esos materiales en el entorno.
Los datos disponibles sobre el transporte, distribución y
transformación de fibras orgánicas en el medio ambiente se limitan a
la identificación de los productos que resultan de la incineración
municipal de los desechos a partir de compuestos que contienen
fibras de carbono y de productos de la descomposición pirolítica de
fibras de carbono y aramidas. Durante la simulación de la
incineración municipal se redujeron tanto el diámetro como la
longitud de las fibras de carbono. Entre los principales productos
de la descomposición pirolítica de fibras de carbono y aramida
figuran hidrocarburos aromáticos, dióxido de carbono, monóxido de
carbono, y cianuros.
3. Niveles ambientales y exposición humana
En el lugar de trabajo se liberan fibrillas orgánicas
sintéticas durante operaciones tales como la producción, bobinado,
troceado, entrelazado, corte y maquinado de las fibras, así como
durante la formación y manipulación de materiales compuestos.
En el caso de las fibras de carbono/grafito, las
concentraciones de fibra respirable son por lo general inferiores a
0,1 fibras/ml, pero se han detectado concentraciones de hasta
0,3 fibras/ml en las proximidades de instalaciones de troceado o
bobinado. También pueden liberarse fibras durante el maquinado
(perforación, aserrado, etc.) de compuestos de fibra de carbono, si
bien la mayor parte del material respirable generado en esos casos
no es de carácter fibroso.
Se ha notificado que las concentraciones medias de fibrillas de
para-aramida en suspensión en el aire del lugar de trabajo son
inferiores a 0,1 fibrillas/ml trabajando filamentos, y de menos de
0,2 fibrillas/ml en las instalaciones de corte de vedijas y
fabricación de pulpa. Durante el procesamiento de la hilaza, la
concentración media de fibrillas en suspensión en el aire es
generalmente inferior a 0,5 fibrillas/ml, si bien se han notificado
niveles de hasta aproximadamente 2,0 fibrillas/ml. Otras
exposiciones asociadas a usos finales en el lugar de trabajo son
normalmente inferiores a 0,1 fibrillas/ml como promedio, cifrándose
las exposiciones máximas en 0,3 fibrillas/ml. Se ha demostrado que
conlleva un riesgo especial de exposición el corte de materiales
compuestos por chorro de agua hiperbárico, operación en la que se
han detectado niveles de hasta 2,91 fibrillas/ml. Se han generado
partículas de un diámetro aerodinámico medio de 0,21 µm durante el
corte mediante láser de epoxiplásticos reforzados con fibras de
aramida, pero no se ha notificado el contenido de fibra del polvo.
Durante esas operaciones se producen también determinados compuestos
orgánicos volátiles (en particular benceno, tolueno, benzonitrilo y
estireno) y otros gases (cianuro de hidrógeno, monóxido de carbono y
dióxido de nitrógeno).
Los datos limitados obtenidos en una fábrica de producción de
fibras de polipropileno muestran que en el caso de las fibras de más
de 5 µm de longitud su concentración máxima en el aire era de 0,5
fibras/ml, siendo la mayoría de los valores inferiores a
0,1 fibras/ml. La microscopía electrónica de barrido mostró que el
tamaño de las fibras suspendidas en el aire oscilaba entre 0,25 y
3,5 µm de diámetro y 1,7 y 69 µm de longitud. En una sola muestra
ambiental recogida cerca de una tejeduría de fibra de carbono, se
detectó una concentración de 0,0003 fibras/ml. La dimensión de esas
fibras era como promedio de 706 µm por 3,9 µm. Se ha notificado
también la liberación de fibra de carbono en el lugar de colisión de
dos aeronaves militares, de resultas de la combustión del compuesto
de fibra de carbono empleado en su construcción. No se obtuvo ningún
otro dato de interés sobre las concentraciones presentes en el medio
ambiente.
4. Depósito, eliminación, retención, durabilidad y translocación
La información obtenida acerca de fibras orgánicas sintéticas
específicas es escasa. Los datos referentes a las fibras de
para-aramida inhaladas (Kevlar) indican que éstas se depositan en
las bifurcaciones de los conductos alveolares. Hay también indicios
de translocación a los nódulos linfáticos traqueobronquiales.
5. Efectos en los animales de experimentación y en los sistemas de
prueba in vitro
En lo que respecta a los tipos de fibra orgánica sintética aquí
analizados, son muy pocos los datos de buena calidad aportados por
los estudios experimentales realizados al efecto.
No hay ningún estudio en que se haya examinado adecuada mente
el potencial fibrógeno o carcinógeno de las fibras de
carbono/grafito. En ratas expuestas a la inhalación de corta
duración (unos días) de fibras de tamaño respirable obtenidas a
partir de brea se observaron respuestas inflamatorias, un aumento de
la velocidad de recambio de las células parenquimatosas y una
hiperplasia mínima de las células alveolares de tipo II. Se
considera que los resultados de un estudio realizado mediante
instilación intratraqueal e inyección intraperitoneal no se prestan
a evaluación, debido a la insuficiente caracterización del material
de ensayo y a la falta de documentación adecuada sobre el protocolo
y los resultados. Un estudio realizado con ratones a los que se
pintó la piel con cuatro tipos de fibra de carbono suspendidos en
benceno resultó inadecuado para evaluar la actividad carcinógena.
En el caso de las fibras de paraaramida, la mayor parte de los
datos proceden de experimentos realizados con Kevlar. Los estudios
efectuados sobre los efectos de la inhalación de corta duración
(2 semanas) de polvo de Kevlar han puesto de manifiesto una
respuesta macrofágica pulmonar cuya gravedad disminuye tras la
interrupción de la exposición. Los estudios de corta duración
realizados con fibrillas de Kevlar ultrafinas han revelado una
respuesta macrofágica parecida y un espesamiento desigual de los
conductos alveolares. Esos dos tipos de lesiones remitieron también
tras la exposición, pero a los 3-6 meses persistía todavía un grado
mínimo de fibrosis. En un estudio realizado con ratas a las que se
sometió durante dos años a inhalación de fibrillas de Kevlar se
observó la aparición, relacionada con esa exposición, de fibrosis
pulmonar (a concentraciones superiores a 25 fibras/ml) y de
neoplasias pulmonares de un tipo inusitado (carcinoma escamoso
quístico queratinizante) en un 11% de hembras a concentraciones de
400 fibras/ml, en un 6% de hembras a concentraciones de 100
fibras/ml; y en un 3% en los machos a concentraciones de 400
fibras/ml. El aumento de mortalidad por toxicidad pulmonar se
observó a la mayor de las concentraciones, lo que sugiere que se
había sobrepasado la Dosis Máxima Tolerada. Existe una considerable
polémica acerca del potencial biológico de esas lesiones y su
trascendencia para la especie humana. Ese estudio, por haber durado
sólo 24 meses, tal vez no haya revelado todo el potencial
carcinógeno de las fibrillas.
La instilación intratraqueal de una sola dosis de papel Nomex
troceado (2,5 mg) que contenía fibras de diámetros comprendidos
entre 2 y 30 µm provocó una respuesta inflamatoria inespecífica; dos
años después de la exposición tuvo lugar una respuesta
granulomatosa. La instilación intratraqueal de una sola dosis de 25
mg de Kevlar provocó una respuesta inflamatoria inespecífica que
remitió al cabo de una semana aproximadamente; más tarde se observó
una respuesta granulomatosa y un grado mínimo de fibrosis.
En tres estudios en que se inyectaron intraperitonealmente
fibras de Kevlar (hasta 25 mg/kg) se observó una respuesta
granulomatosa, pero no así un aumento significativo de la incidencia
de neoplasias. Los autores de esas investigaciones indicaron que si
no se había observado una respuesta neoplásica era posiblemente
porque se había producido una aglomeración de las fibrillas de
Kevlar en la cavidad peritoneal.
No existen estudios en que se haya examinado adecuadamente el
potencial fibrógeno o carcinógeno de las fibras de poliolefina. En
un experimento realizado con ratas sometidas durante 90 días a
inhalación de fibras de polipropileno (hasta 50 fibras/ml)
respirables (46% < 1 µm) se observaron cambios dependientes de la
dosis y duración de la exposición, consistentes en un aumento de la
celularidad y bronquiolitis. No se dispone de datos pertinentes
sobre los efectos de la instilación intratraqueal. En los estudios
efectuados en ratas mediante inyección intraperitoneal de fibras o
polvo de polipropileno no se observó ningún aumento significativo de
la incidencia de tumores peritoneales.
Los datos de que se dispone para evaluar la toxicidad y
genotoxicidad in vitro de las fibras orgánicas sintéticas no son
adecuadas. En el caso de las aramidas, los estudios han demostrado
que las fibrillas cortas y finas de para-aramida tienen propiedades
citotóxicas. Respecto a las fibras de poliolefina, hay algunos
indicios de que las fibras de polipropileno son citotóxicas. Las
pruebas de mutagenicidad realizadas con extractos de gránulos de
polietileno arrojaron resultados negativos.
6. Efectos en el ser humano
En un estudio transversal realizado en 88 de los 110
trabajadores de una fábrica de producción de fibra de carbono de
filamento continuo a partir de poliacrilonitrilo, la exploración
radiográfica y espirométrica y los cuestionarios sobre síntomas
respiratorios no pusieron de manifiesto ningún efecto respiratorio
nocivo. En otros estudios no tan bien documentados se notificó el
hallazgo de efectos adversos en trabajadores dedicados a la
producción de fibras tanto de carbono como de poliamida; los datos
publicados sobre estas investigaciones, sin embargo, eran
insuficientes para determinar la validez de las correlaciones
señaladas.
7. Resumen de la evaluación
La información sobre los niveles de exposición a la mayoría de
las fibras orgánicas sintéticas es limitada. Los datos disponibles
indican en general que los niveles de exposición en el entorno
ocupacional son bajos. Cabe la posibilidad de que en futuras
aplicaciones y usos se alcancen exposiciones más elevadas. No se
sabe prácticamente nada sobre lo que ocurre con esos productos en el
medio ambiente ni sobre su distribución o las exposiciones de la
población general.
A tenor de los escasos datos toxicológicos obtenidos con
animales de laboratorio, cabe concluir que existe la posibilidad de
que la exposición ocupacional a esas fibras orgánicas sintéticas por
inhalación tenga efectos adversos en la salud. El riesgo potencial
para la salud asociado a la exposición a las fibras orgánicas
sintéticas presentes en el entorno general se desconoce por el
momento, pero probablemente sea muy bajo.