Asbestos and other natural mineral fibres (EHC 53, 1986)

INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY

ENVIRONMENTAL HEALTH CRITERIA 53

ASBESTOS AND OTHER NATURAL MINERAL 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

World Health Orgnization
Geneva, 1986

The International Programme on Chemical Safety (IPCS) is a
joint venture of the United Nations Environment Programme, the
International Labour Organisation, and the World Health
Organization. The main objective of the IPCS is to carry out and
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toxicology. Other activities carried out by the IPCS include the
development of know-how for coping with chemical accidents,
coordination of laboratory testing and epidemiological studies, and
promotion of research on the mechanisms of the biological action of
chemicals.

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CONTENTS

ENVIRONMENTAL HEALTH CRITERIA FOR ASBESTOS AND OTHER NATURAL
MINERAL FIBRES

1. SUMMARY AND RECOMMENDATIONS FOR FURTHER RESEARCH

1.1. Summary
1.1.1. Identity; physical and chemical properties,
methods of sampling and analysis
1.1.2. Sources of occupational and environmental exposure
1.1.3. Environmental levels and exposures
1.1.4. Toxicological effects on animals
1.1.5. Effects on man
1.1.6. Evaluation of health risks
1.2. Recommendations for further research

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, SAMPLING AND
ANALYSIS

2.1. Identity; physical and chemical properties
of asbestos minerals
2.1.1. Serpentine group minerals - chrysotile
2.1.2. Amphibole group minerals
2.1.2.1 Crocidolite (Riebeckite asbestos)
2.1.2.2 Amosite (Grunerite asbestos)
2.1.2.3 Anthophyllite asbestos
2.1.2.4 Tremolite and actinolite asbestos
2.2. Identity; physical and chemical properties
of other natural mineral fibres
2.2.1. Fibrous zeolites
2.2.2. Other fibrous silicates (attapulgite,
sepiolite, and wollastonite)
2.3. Sampling and analytical methods
2.3.1. Collection and preparation of samples
2.3.1.1 Air
2.3.1.2 Water
2.3.1.3 Biological tissues
2.3.1.4 Geological samples
2.3.2. Analysis
2.3.2.1 Light microscopy
2.3.2.2 Electron microscopy
2.3.2.3 Gravimetric determination
2.3.3. Other methods
2.3.4. Relationships between fibre, particle, and mass
concentration

3. SOURCES OF OCCUPATIONAL AND ENVIRONMENTAL EXPOSURE

3.1. Natural occurrence
3.2. Man-made sources
3.2.1. Asbestos
3.2.1.1 Production
3.2.1.2 Mining and milling
3.2.1.3 Uses

3.2.2. Other natural mineral fibres
3.2.3. Manufacture of products containing asbestos
3.2.3.1 Asbestos-cement products
3.2.3.2 Vinyl asbestos floor tiles
3.2.3.3 Asbestos paper and felt
3.2.3.4 Friction materials (brake
linings and clutch facings)
3.2.3.5 Asbestos textiles
3.2.4. Use of products containing asbestos

4. TRANSPORT AND ENVIRONMENTAL FATE

4.1. Transport and distribution
4.1.1. Transport and distribution in air
4.1.2. Transport and distribution in water
4.2. Environmental transformation, interaction, and
degradation processes

5. ENVIRONMENTAL EXPOSURE LEVELS

5.1. Air
5.1.1. Occupational exposure
5.1.2. Para-occupational exposure
5.1.3. Ambient air
5.2. Levels in other media

6. DEPOSITION, TRANSLOCATION, AND CLEARANCE

6.1. Inhalation
6.1.1. Asbestos
6.1.1.1 Fibre deposition
6.1.1.2 Fibre clearance, retention,and translocation
6.1.2. Ferruginous bodies
6.1.3. Content of fibres in the respiratory tract
6.2. Ingestion

7. EFFECTS ON ANIMALS AND CELLS

7.1. Asbestos
7.1.1. Fibrogenicity
7.1.1.1 Inhalation
7.1.1.2 Intrapleural and intraperitoneal injection
7.1.1.3 Ingestion
7.1.2. Carcinogenicity
7.1.2.1 Inhalation
7.1.2.2 Intratracheal instillation
7.1.2.3 Direct administration into body cavities
7.1.2.4 Ingestion
7.1.3. In vitro studies
7.1.3.1 Haemolysis
7.1.3.2 Macrophages
7.1.3.3 Fibroblasts
7.1.3.4 Cell-lines and interaction with DNA
7.1.3.5 Mechanisms of fibrogenic and carcinogenic
action of asbestos

7.1.3.6 Factors modifying carcinogenicity
7.2. Other natural mineral fibres
7.2.1. Fibrous clays
7.2.1.1 Palygorskite (Attapulgite)
7.2.1.2 Sepiolite
7.2.2. Wollastonite
7.2.3. Fibrous zeolites - erionite
7.2.4. Assessment

8. EFFECTS ON MAN

8.1. Asbestos
8.1.1. Occupational exposure
8.1.1.1 Asbestosis
8.1.1.2 Pleural thickening, visceral, and parietal
8.1.1.3 Bronchial cancer
8.1.1.4 Mesothelioma
8.1.1.5 Other cancers
8.1.1.6 Effects on the immune system
8.1.2. Para-occupational exposure
8.1.2.1 Neighbourhood exposure
8.1.2.2 Household exposure
8.1.3. General population exposure
8.2. Other natural mineral fibres
8.2.1. Fibrous clays
8.2.1.1 Palygorskite (Attapulgite)
8.2.1.2 Sepiolite
8.2.2. Wollastonite
8.2.3. Fibrous zeolites - erionite

9. EVALUATION OF HEALTH RISKS FOR MAN FROM EXPOSURE TO ASBESTOS
AND OTHER NATURAL MINERAL FIBRES

9.1. Asbestos
9.1.1. General considerations
9.1.2. Qualitative approach
9.1.2.1 Occupational
9.1.2.2 Para-occupational exposure
9.1.2.3 General population exposure
9.1.3. Quantitative approach
9.1.3.1 Bronchial cancer
9.1.3.2 Mesothelioma
9.1.3.3 Risk assessment based on mesothelioma
incidence in women
9.1.4. Estimating the risk of gastrointestinal cancer
9.2. Other natural mineral fibres
9.3. Conclusions
9.3.1. Asbestos
9.3.1.1 Occupational risks
9.3.1.2 Para-occupational risks
9.3.1.3 General population risks
9.3.2. Other mineral fibres

10. PREVIOUS EVALUATIONS BY INTERNATIONAL BODIES

10.1. IARC
10.2. CEC

REFERENCES

WHO TASK GROUP ON ASBESTOS AND OTHER NATURAL MINERAL FIBRES

Members

Dr I.M. Ferreira, Department of Preventive and Social Medicine,
Unicamp, Campinas, Brazil

Dr J.C. Gilson, Hembury Hill Farm, Honiton, Devon, United Kingdom
(Chairman)

Professor M. Ikeda, Department of Environmental Health, Tohoku
University School of Medicine, Sendai, Japan

Dr V. Kodat, Department of Hygiene and Epidemiology, Ministry of
Health of the Czech Socialist Republic, Prague, Vinohrady,
Czechoslovakia

Dr A.M. Langer, Environmental Sciences Laboratory, Mount Sinai
School of Medicine, New York, New York, USA

Dr F. Mansour, Amiantit, Saudi Arabia and Middle East, Damman,
Saudi Arabia

Ms M.E. Meek, Health and Welfare Canada, Health Protection Branch,
Environmental Health Centre, Tunney's Pasture, Ottawa, Ontario,
Canada (Rapporteur)

Ms C. Sonich-Mullin, US Environmental Protection Agency, ECAO,
Cincinnati, Ohio, USA

Dr U.G. Oleru, College of Medicine, University of Lagos, Lagos,
Nigeria (Vice-Chairman)

Professor K. Robock, Institute for Applied Fibrous Dust Research,
Neuss, Federal Republic of Germany

Members from Other Organizations

Dr A. Berlin, Commission of the European Communities, Luxembourg

Dr A.R. Kolff van Oosterwijk, Commission of European Communities,
Luxembourg

Observers

Dr K. Browne, Asbestos International Association, London, United
Kingdom

Dr E. Costa, Asbestos International Association (London), Genoa,
Italy

Dr J. Dunnigan, L'Institut de l'Amiante, Sherbrooke, Canada

Dr Fischer, Federal Health Office, Berlin (West)

Dr R. Konstanty, German Trade Union Congress, Düsseldorf, Federal
Republic of Germany

Mr L. Mazzuckelli, National Institute for Occupational Safety and
Health, Cincinnati, Ohio, USA

Dr E. Meyer, Federal Health Office, Institute for Hygiene of Water,
Soil, and Air, Berlin (West)

Dr H.-J. Nantke, Umweltbundesamt, Berlin (West)

Secretariat

Professor F. Valic, IPCS Consultant, World Health Organization,
Geneva, Switzerland (Secretary)^a

Dr A. David, International Labour Office, Geneva, Switzerland

Mr A. Fletcher, International Agency for Research on Cancer, Lyons,
France^b

Ms B. Goelzer, Office of Occupational Health, World Health
Organization, Geneva, Switzerland

Dr H. Muhle, Fraunhofer Institute for Toxicology and Aerosol
Research, Hanover, Federal Republic of Germany (Temporary
Adviser)

---------------------------------------------------------------------------
^a Department of Public Health, Andrija Stampar School of
Public Health, University of Zagreb, Zagreb, Yugoslavia
^b Present for only part of meeting.

NOTE TO READERS OF THE CRITERIA DOCUMENTS

Every effort has been made to present information in the
criteria documents as accurately as possible without unduly
delaying their publication. In the interest of all users of the
environmental health criteria documents, readers are kindly
requested to communicate any errors that may have occurred to the
Manager of the International Programme on Chemical Safety, World
Health Organization, Geneva, Switzerland, in order that they may be
included in corrigenda, which will appear in subsequent volumes.

ENVIRONMENTAL HEALTH CRITERIA FOR ASBESTOS AND OTHER NATURAL
MINERAL FIBRES

Following the recommendations of the United Nations Conference
on the Human Environment held in Stockholm in 1972, and in response
to a number of resolutions of the World Health Assembly and a
recommendation of the Governing Council of the United Nations
Environment Programme, a programme on the integrated assessment of
the health effects of environmental pollution was initiated in
1973. The programme, known as the WHO Environmental Health
Criteria Programme, has been implemented with the support of the
Environment Fund of the United Nations Environment Programme. In
1980, the Environmental Health Criteria Programme was incorporated
into the International Programme on Chemical Safety (IPCS), a joint
venture of the United Nations Environment Programme, the
International Labour Organisation, and the World Health
Organization. The Programme is responsible for the publication of
a series of criteria documents.

A WHO Task Group on Environmental Health Criteria for Asbestos
and Other Natural Mineral Fibres was held at the Fraunhofer
Institute for Toxicology and Aerosol Research, Hanover, Federal
Republic of Germany from 15-22 July 1985. Professor W. Stöber
opened the meeting and greeted the members on behalf of the host
institution, and Dr U. Schlottmann spoke on behalf of the
Government. Professor F. Valic addressed the meeting on behalf of
the three co-sponsoring organizations of the IPCS (WHO/ILO/UNEP).
The Task Group reviewed and revised the draft criteria document and
made an evaluation of the risks for human health from exposure to
asbestos and other natural mineral fibres.

The first draft of the document was a combination of texts
prepared by DR H. MUHLE and DR K. SPURNY of the Fraunhofer
Institute for Toxicology and Aerosol Research, Hanover, Federal
Republic of Germany, PROFESSOR F. POTT of the Medical Institute for
Environmental Hygiene, Düsseldorf, Federal Republic of Germany,
PROFESSOR J. PETO, of the Institute of Cancer, University of
London, London, United Kingdom, PROFESSOR M. LIPPMANN, of the
Institute of Environmental Medicine, New York University Medical
Center, New York, USA, MS M.E. MEEK, Department of National Health
and Welfare, Ottawa, Canada, and DR J.F. STARA and MS C. SONICH-
MULLIN, of the US Environmental Protection Agency, Cincinnati,
Ohio, USA.

A Working Group consisting of PROFESSOR C. McDONALD, MS M.E.
MEEK, DR H. MUHLE, MS J. HUGHES, and PROFESSOR F. VALIC reviewed
the first, and developed the second, draft.

The efforts of all who helped in the preparation and
finalization of the document are gratefully acknowledged.

1. SUMMARY AND RECOMMENDATIONS FOR FURTHER RESEARCH

1.1. Summary

1.1.1. Identity; physical and chemical properties, methods
of sampling and analysis

The commercial term asbestos refers to a group of fibrous
serpentine and amphibole minerals that have extraordinary tensile
strength, conduct heat poorly, and are relatively resistant to
chemical attack. The principal varieties of asbestos used in
commerce are chrysotile, a serpentine mineral, and crocidolite and
amosite, both of which are amphiboles. Anthophyllite, tremolite,
and actinolite asbestos are also amphiboles, but they are rare, and
the commercial exploitation of anthophyllite asbestos has been
discontinued. Other natural mineral fibres that are considered
potentially hazardous because of their physical and chemical
properties are erionite, wollastonite, attapulgite, and sepiolite.

Chrysotile fibres consist of aggregates of long, thin, flexible
fibrils that resemble scrolls or cylinders. The dimensions of
individual chrysotile fibres depend on the extent to which the
sample has been manipulated. Amphibole fibres generally tend to be
straight and splintery. Crocidolite fibrils are shorter with a
smaller diameter than other amphibole fibrils, but they are not as
narrow as fibrils of chrysotile. Amosite fibrils are larger in
diameter than those of both crocidolite and chrysotile. Respirable
fractions of asbestos dust vary according to fibre type and
manipulation.

Several methods involving optical phase contrast microscopy
have been developed for determining levels of asbestos fibres in
the air of work-places. Only fibres over 5 µm in length with an
aspect ratio > 3:1 and a diameter of less than 3 µm are counted.
Thus, the resulting fibre count can be regarded only as an index of
actual numbers of fibres present in the sample (fibres with
diameters less than the resolution of the light microscope are not
included in this assay). Fibres with diameters smaller than
approximately 0.25 µm cannot be seen by light microscopy, and an
electron microscope is necessary for counting and identifying these
fibres. Electron microscopes that are equipped with auxiliary
equipment can provide information on both structure and elemental
composition.

The results of analysis using light microscopy can be compared
with those using transmission or scanning electron microscopy, but
only if the same counting criteria are used.

1.1.2. Sources of occupational and environmental exposure

Asbestos is widely distributed in the earth's crust.
Chrysotile, which accounts for more than 95% of the world asbestos
trade, occurs in virtually all serpentine rocks. The remainder
consists of the amphiboles (amosite and crocidolite). Chrysotile
deposits are currently exploited in more than 40 countries; most of
these reserves are found in southern Africa, Canada, China, and the

USSR. There are, reportedly, thousands of commercial and
industrial applications of asbestos.

Dissemination of asbestos and other mineral fibres from natural
deposits may be a source of exposure for the general population.
Unfortunately, few quantitative data are available. Most of the
asbestos present in the atmosphere and ambient water probably
results from the mining, milling, and manufacture of asbestos or
from the deterioration or breakage of asbestos-containing
materials.

1.1.3. Environmental levels and exposures

Asbestos is ubiquitous in the environment because of its
extensive industrial use and the dissemination of fibres from
natural sources. Available data using currently-accepted methods
of sampling and analysis indicate that fibre levels (fibres > 5 µm
in length) at remote rural locations are generally below the
detection limit (less than 1 fibre/litre), while those in urban air
range from < 1 to 10 fibres/litre or occasionally higher.
Airborne levels in residential areas in the vicinity of industrial
sources have been found to be within the range of those in urban
areas or occasionally slightly higher. Non-occupational indoor
levels are generally within the range found in the ambient air.
Occupational exposure levels vary depending on the effectiveness of
dust-control measures; they may be up to several hundred fibres/ml
in industry or mines without or with poor dust control, but are
generally well below 2 fibres/ml in modern industry.

Reported concentrations in drinking-water range up to 200 x 10⁶
fibres/litre (all fibre lengths).

1.1.4. Toxicological effects on animals

Fibrosis in many animal species, and bronchial carcinomas and
pleural mesotheliomas in the rat, have been observed following
inhalation of both chrysotile and amphibole asbestos. In these
studies, there were no consistent increases in tumour incidence at
other sites, and there is no convincing evidence that ingested
asbestos is carcinogenic in animals. Data from the inhalation
studies have shown that shorter asbestos fibres are less fibrogenic
and carcinogenic.

Few data are available concerning the pathogenicity of the
other natural mineral fibres. Fibrosis in rats has been observed
following inhalation of attapulgite and sepiolite; a remarkably
high incidence of mesotheliomas occurred in rats following
inhalation of erionite. Long-fibred attapulgite induced
mesotheliomas following intrapleural and intraperitoneal
administration. Wollastonite also induced mesothelioma after
intrapleural administration. Erionite induced extremely high
incidences of mesotheliomas following inhalation exposure and
intrapleural and intraperitoneal administration.

The length, diameter, and chemical composition of fibres are
important determinants of their deposition, clearance, and
translocation within the body. Available data also indicate that
the potential of fibres to induce mesotheliomas following
intrapleural or intraperitoneal injection in animal species is
mainly a function of fibre length and diameter; in general, fibres
with maximum carcinogenic potency have been reported to be longer
than 8 µm and less than 1.5 µm in diameter.

1.1.5. Effects on man

Epidemiological studies, mainly on occupational groups, have
established that all types of asbestos fibres are associated with
diffuse pulmonary fibrosis (asbestosis), bronchial carcinoma, and
primary malignant tumours of the pleura and peritoneum
(mesothelioma). That asbestos causes cancers at other sites is
less well established. Gastrointestinal and laryngeal cancer are
possible, but the causal relationship with asbestos exposure has
not yet been firmly established; there is no substantial supporting
evidence for cancer at other sites. Asbestos exposure may cause
visceral and parietal pleural changes.

Cigarette smoking increases the asbestosis mortality and the
risk of lung cancer in persons exposed to asbestos but not the risk
of mesothelioma. Generally, cases of malignant mesothelioma are
rapidly fatal. The observed incidence of these tumours, which was
low until about 30 years ago, has been increasing rapidly in males
in industrial countries. As asbestos-related mesothelioma became
more widely accepted and known to pathologists in western
countries, reports of mesothelioma increased. The incidence of
mesothelioma prior to, e.g., 1960, is not known. Mesotheliomas
have seldom followed exposure to chrysotile asbestos only. Most,
but not all, cases of mesothelioma have a history of occupational
exposure to amphibole asbestos, principally crocidolite, either
alone or in amphibole-chrysotile mixtures.

There is strong evidence that one non-asbestos fibrous mineral
(erionite) is carcinogenic in man. This fibrous zeolite is likely
to be the cause of localized endemic mesothelioma in Turkey.

Non-malignant thickening of the visceral pleura is frequently
associated with asbestosis. Thickening of the parietal pleura,
sometimes with calcification, may occur in the absence of
detectable asbestosis. It is seen in those occupationally exposed
to asbestos and also occurs endemically in a number of countries,
but the causes have not been fully established. Tremolite fibre
has been implicated as an etiological agent in some regions.

1.1.6. Evaluation of health risks

At present, past exposure to asbestos in industry or in the
general population has not been sufficiently well defined to make
an accurate assessment of the risks from future levels of exposure,
which are likely to be low.

A simple risk assessment is not possible for asbestos. In
making an assessment, the emphasis is placed on the incidence of
lung cancer and mesothelioma, the principal hazards. Two
approaches are possible, one based on a comparative and qualitative
evaluation of the literature (qualitative assessment), the other
based on an underlying mathematical model to link fibre exposure to
the incidence of cancer (quantitative assessment). Attempts to
derive the mathematical model have had limited success. Data from
several studies support a linear relationship with cumulative dose
for lung cancer and an exponential relationship with time since
first exposure for mesothelioma. However, the derived
"coefficients" within these equations cover a wide range of values
from zero upwards. This numerical variability reflects the
uncertainty of many factors including historical concentration
measurements, fibre size distributions associated with a given
fibre level, and variations in the activity of different fibre
types. Furthermore, smoking habits are rarely well defined in
relation to bronchial cancer. The variability may also reflect
uncertainty in the validity of the models. These factors have
complicated the quantitative extrapolation of the risk of
developing these diseases to levels of exposure such as those in
the general environment, which are orders of magnitude below levels
of exposure in the populations from which the estimates have
derived.

The following conclusions can be drawn on the basis of
qualitative assessment:

(a) Among occupational groups, exposure to asbestos poses a
health hazard that may result in asbestosis, lung cancer,
and mesothelioma. The incidence of these diseases is
related to fibre type, fibre dose, and industrial
processing. Adequate control measures should significantly
reduce these risks.

(b) In para-occupational groups including persons with
household contact, those living in the vicinity of
asbestos-producing and -using plants, and others, the risks
of mesothelioma and lung cancer are generally much lower
than for occupational groups. The risk of asbestosis is
very low. These risks are being further reduced as a
result of improved control practices.

(c) In the general population, the risks of mesothelioma and
lung cancer, attributable to asbestos, cannot be quantified
reliably and are probably undetectably low. Cigarette
smoking is the major etiological factor in the production
of lung cancer in the general population. The risk of
asbestosis is virtually zero.

(d) On the basis of available data, it is not possible to
assess the risks associated with exposure to the majority
of other natural mineral fibres in the occupational or
general environment. The only exception is erionite for
which a high incidence of mesothelioma in a local
population has been associated with exposure.

1.2. Recommendations for Further Research

The molecular and cellular mechanisms associated with both the
fibrogenic and carcinogenic action of asbestos are not known. In
addition, precise epidemiological data and reliable exposure data
to establish dose-response relationships for asbestos fibres are
lacking. There should be further studies on:

(a) the significance of the physical and chemical properties
of asbestos and other mineral fibres (fibre dimension,
surface properties, and contaminants) with respect to their
biological effects;

(b) the biological significance of the durability of mineral
fibres in the body;

(d) the induction of malignant tumours by well-characterized
samples of other natural mineral fibres, especially
asbestos substitutes;

(e) immunological, cellular, and biochemical responses to
natural mineral fibres (including their action as initiator
and/or promotor);

(f) prevalence and incidence of disease in large cohorts of
more recent workers with reliably-measured exposure; and

(g) improvement and international standardization of methods of
monitoring exposure to asbestos and other fibrous
materials.

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, SAMPLING AND ANALYSIS

2.1. Identity; Physical and Chemical Properties of Asbestos Minerals

Asbestos is a collective name given to minerals that occur
naturally as fibre bundles and possess unusually high tensile
strength, flexibility, and chemical and physical durability. Fibre
bundles may be several centimetres long. Bundle diameters may
vary significantly, but tend to be in the millimeter range. This
has given rise to a technical grading based on fibre bundles,
lengths, and diameters. However, when these fibre bundles are
manipulated, they may break down into smaller units, a portion of
which have dimensions in the submicron range.

The asbestos minerals are not classified on a mineralogical
basis, but rather on a commercial basis because of their unique
properties. Therefore, the asbestos variety commercially known as
crocidolite is referred to in the mineralogical literature as
riebeckite. The asbestos variety called amosite is known
mineralogically as grunerite. All other asbestos types are
referred to by their proper mineral names.

The properties usually attributed to asbestos as controlling
both its stability in the environment, and its biological
behaviour, include fibre length and diameter, surface area,
chemical nature, surface properties, and stability of the mineral
within a biological host. The physical and chemical properties of
asbestos have been widely discussed in the literature (Allison et
al., 1975; Selikoff & Lee, 1978; Michaels & Chissick, 1979; US
NRC/NAS, 1984; Langer & Nolan, 1985).

Two basic mineral groups, serpentine and amphibole, contain
important asbestos minerals including the 6 minerals of special
interest listed in Table 1. These groups are hydrated silicates
with complex crystal structures. The typical chemical composition
of the individual types of asbestos within these groups is provided
in Table 1.

2.1.1. Serpentine group minerals - chrysotile

Chrysotile is a sheet silicate composed of planar-linked silica
tetrahedra with an overlying layer of brucite. The silica-brucite
sheets are slightly warped because of a structural mismatch,
resulting in the propagation of a rolled scroll that forms a long
hollow tube. These tubes form the composite fibre bundle of
chrysotile.

Table 1. Physical and chemical properties of common asbestos minerals^a
-------------------------------------------------------------------------------------------------------------
Characteristic Chrysotile Crocidolite^b Amosite^c Antho- Tremolite^d Actinolite^d
phyllite^d
-------------------------------------------------------------------------------------------------------------
Theoretical Mg₃ Na₂FeII₃FeIII₂ (Fe, Mg)₇ (Mg, Fe)₇ Ca₂Mg₅ Ca₂(Mg, Fe)₅
formula (Si₂O₅)(OH) (Si₈O₂₂)(OH)₂ (Si₈O₂₂)(OH)₂ (Si₈O₂₂)(OH)₂ (Si₈O₂₂)(OH)₂ (Si₈O₂₂)(OH)₂
-------------------------------------------------------------------------------------------------------------
Chemical analysis
(range of major consitutents (%))

SiO₂ 38 - 42 49 - 56 49 - 52 53 - 60 55 - 60 51 - 56

Al₂O₃ (0 - 2)^e (0 - 1) (0 - 1) (0 - 3) (0 - 3) (0 - 3)

Fe₂O₃ (0 - 5) 13 - 18 (0 - 5) (0 - 5) (0 - 5) (0 - 5)

FeO (0 - 3) 3 - 21 35 - 40 3 - 20 (0 - 5) 5 - 15

MgO 38 - 42 (0 - 13) 5 - 7 17 - 31 20 - 25 12 - 20

CaO (0 - 2) (0 - 2) (0 - 2) (0 - 3) 10 - 15 10 - 13

Na₂O (0 - 1) 4 - 8 (0 - 1) (0 - 1) (0 - 2) (0 - 2)

N₂O⁺ 11.5 - 13 1.7 - 2.8 1.8 - 2.4 1.5 - 3.0 1.5 - 2.5 1.8 - 2.3
-------------------------------------------------------------------------------------------------------------
Colour usually white blue light grey white to white to pale to
to pale green to pale grey pale grey dark green
yellow^f, brown brown
pink^f

Decomposition 450 - 700 400 - 600 600 - 800 600 - 850 950 - 1040 620 - 960
temperature^g (°C)

Fusion 1500 1200 1400 1450 1315 1400
temperature of
residual
material (°C)
-------------------------------------------------------------------------------------------------------------

Resistance undergoes good attacked very good very good attacked
to acids fairly rapid slowly slowly
attack

Resistance very good good good very good good good
to alkalis
-------------------------------------------------------------------------------------------------------------
Mechanical properties of fibre as
taken from rock samples

Tensile strength 31 35 17 (< 7) 5 5
(10³ kg/cm²)

(Average) (440) (495) (250) (< 100) (< 70) (< 70)
(10³ psi)

Young's modulus 1620 1860 1620 - - -
(10³ kg/cm²)

(Average) (23) (27) (23)
(10⁴ psi)
-------------------------------------------------------------------------------------------------------------
Texture usually flexible to usually usually usually
flexible, brittle and brittle brittle brittle
silky, and tough
tough
-------------------------------------------------------------------------------------------------------------

Table 1 (contd).
-------------------------------------------------------------------------------------------------------------
Characteristic Chrysotile Crocidolite^b Amosite^c Antho- Tremolite^d Actinolite^d
phyllite^d
-------------------------------------------------------------------------------------------------------------
Main producing Canada, South Africa South Africa Mozambique Italy
countries China, USA USA
Italy,
South Africa,
Swaziland,
USA,
USSR,
Zimbabwe
-------------------------------------------------------------------------------------------------------------
^a From: CEC (1977).
^b Mineralogical name of crocidolite is riebeckite.
^c Mineralogical name of amosite is grunerite.
^d Anthophyllite asbestos is the proper term, as with tremolite and actinolite.
^e Bracketed figures denote common elemental substitution found in asbestos minerals.
^f From serpentinized dolomite deposits.
^g Dehydroxylation or dehydrogenation accompanied by disruption of crystal lattice and major loss of
strength.
^h Commercial exploitation of anthophyllite discontinued.
The chemical composition is uniform in contrast to that of the
amphibole asbestos varieties. Some trace oxides (Table 1) are
always present as a result of contamination during the formation of
the mineral in the host rock. Some of these trace elements may be
structurally accommodated within the tetrahedral site of the silica
layer (as in the case of aluminum substituting for silicon), or the
octahedral site of the brucite layer (as in the case of nickel or
iron substituting for magnesium), or may exist as major elements
within minor concentrations of discrete mineral phases intercalated
in the fibre bundle (e.g., magnetite). Organic impurities have not
been observed in virgin chrysotile (Harington, 1962).

Chrysotile fibrils are long, flexible, and curved, and they
tend to form bundles that are often curvilinear with splayed ends.
Such bundles are held together by hydrogen bonding and/or
extrafibril solid matter. Chrysotile fibres naturally occur in
lengths varying from 1 to 20 mm, with occasional specimens as long
as 100 mm. Some of the physical properties of chrysotile are shown
in Table 1.

Exposure to acid results in the liberation of magnesium ions
and the formation of a siliceous residue. Chrysotile fibres are
almost completely destroyed within 1 h when placed in 1 N
hydrochloric acid at 95 °C (Speil & Leineweber, 1969). Chrysotile
is highly susceptible to acid attack, yet is more resistant to
attack by sodium hydroxide than any of the amphibole fibres.

Chrysotile dehydroxylates partially and gradually;
dehydroxylation mainly occurs at approximately 600 - 650 °C
followed by recrystallization to fosterite and silica at about 810
- 820 °C.

2.1.2. Amphibole group minerals

The amphibole minerals are double chains of silica tetrahedra,
cross-linked with bridging cations. The hollow central core
typical for chrysotile is lacking.

Magnesium, iron, calcium, and sodium have been reported to be
the principal cations in the amphibole structure (Speil &
Leineweber, 1969). Some physical properties are summarized in
Table 1.

The amphibole structure allows great latitude in cation
replacement, and the chemical composition and physical properties
of various amphibole asbestos fibres cover a wide range. Only
rarely does the composition of a field sample coincide with the
assigned theoretical or idealized formula. However, theoretical
compositions are used for identifying the various fibres as a
matter of convenience (Table 1).

Whereas the comminution of chrysotile fibres may produce
separated unit fibrils (which are bound by weak proton forces
and/or interfibril amorphous magnesium silicate material), the
breakage (both parting and cleavage) of amphiboles occurs along

defined crystallographic planes. Parting along some of these
surfaces may result in fibrils of amphibole, 4.0 nm in diameter
(Langer & Nolan, 1985).

These mechanisms of amphibole breakage are important
biologically with regard to resultant particle number, surface
area, and general respirability (all of which control penetration
to target cells and delivered dose), and also with regard to
expressed chemical information contained on the fibre surface
(Harlow et al., 1985). In a crystallographic study of amosite
asbestos and its physically-different counterpart, grunerite, size
distributions were different when they were comminuted in an
identical manner. This factor controls both quantity and quality
of dose (Harlow et al., 1985).

2.1.2.1 Crocidolite (Riebeckite asbestos)

Crocidolite is represented by the "idealized" empirical formula
provided in Table 1. Iron can be partially substituted by Mg²⁺
within the structure. Typical crocidolite fibre bundles easily
disperse into fibres that are shorter and thinner than other
amphibole asbestos fibres, similarly dispersed. However, these
ultimate fibrils are generally not as small in diameter as fibrils
of chrysotile. In comparison with other amphiboles or chrysotile,
crocidolite has a relatively poor resistance to heat, but its
fibres are used extensively in applications requiring good
resistance to acids. Crocidolite fibres have fair to good
flexibility, fair spinnability, and a texture ranging from soft to
harsh. Unlike chrysotile, crocidolite is usually associated with
organic impurities, including low levels of polycyclic aromatic
hydrocarbons such as benzo( a )pyrene (Harington, 1962). Only about
4% of asbestos being mined at present is crocidolite.

2.1.2.2 Amosite (Grunerite asbestos)

The characteristics of amosite are given in Table 1. The Fe²⁺
to Mg²⁺ ratio varies, but is usually about 5.5:1.5. Amosite fibrils
are generally larger than those of crocidolite, but smaller than
particles of anthophyllite asbestos similarly comminuted. Most
amosite fibrils have straight edges and characteristic right-angle
fibre axis terminations.

2.1.2.3 Anthophyllite asbestos

Anthophyllite asbestos is a relatively rare, fibrous,
orthorhombic, magnesium-iron amphibole (Table 1), which
occasionally occurs as a contaminant in talc deposits. Typically,
anthophyllite fibrils are more massive than other common forms of
asbestos.

2.1.2.4 Tremolite and actinolite asbestos

The other fibres mentioned in the text include tremolite
asbestos, a monoclinic calcium-magnesium amphibole, and its iron-
substituted derivative, actinolite asbestos. Both rarely occur in

the asbestos habit, but are common as contaminants of other
asbestos deposits; actinolite asbestos occurs as a contaminant
fibre in amosite deposits and tremolite asbestos as a contaminant
of both chrysotile and talc deposits. Tremolite asbestos fibrils
range in size but may approach the dimensions of fibrils of
crocidolite and amosite.

2.2. Identity; Physical and Chemical Properties of Other
Natural Mineral Fibres

Many minerals, other than asbestos, exist in nature with a
fibrous habit. Still others comminute to produce particles with a
fibrous form. Some enter the environment through human activities
and others through natural erosion processes. These have become
increasingly important because they have been linked with human
disease in a limited number of instances (as with the case of
erionite associated with mesothelioma in Turkey) and because they
have been suggested as substitutes for asbestos.

2.2.1. Fibrous zeolites

Zeolites are crystalline aluminosilicates in which the primary
"building blocks" are tetrahedra consisting of either silicon or
aluminium atoms surrounded by four oxygen atoms. These tetrahedra
combine, linked together by oxygen bridges and cations, to yield
ordered three-dimensional frameworks. Although there are more than
30 known natural zeolites, only part of them are fibrous, including erionite,
mesolite, mordenite, natrolites, scolecite and thomsonite (Table 2) (Wright
et al.,1983;Gottardi & Galli, 1985).

Erionite fibres are similar in dimension to asbestos fibres,
though they are probably shorter in length on average (Suzuki,
1982; Wright et al., 1983).

Table 2. Typical formulae of some fibrous zeolites^a
------------------------------------------------------
Erionite (Na₂K₂CaMg)_4.5(Al₉Si₂₇O₇₂) x 27 H₂O

Mesolite Na₂Ca₂Al₆Si₉O₃₀ x 8H₂O

Mordenite (Ca,Na₂,K₂)Al₂Si₁₀O₂₄ x 7(H₂O)

Natrolite Na₂Al₂Si₃O₁₀ x 2H₂O

Paranatrolite Na₂Al₂Si₃O₁₀ x 3H₂O

Tetranatrolite Na₂Al₂Si₃O₁₀ x 2H₂O

Scolecite CaAl₂Si₃O₁₀ x 3H₂O

Thomsonite NaCa₂Al₅Si₅O₂₀ x 6H₂O

------------------------------------------------------
^a From: Mumpton (1979).

2.2.2. Other fibrous silicates (attapulgite, sepiolite, and
wollastonite)

The chemical composition of these minerals is:

palygorskite (attapulgite):
Mg₅Si₈O₂₀(OH)₂(H₂O)₄ x 4H₂O (Barrer, 1978);

sepiolite:
Mg₈Si₁₂O₃₀(OH)₄(H₂O)₄ x 8H₂O (Barrer, 1978);

wollastonite:
CaSiO₃ (Ullmann, 1982).

Certain clay minerals, such as sepiolite and, especially,
attapulgite, may occur in forms that are similar to both chrysotile
and amphibole asbestos fibrils. Under the electron microscope,
they may appear to have a hollow tube structure, or have an
appearance of an amphibole lath. Meerschaum represents a massive
form of fibrous sepiolite. The surface of attapulgite resembles
that of chrysotile in that it is hydrated and protonated.
Attapulgite consists principally of short fibres of the mineral
palygorskite (Bignon et al., 1980).

Wollastonite has received considerable attention as a possible
substitute for asbestos. The basic structure of this mineral is an
infinite silicon oxygen chain (SiO₃). Calcium cations link the
infinite chains together (Leineweber, 1980). The properties of
wollastonite as well as its biological effects have been discussed
in several papers (Korhonen & Tossavainen, 1981; Huuskonen et al.,
1983a,b).

Relevance of physical and chemical properties to biological effects

For respirability, the most important single property of both
asbestos and other fibrous minerals appears to be fibre diameter.
The smaller the fibre diameter, the greater the particle number per
unit mass of dust; the more stable the dust aerosol, the greater
the inhalation potential and penetration to distal portions of the
lung. Once within the tissue, fibre length, surface chemistry, and
physical and chemical properties are the likely factors controlling
biological activity (Langer & Nolan, 1985).

2.3. Sampling and Analytical Methods

Collection and preparation of samples from the environment and
subsequent analysis of asbestos and other natural mineral fibres or
application of direct measuring methods are required for the
assessment of human exposure, evaluation of control measures, and
control of compliance with regulations. Sampling strategies and
analytical procedures must be adequately planned and conducted.
Calibration of instruments and quality control are essential to
ensure accuracy and precision. Detailed descriptions of the
collection and preparation of samples and of analytical procedures
are beyond the scope of this document (Asbestos International
Association, 1982, 1984; EEC, 1983; ILO, 1984).

2.3.1. Collection and preparation of samples

The collection and preparation of samples from air, water, and
biological and geological media require different strategies and
specimen preparation techniques. However, once in a suitable form
for analysis, the instrumental methods required are virtually
identical.

2.3.1.1 Air

The identity of fibres in the work-place is usually known.
This is not true in the general environment, where fibre
identification is generally necessary. The ratio of asbestos
fibres to total respirable particles varies widely, ranging from
1:10³ to 1:10⁷ (Nicholson & Pundsack, 1973; Lanting & den
Boeft,1979).

In addition to fibre identification and concentration, it is
important to focus on fibre size and its relation to inspirability
and respirability (Fig. 1).

The upper limit of the geometric diameter of respirable
asbestos fibres is 3 µm, obtained from the cut-off of the alveolar
fraction of spherical particles (aerodynamic diameter of 10 µm;
specific gravity 1 g/cm³) (Fig. 1) and the average specific gravity
of asbestos (3 g/cm³). While, in some countries, the inspirable
fraction as a whole is covered when measuring the concentration of
airborne asbestos, only the alveolar fraction (termed "respirable
dust") is used in the majority of countries (ILO, 1984).

The concentration of airborne fibres is expressed either as
fibre number concentration, i.e., fibres/ml, fibres/litre, or
fibres/m³ (alveolar fraction) in the work-place and/or general
environment, or as mass concentration, i.e., mg/m³, in the work-
place environment and for emission control (inspirable or alveolar
fraction) (EEC, 1983; ILO, 1984), or ng/m³ in the general
environment (alveolar fraction).

When fibre number concentrations are determined by optical
microscopy, particles having a diameter of less than 3 µm, a
length-to-diameter ratio greater than 3:1, and a length greater
than 5 µm are counted, since they are thought to be the most

biologically-relevant part of the alveolar fraction (EEC, 1983;
ILO, 1984). However, this conclusion is based mainly on studies on
animals involving intrapleural or intraperitoneal administration of
fibres, or intratracheal administration. In addition, alveolar
deposition is relevant for the induction of pleural and peritoneal
mesotheliomas and interstitial fibrosis, but not for the production
of bronchial carcinomas in man, most of which develop in the large
bronchi.

In the past, sampling strategies have not always been
representative of workers' exposures. As an initial step, an
inventory of the work-place exposure conditions should be
undertaken. The sampling strategy should be determined by the
nature of probable exposure at different work locations. An
adequate sampling strategy can, and must be, designed and strictly
followed, and should include decisions on "where", "when", and "for
how long" to sample, as well as on the acceptable number of
samples. The sampling procedure must also be considered so that a
sampling plan can be established. Details of sampling strategies
and procedures can be found in the literature (US NIOSH, 1973,
1977; Robock & Teichert, 1978; Rajhans & Sullivan, 1981; Asbestos
International Association, 1982, 1984; Robock, 1982; Valic, 1983;
ILO, 1984; WHO, 1984).

Specific procedures for the evaluation of airborne asbestos
have been developed and some have been standardized and used in
different countries (US EPA, 1978; US NIOSH, 1984; Asbestos
International Association, 1982, 1984; EEC, 1983; ILO, 1984; ISO,
1984; OECD, 1984). These procedures usually provide guidelines for
sampling strategy in addition to collection and analytical
procedures.

Samples are collected by drawing a given volume of air through
a filter for a given length of time, using pumps that are able to
provide a constant and measureable rate of flow. The concentration
of the fibres deposited on the filter is subsequently determined.

Personal sampling within the worker's breathing zone, as well
as static sampling at fixed locations, can be conducted, depending
on the purpose of the evaluation. Personal sampling should be used
to assess a worker's exposure (e.g., for compliance control and
for epidemiological studies). Static sampling is widely applied
for the evaluation of engineering control.

Basically, the same principles should be applied in collecting
samples for the determination of airborne fibre concentrations in
ambient-air (Asbestos International Association, 1984; VDI, 1984).
However, the sampling strategy (e.g., location of sample collection
points, duration of sampling, etc.) varies from that in the
occupational environment (VDI, 1984).

The same principles should also be applied in the collection of
samples at the work-place to determine mass concentrations (mg/m³)
by gravimetric methods (ILO, 1984).

2.3.1.2 Water

Available technology for determining asbestos in water is
described in a US EPA report (US EPA, 1983). The water sample to
be analysed is initially treated with ozone and ultraviolet
radiation to oxidize suspended organic material. A capillary pore
polycarbonate filter (0.1 µm pore size) is then used to filter the
water sample. The filter is prepared by carbon extraction
replication and then examined with a transmission electron
microscope (TEM).

Since some problems may require less sophisticated
instrumentation, depending on fibre size, type, and concentration,
and to minimize expenditure, a more inexpensive rapid method has
been developed to evaluate the need for the detailed analysis of
water samples suspected of containing asbestos fibres. This method
is not yet in common use. Details of both the full method and the
rapid method are given in US EPA (1983).

2.3.1.3 Biological tissues

Many techniques have been developed for the recovery of mineral
dust from human tissues (Langer et al., 1973; Gaudichet et al.,
1980; Pooley & Clark, 1980). These include wet chemistry methods
(e.g., formamide, glacial acetic and other acids, enzyme, alkali,
and sodium hypochloride digestion), and physical methods (e.g.,
ashing using both low and high temperatures) for tissue
destruction. The recovered residues can be assayed
gravimetrically, by light microscopy or by electron beam
instrumentation (Langer et al., 1973). In addition, with the
development of the carbon-extraction replication technique, it is
possible to analyse, in situ, minerals in tissue slides (Langer et
al., 1972).

2.3.1.4 Geological samples

The preparation of geological specimens (rocks, soils, powdered
mineral specimens, etc.) for fibre analysis follows standard
geological techniques for sample selection, splitting, and
chemical-physical mineral separation. Detailed descriptions of the
many techniques available is beyond the scope of this document
(Bowes et al., 1977).

2.3.2. Analysis

In general, the analytical procedures for fibre quantification
and identification are applicable to all types of samples.

2.3.2.1 Light microscopy

Several versions of a method for counting respirable fibres on
filters, based on phase contrast light microscopy, have been
developed (Asbestos Research Council, 1971; Asbestos International
Association, 1982; US NIOSH, 1984). These are most appropriate for
analysis in the occupational environment, where fibre
identification is unnecessary. The most widely recommended
procedure is the Membrane Filter Method, based on the Asbestos

International Association/RTMI method, which has also been adopted
by the European Economic Communities (EEC, 1983) and the
International Labour Office (ILO, 1984). The same principles are
now under discussion for acceptance by the International Standards
Organization (ISO, 1984). The determination of fibres by phase
contrast microscopy has been widely discussed in the literature
(Rooker et al., 1982; Walton, 1982; ILO, 1984; Taylor et al.,
1984).

Mineral fibres down to about 0.25 µm in diameter (lower for
amphiboles than for chrysotile) are visible and countable by this
method. Identification of specific fibre types is not possible
using this technique and, therefore, every fibre is counted as
"asbestos". The detection limit of the method, defined as the
minimum fibre concentration that can be detected above the
background fibre count, is usually 0.1 fibre/ml. Theoretically,
the detection limit can be lowered by increased sampling time, but
this cannot normally be achieved in industrial situations because
ambient dust levels lead to overloading of the filter.

Large systematic and random observer differences in optical
fibre counts have been reported using the Membrane Filter Method.
These can be reduced by selection of the proper equipment, training
of personnel, and inter-laboratory comparisons.

Improvement in the counting of fibres can be effected by the
automatic evaluation of filter samples. In principle, such
evaluations can be conducted using image analysing systems (Dixon &
Taylor, 1979) or magnetic alignment combined with scattered light
measurements (Gale & Timbrell, 1980).

Finally, it must be stressed that the development, improvement,
and refinement of the Membrane Filter Method in recent years have
led to higher sensitivity and thus to more reliable assessment of
levels in the work-place.

2.3.2.2 Electron microscopy

Asbestos fibres may represent a very small part of the total
number of particles in the general environment, water, and
biological and geological samples. Moreover, the types of fibres
may not be known, and the diameters of asbestos fibres found may be
smaller than those found in the work-place environment. Thus, an
electron microscopic technique is preferred for the analysis of
these filter samples. For example, scanning electron microscopy
(SEM), transmission electron microscopy (TEM, STEM) with energy
dispersive X-ray analyser (EDXA), and selected area electron
diffraction (SAED) (so-called analytical electron microscopy) can
be applied. Analytical electron microscopy has been discussed in
the specialized literature (Clark, 1982; Lee et al., 1982; Steel et
al., 1982).

In order to establish a correlation with the results obtained
by phase contrast microscopy, the results of any fibre count

(aspect ratio > 3:1) must contain the following size fraction:

- fibres greater in length than 5 µm with diameters
between 0.25 µm and 3 µm, which represent the size
fraction recommended for counting by phase contrast
microscopy.

When required, the following size fractions can also be
considered:

- fibres greater in length than 5 µm with diameters of
less than 0.25 µm; and

- fibres shorter in length than 5 µm with diameters
greater than and/or smaller than 0.25 µm.

The results obtained by the electron microscopic assessment of
concentrations of total fibrous particles and/or asbestos particles
have often only been published for an aspect ratio greater than
3:1, independent of length and diameter. These results cannot be
compared, since there are few data on the lower visibility limit
(magnification) and identification limit with regard to the
diameter, and since no correlation with the evaluation criteria for
measurements in work-place environments can be established.

(a) Scanning electron microscopy

Fibres with diameters as small as 0.03 - 0.04 µm may be visible
with this instrument, depending on preparation and instrumentation
techniques (Cherry, 1983). The scanning electron microscope can be
used routinely to identify fibres down to a diameter of 0.2 µm,
when equipped with an energy dispersive X-ray spectrometry system
(EDXA) in environments where fibres are known. Limitations may be
encountered in environments where different minerals have identical
elemental ratios; in this case, selected area electron diffraction
(SAED) is required for identification.

One advantage of SEM is that the filter (membrane or Nuclepore)
can be examined directly within the microscope, without the
generation of preparation artifacts.

(b) Transmission electron microscopy

A modern Transmission Electron Microscope has a resolution of
about 0.0002 µm, which is more than adequate for resolving unit
fibrils of any mineral. The TEM, if equipped with EDXA, can
chemically characterize fibres down to a diameter of 0.01 µm. In
addition, SAED permits the determination of structural elements of
crystalline substances. When samples containing large fibres are
analysed under similar conditions, the detection limits are
comparable for TEM and SEM. As the sensitivity of analytical
instruments increases, so does the possibility of error in
measurement, e.g., the incorporation of adventitious mineral
grains. This may result in erroneous fibre counts, especially in
the analysis of samples with a low mineral fibre content.

The application of the TEM is very advantageous because of the
possibility of structural characterization by means of SAED, which
increases identification accuracy (Beaman & Walker, 1978).

2.3.2.3 Gravimetric determination

Various generally-known methods are available for the
gravimetric evaluation of filter samples (mg/m³) from the work-
place environment and exhaust emissions, including the weighing of
the filter before and after dust sampling or absorption of ionizing
radiation. Qualitative and quantitative infrared spectrometry or X-
ray diffraction analysis (Taylor, 1978; Lange & Haartz, 1979), to
determine the composition of dust, can be carried out on such
filter samples. These filters must contain a relatively large mass
of dust. The disadvantage of gravimetric determination is that
there is no discrimination between fibrous and non-fibrous dusts,
and therefore, it is thought to provide a poor index of the health
hazards posed by asbestos-containing dust.

2.3.3. Other methods

Optical dust-measuring instruments, such as the Tyndallo-meter,
the Fibrous Aerosol Monitor, and the Royco particle counter (ACGIH,
1983), apply the light scattering principle for measuring dust
concentrations in the work-place environment and in stacks of
central dust collectors. They are direct-reading instruments to
which a recorder can be connected.

The advantages of these instruments are:

(a) immediate location of dust sources;

(b) instant determination of the efficiency of
dust-suppression measures;

(d) determination of short-time peak concentrations.

However, these techniques are limited by dust concentration,
particle morphology, and the lack of specificity in terms of
particle identity.

These direct-reading instruments are used mainly for static
monitoring, and for the evaluation of engineering control measures.
For reliable evaluation of work-place air levels, these instruments
should be calibrated with work-place dust samples of known
concentration.

2.3.4. Relationships between fibre, particle, and mass concentration

There is no general relationship between the results of fibre
counts and mass measurements in the assessment of the concentration
of asbestos and other natural mineral fibres in the various types
of environmental media.

Several attempts have been made to establish conversion factors
between mass measurements and fibre counts (Bruckman & Rubino,
1975; Gibbs & Hwang, 1980). Although relationships for individual
work-places and specific work practices have been determined, these
factors cannot be applied generally. The very wide range of numbers
of fibres per unit weight for a given density as a function of
fibre size has been calculated by Pott (1978) on a theoretical
basis (Table 3). In early analyses for asbestos using electron
microscopy, the sample-preparation technique artificially increased
the number of fibres, and therefore, the authors usually
reconverted fibre counts to mass units. However, using electron
microscopy, it is now possible to measure asbestos fibres unchanged
and, thus, the conversion is not warranted.

Conversion of the results of measurements of number of
particles per unit volume (mppcf - millions of particles per cubic
foot) obtained with the Midget Impinger into number of fibres per
unit volume (F/ml) has presented similar problems (Robock, 1984).
While the calculated mean ratios (F/cm³/mppcf) for various
industrial settings varied only between 3 and 8, there were large
variations within each industry; for example, in the textile
industry, the experimentally-determined ratio varied from 1.2 to
11.6 and, in mines, between 0.5 and 47.4 (Robock, 1984).
Table 3. The numbers of fibres per ng for different size categories
(cylindrical fibre shape, density 2.5); diameter/length ratios in the second
line^a
-------------------------------------------------------------------------------
Diameter Length (µm)
(µm) 0.625 1.25 2.5 5 10 20 40 80
-------------------------------------------------------------------------------
0.031 819 200 409 600 204 800 102 400
1:20 1:40 1:80 1:160

0.0625 204 800 102 400 51 000 25 600 12 800
1:10 1:20 1:40 1:80 1:160

0.125 51 200 25 600 12 800 6400 3200 1600
1:5 1:10 1:20 1:40 1:80 1:160

0.25 12 800 6400 3200 1600 800 400 200
1:2.5 1.5 1.10 1:20 1:40 1:80 1:160

0.5 1600 800 400 200 100 50 25
1:2.5 1:5 1:10 1:20 1:40 1:80 1:160

1.0 200 100 50 25 12.5 6.25
1:2.5 1:5 1:10 1:20 1:40 1:80

2.0 25 12.5 6.25 3.2 1.6
1:2.5 1:5 1:10 1:20 1.40
-------------------------------------------------------------------------------
^a From: Pott (1978).
3. SOURCES OF OCCUPATIONAL AND ENVIRONMENTAL EXPOSURE

Once liberated into the environment, asbestos persists for an
unknown length of time. The release of free fibres into the air
through both natural and human activities is the most important
mode to be considered. The main potential exposure sources are the
handling, processing, and disposal of dry asbestos and asbestos-
containing products. Fibres can also be released through the
weathering of geological formations in which asbestos occurs or as
a result of the disturbance of these formations by man.

3.1. Natural Occurrence

Asbestos is widely distributed throughout the lithosphere, and
is found in many soils. Chrysotile, the most abundant and
economically-important form, is present in most serpentine rock
formations in the earth's crust and workable deposits are present
in over 40 nations; however, Canada, South Africa, the USSR, and
Zimbabwe, have 90% of the established world reserves (Shride,
1973). On the other hand, the various amphibole asbestos mineral
types have a comparatively limited geographical distribution,
principally in Australia and South Africa.

The presence of asbestos minerals as accessory minerals in
geological formations is quite common throughout the world.
However, only a few of these deposits are commercially exploitable.
In Europe, the serpentine belt of the Alpine mountain chain
contains chrysotile as well as other mineral fibres. These rocks
can be disturbed by weathering, land-slides, or by man during such
activities as mining, road construction, and tilling of the soil.

The total amount of asbestos emitted from natural sources is
probably greater than that emitted from industrial sources.
However, no measurements concerning the extent of release of
airborne fibres through natural weathering processes are available.

A study of the mineral content of the Greenland ice cap showed
that airborne chrysotile existed long before it was used
commercially on a large scale. The earliest dating in the ice
cores showed the presence of chrysotile in 1750 (Bowes et al.,
1977).

There are also some data on levels of asbestos in water
supplies due mainly to erosion from natural sources (e.g.,
drinking-water in areas such as San Francisco, California;
Sherbrooke, Quebec; and Seattle, Washington).

Increases in the incidence of asbestos-related diseases (e.g.,
pleural calcification and mesothelioma) in areas in Bulgaria,
Czechoslovakia, Finland, Greece, and Turkey have served as a
surrogate indicator of exposure to other natural mineral fibres
(e.g., anthophyllite, tremolite, sepiolite, and erionite). The
results of such studies are discussed more fully in section 8
(Burilkov & Michailova, 1970; Constantopoulos et al., 1985).

In the Federal Republic of Germany and the USA, asbestos
emissions have been detected in quarries (Carter, 1977; Spurny et

al., 1979b), and from quarried rocks used as road gravel (Rohl et
al., 1977).

3.2. Man-Made Sources

3.2.1. Asbestos

Activities resulting in potential asbestos exposure can be
divided into four broad categories. The first category is the
mining and milling of asbestos. The second is the inclusion of
asbestos in products that are currently being developed or
manufactured such as brake shoes, thermal insulation, floor tiles,
and cement articles, and the manipulation of these products (e.g.,
replacement of brake shoes and insulation materials). The third
potential source includes construction activities (cutting and
other manipulations), particularly the removal (e.g, tear-out or
stripping) or maintenance of previously-installed asbestos in
buildings or structures, and the demolition of asbestos-containing
buildings or structures. The fourth is the transportation, use, and
disposal of asbestos or asbestos-containing products. In each case,
appropriate work practices and control measures to prevent or
control the release of asbestos must be implemented (ILO, 1984).

3.2.1.1 Production

The world production of asbestos increased by 50% between 1964
and 1973, when it reached a level of nearly 5 million tonnes. The
projected world demand for asbestos, based on historical
consumption figures and usage patterns through the mid-1970s,
indicates more than a doubling by the year 2000. However, world
production figures for the period 1979-83 showed a decline in
production (Table 4). Fig. 2 shows a drastic decline in major
asbestos uses in the USA in the period 1977-83. The only
substantial increase in asbestos demand seems to be occurring in
developing countries (Clifton, 1980), and in some European
countries. Industrial Minerals (1978) reported that the market for
some natural mineral fibres, other than asbestos, is growing
rapidly as a result of the constant search for asbestos
substitutes. This is, in part, a result of the legislative
restrictions on asbestos in some countries.

Table 4. World production figures on asbestos (tonnes)^a
---------------------------------------------------------------------------
Country 1979 1980 1981 1982 1983
---------------------------------------------------------------------------
Afghanistan 4000
Argentina 1371 1261 1280 1218 1350
Australia
Chrysotile 79 721 92 418 45 494 18 587 20 000
Brazil 138 457 170 403 138 417 145 998 158 855
Bulgaria 600 700 400 600 700
Canada
Chrysotile 1 492 719 1 323 053 1 121 845 834 249 820 000
China 140 000 131 700 106 000 110 000 110 000
---------------------------------------------------------------------------

Table 4 (contd.)
---------------------------------------------------------------------------
Country 1979 1980 1981 1982 1983
---------------------------------------------------------------------------
Cyprus
Chrysotile 35 472 35 535 24 703 18 997 17 288
Czechoslovakia 564 617 388 342 325
Egypt 238 316 325 310 325
India
Amphibole 32 094 33 716 27 521 19 997 17 288
Italy 143 931 157 794 137 000 116 410 139 054
Japan
Chrysotile 3362 3897 3950 4135 4000
Korea, Republic of 14 804 9854 14 084 15 933 12 506
Mozambique 789 800 800 800 800
South Africa
Amosite 39 058 51 646 56 834 43 457 40 656
Crocidolite 118 301 119 148 102 337 87 263 87 439
Chrysotile 91 828 106 940 76 772 81 140 93 016
Swaziland
Chrysotile 34 294 32 833 35 264 30 145 28 287
Taiwan 2957 683 2317 2392 2819
Turkey 38 967 8882 2833 23 283 22 596
USA^b 93 354 80 079 75 618 63 515 69 906
USSR 2 020 000 2 070 000 1 105 000 2 180 000 2 250 000
Yugoslavia 9959 10 468 12 206 10 748 9663
Zimbabwe
Chrysotile 259 891 250 949 247 503 197 682 153 221

World Total 4 800 000 4 700 000 4 300 000 4 000 000 4 100 000
---------------------------------------------------------------------------
^a From: BGS (1983).
^b Sold or used by producers.

Note: In addition to the countries listed, the Democratic Peoples
Republic of Korea and Romania are also believed to produce asbestos.

3.2.1.2 Mining and milling

Asbestos ore is usually mined in open-pit operations. Possible
sources of particulate (asbestos) emissions include: drilling,
blasting, loading broken rock, and transporting ore to the primary
crusher or waste to dumps. Subsequently, the ore is crushed and
may lead to exposure from the following emission sources: unloading
ore from the open pit, primary crushing, screening, secondary
crushing, conveying and stockpiling wet ore. A drying step
follows, which involves conveying the ore to the dryer building,
screening, drying, tertiary crushing, conveying ore to dry-rock
storage building, and dry-rock storage. The next step is the
milling of the ore. In well-controlled mills, this is largely
confined to the mill building and presents very little emission to
the air because the mill air is collected and, usually, ducted
through some particulate matter control device.

Few attempts have been made to quantify fibre emissions from
mining and milling operations.

3.2.1.3 Uses

Asbestos has been used in thousands of applications (Shride,
1973). The way in which asbestos has been incorporated into
various end-products is illustrated in Fig. 3. There are wide
variations in the pattern of use of asbestos in various countries.
For example, in some countries, the production and application of
some of these asbestos products has been discontinued, in part,
because of serious health risks associated with their production.
In some countries, there are also secular trends in the pattern of
usage, i.e., decrease in the production of insulation and increase
in the manufacture of friction materials. The products in group I
cannot all be regarded as end-products but are generally used in
conjunction with water as insulating plasters, cement, or spray
mixtures. The greatest use of asbestos fibres lies in the
manufacture of composites (group II). The cement variety, i.e.,
asbestos cement, constitutes a major component of this group.
Other products of major importance are friction materials,
insulation boards, millboard and paper, reinforced plastics, and
vinyl tiles and sheets. Asbestos can be spun into yarn and woven
into cloth. The resulting textile products (group III) can be used
for further processing into friction materials, packings, and
laminates, or may find direct applications such as insulation
cloth, protective clothing, fire protection, and electrical
insulation.

A list of the most important asbestos-containing products and
their approximate fibre contents is given in Table 5. The
references in the right-hand column refer to Fig. 3.

It should be noted that the extent to which respirable fibres
are produced depends on the type of asbestos product and how it is
manipulated.

3.2.2. Other natural mineral fibres

Other natural mineral fibres may be present in air in
respirable form or may become respirable as a result of
manipulation. The dimensions of these fibres are comparable with
those of asbestos.

(a) Fibrous zeolites

Erionite has been mined in the USA for use in ion-exchange
processes, for the retention of nitrogen in fertilizers, and for
use in concrete aggregate or road surfacing. Some of these
applications, as well as natural weathering, may lead to
significant fibre concentrations in the local air (US NRC/NAS,
1984). Fibres may also be found in drinking-water as a result of
natural weathering.

Table 5. Asbestos products and asbestos contents^a
------------------------------------------------------------------
Approximate Asbestos Reference
asbestos fibre to Fig. 3
content type^b
(% weight)
------------------------------------------------------------------
1. Asbestos-cement 10 - 15 C, A, Cr II-6
building products

2. Asbestos-cement 12 - 15 C, Cr, A II-6
pressure, sewage,
and drainage pipes

3. Fire-resistant 25 - 40 A, C II-6, II-5
insulation boards

4. Insulation products 12 - 100 A, C, Cr I-1, I-2, I-3,
including spray I-4, II-5

5. Jointings and 25 - 85 C, Cr II-8, III-18
packings

6. Friction materials 15 - 70 C II-10

7. Textile products 65 - 100 C, Cr III
not included in (6)

8. Floor tiles and 5 - 7.5 C II-9
sheets

9. Moulded plastics 55 - 70 C, Cr II-9, II-10
and battery boxes

10. Fillers and rein- 25 - 98 C, Cr II-7, II-11
forcements and
products made
thereof (felts,
millboard, paper,
filter pads for
wines and beers,
underseals, mastics,
adhesives, coatings,
etc.
------------------------------------------------------------------
^a From: CEC (1977).
^b A = amosite (not used in all countries); C = chrysotile;
Cr = crocidolite (not used in all countries).

(b) Palygorskite (attapulgite)

Available data on the production of attapulgite in various
countries are presented in Table 6.

Table 6. World production of attapulgite and sepiolite^a
----------------------------------------------------------
Country Annual production of Annual production of
attapulgite (tonnes) sepiolite (tonnes)
----------------------------------------------------------
France unknown 2500

India 10 000

Senegal 16 700

Spain 50 000 236 000

USA 700 000
----------------------------------------------------------
^a Modified from: Bignon et al. (1980).

The USA is the biggest producer and consumer of attapulgite;
consumption currently exceeds 700 000 tonnes and is almost triple
that of asbestos. The consumption figures for various uses of
attapulgite in the USA are listed in Table 7. An additional 100 000
tonnes is exported from the USA each year (US Bureau of Mines,
1982). Similar data for other countries are not available.

Table 7. Uses of attapulgite in the USA^a
-----------------------------------------------------
Use 1981 consumption
(1000 tonnes)
-----------------------------------------------------
Drilling mud 173.5
Fertilizers 50.2
Filtering (oil and grease) 18.7
Oil and grease adsorbents 178.2
Pesticides and related products 106.5
Pet waste adsorbent 105.8
Medical, pharmaceutical, 0.06
cosmetic ingredients
Other uses 79.5

Total 712.46
-----------------------------------------------------
^a From: US Bureau of Mines (1982).

In France, attapulgite is used in drugs for the treatment of
gastrointestinal diseases (Bignon et al., 1980); in the USA, it is
a component of non-prescription antidiarrhoeal drugs (Physicians'
Desk Reference, 1983).

The potential environmental effects of attapulgite were
reviewed by the US NRC/NAS (1984). It was stated that, when used
in such products as pet waste adsorbents, fertilizers, and
pesticides, substantial amounts of attapulgite could be released
into the air. Attapulgite has also been found in water supplies
(Millette et al., 1979b).

Available data on the production of sepiolite in several
countries are presented in Table 6.

Minerals that contain sepiolite are used as cat litter.

3.2.3 Manufacture of products containing asbestos

3.2.3.1 Asbestos-cement products

Throughout the world, the asbestos-cement industry is the
largest user of asbestos fibres. Asbestos-cement products contain
10 - 15% asbestos, mostly in the form of chrysotile, though limited
amounts of crocidolite may be used in large-size asbestos-cement
pipes, to give the required strength as well as to increase the
speed of production. The most important products are asbestos-
cement pipes and sheets. Products are primarily manufactured in
wet processes.

Possible emission sources are: (a) the feeding of asbestos
fibres into the mix; (b) blending the mix; and (c) cutting or
machining end products. Emissions may range from negligible to
significant according to the dust control measures and technology.
Emissions can also occur from sources other than processing
operations, such as the improper handling and/or shipment of dry
materials containing asbestos and during the cutting or machining
of end-products. Recently, control measures have been developed
and approved in the Federal Republic of Germany
(Berufsgenossenschaftliches Institut für Arbeitssicherheit, 1985),
which have reduced airborne levels in the immediate vicinity by 1 -
2 orders of magnitude, generally, to less than 1000 fibres/litre.

3.2.3.2 Vinyl asbestos floor tiles

The second largest user of asbestos fibres in the USA is the
asphalt and vinyl floor tile manufacturing industry. This type of
tile has found increased use in many countries because of its
durability and impermeability to water.

3.2.3.3 Asbestos paper and felt

Products classified as asbestos paper and felt range from thin
paper to 1 cm thick millboard, which contains up to 97% asbestos.
The feed for paper machines is prepared by mixing short chrysotile
fibres with water and binders. Since papermaking is a wet process,
little asbestos dust is generated during manufacture. However,
finishing operations, such as slitting and calendering, may be
sources of dust emission. The use of asbestos paper and felt is
declining in some countries.

3.2.3.4 Friction materials (brake linings and clutch facings)

Moulded brake linings are used on disc and drum-type car
brakes. Woven brake linings and clutch facings for heavy use are
made from high-strength asbestos yarn and fabric reinforced with

wire; this material is dried and impregnated with resin. In the
moulding process, the asbestos fibres and other constituents are
combined with the resin, which is thermoset. Final treatment
involves curing by baking, and grinding to customer specifications.
Emissions may range from negligible to significant depending on
dust control measures and technology.

3.2.3.5 Asbestos textiles

Asbestos textiles are used in the manufacture of fire-resistant
garments, sealing materials, wicks, and thermal insulation, or as
an intermediate product in brake linings, clutch facing,
insulation, and gaskets. Asbestos textile manufacturing is the
dustiest of all asbestos-manufacturing processes, and dust
emanating from this process is more difficult and costly to
control. However, during the past decade, emissions have been
substantially reduced in countries in which improved control
measures and technology have been implemented.

3.2.4 Use of products containing asbestos

Few data are available on fibre emissions during the use of
products containing asbestos or other mineral fibres. In most
construction materials and consumer products, the fibres are firmly
bound or encased in a solid matrix and are not expected to be
released under normal conditions, but may be emitted during
manipulation or renovation of such materials or products (e.g.,
fibre levels measured by light microscopy in the vicinity of such
activities as removal of pipe lagging containing asbestos or the
sanding of asbestos-containing drywall topping and spackling
compounds may approach or exceed current occupational exposure
limits) (Fischbein et al., 1979; Sawyer & Spooner, 1979).

4. TRANSPORT AND ENVIRONMENTAL FATE

4.1 Transport and Distribution

Once in the environment, fibres are mainly transported and
distributed via air and water.

4.1.1 Transport and distribution in air

Airborne mineral fibres are stable and may travel significant
distances from the site of origin. Airborne asbestos fibres, for
example, have aerodynamic diameters that are generally less than
0.3 µm and, therefore, their sedimentation velocities are very
low. Measurements concerning the transport and distribution of
specific mineral fibres have been made under certain environmental
conditions and at specific locations (Laamanen et al., 1965;
Heffelfinger et al., 1972; Harwood & Blaszak, 1974; US EPA, 1974).

Calculations using a dispersion model from a point source
(Harwood & Blaszak, 1974) indicated that concentrations of airborne
fibres of small dimension decreased very slowly with increasing
distance. This study underscores two important characteristics of
ambient air fibre burden:

(a) fibres are transported great distances from point
sources; and

(b) fibres in ambient air are small in size, requiring
electron beam instrumentation for detection.

4.1.2 Transport and distribution in water

Long-range transport of asbestiform fibres in water has been
reported. Cooper & Murchio (1974) concluded that chrysotile
fibres present in tap-water in San Francisco, California, were
actually introduced at a reservoir many km south of the city.
Nicholson (1974) attributed the presence of amphibole fibres in the
municipal water supply of Duluth, Minnesota, to the transport, over
96 km, of taconite tailings from a Silver Bay mining operation. In
this instance, transport resulted from bottom currents in Lake
Superior.

4.2 Environmental Transformation, Interaction, and Degradation
Processes

Mineral fibres are relatively stable and tend to persist under
typical environmental conditions. However, asbestos fibres may
undergo chemical alteration as well as changes in dimension. For
example, chrysotile, and to a lesser extent amphibole, asbestos
fibres are capable of chemical alteration in aqueous media. The
magnesium hydroxide content of chrysotile is partially or wholly
removed by solution, depending on time, temperature, and pH. An
insoluble silica skeleton of the fibre remains. Grunerite fibres,
of which amosite is the known commercial form, have been reported
to react with water, losing some iron on extended exposure to lake

water; the fibres appeared partially degraded and broken when
examined microscopically (Kramer et al., 1974).

The comparative solubility of selected mineral fibres has been
studied and a general trend determined: chrysotile > amosite >
actinolite > crocidolite > anthophyllite > tremolite (US
NRC/NAS, 1977). Because of their high adsorption properties, it is
thought that some mineral fibres may adsorb and carry various
organic agents present in the environment.

5. ENVIRONMENTAL EXPOSURE LEVELS

Asbestos is ubiquitous in the environment because of its
extensive industrial use and its dissemination through erosion from
natural sources. Other natural mineral fibres also occur in the
environment and may, at times, be present at similar or even higher
concentrations than asbestos, depending on local conditions. Since
the size distributions of such fibres are often similar to those of
asbestos, it is likely that distribution patterns in the
environment will also be similar.

It is difficult to compare available data on airborne fibre
levels because of inconsistencies in both the methods of sampling
and analysis, and the expression of results. In most countries,
for instance, airborne fibre concentrations in the work-place are
expressed as fibre/ml or mg/m³. For concentrations in ambient
air, fibre/litre, fibre/m³, and ng/m³ are commonly used. Fibre
concentrations in biological materials are usually expressed in
fibre/g or in µg/g of the dry tissue.

In this section, the available data will be discussed in terms
of occupational, para-occupational (household and neighbourhood),
and general environmental (air and other media) exposure.

5.1 Air

5.1.1 Occupational exposure

Exposure levels for different types of asbestos and other
mineral fibres vary considerably within and between industries.

This discussion will be limited to data obtained by the
Membrane Filter Method and expressed as fibre/ml. On the basis of a
review of historical data, ranges of levels in various industries
without or with poor dust suppression measures are illustrated in
Fig. 4. In recent years, concentrations in many countries have been
much lower than those illustrated because of the introduction of
engineering controls. For example, results of more recent personal
exposure measurements made during various operations involving the
manufacture of asbestos-containing products in the United Kingdom
between 1972 and 1978 indicate that, in most cases (54 - 86.5%),
levels were below 0.5 fibres/ml (Table 8). Data from various
branches of the asbestos industry in France (Table 9), indicate
levels that are achievable by current dust control methods.

The reduction in levels over time is even greater than is
reflected by the data, because of the increased sensitivity (3x) of
the currently-used Membrane Filter Method, compared with the
sensitivity of previously-used methods for the determination of
airborne asbestos.

However, it should be noted that there are countries in which
effective dust control measures have not been introduced; current
levels in these countries may approach those illustrated in Fig. 4
(Oleru, 1980).

Table 8. Asbestos levels in different manufacturing
industries in the United Kingdom, 1972-78^a
---------------------------------------------------------
Industry Number of Percentage of results^b
results < 0.5 < 1.0 < 2.0
(fibres/ml)
---------------------------------------------------------
Asbestos cement 845 86.5 95.0 98.5
Millboard/paper 135 87.0 98.2 99.6
Friction materials 900 71.0 85.5 95.0
Textiles 1304 58.5 80.7 95.0
Insulation board 545 54.0 72.5 88.6
---------------------------------------------------------
^a From: Health and Safety Commission (1979).
^b 4-h samples.

Table 9. Asbestos fibre concentrations in 1984 in various
branches of the asbestos industry in France^a
------------------------------------------------------------------
Branch Fibre concentrations (fibre/ml) Total
------------------------------------ number of
< 0.5 0.5 - 1 1 - 2 > 2 points
------------------------------------------------------------------
Asbestos cement

Numbers^b 261 11 6 1 279
Percentage 93.5 3.9 2.1 0.3

Friction materials

Numbers 249 84 55 8 396
Percentage 62.8 21.2 13.8 2.0

Textile

Numbers 81 25 17 1 124
Percentage 65.3 20.1 13.7 0.8

Others

Numbers 41 14 0 1 56
Percentage 73.2 25.0 0 1.7
------------------------------------------------------------------
Total

Numbers 632 134 78 11 855
Percentage 73.9 15.6 9.1 1.2
------------------------------------------------------------------
^a From: AFA (1985).
^b Numbers of points in work-place areas.

5.1.2 Para-occupational exposure

Members of the families of asbestos workers handling
contaminated work clothes (a practice which should be discouraged),
and, in some cases, members of the the general population may be
exposed to elevated concentrations of airborne asbestos fibres.
Asbestos has been used widely in building materials for domestic
application (e.g., asbestos-cement products and floor tiles), and
elevated airborne levels have been measured during the manipulation
of these materials (e.g., home construction and renovation by the
homeowner).

In this and the following section, only data obtained by
electron microscopy will be considered, because of the necessity of
identifying asbestos and distinguishing it from other inorganic
fibres that may also be present in ambient air. In addition, only
data obtained using direct preparation methods without alteration
of the fibrous material and reported as fibre number concentrations
will be included.

Asbestos levels in the air of mining towns in Quebec have been
determined recently by transmission electron microscopy using
direct transfer sample preparation techniques. Samples were
collected in June 1983 at 11 sites in 5 mining communities located
downwind from asbestos mines. Sampling was also conducted at a
control site in Sherbrooke, Quebec. The overall mean asbestos
concentrations in the samples from the mining towns were 47.2
fibres/litre (total) and 7.8 fibres/litre (> 5 µm). Mean values
for each of the sites sampled ranged up to 97.5 fibres/litre
(total) and 20.6 fibres/litre (> 5 µm). For the control
community, the mean values were lower - 14.7 fibres/litre (total)
and 0.7 fibres/litre (> 5 µm) (Lebel, 1984).

Measurements were carried out in 1983 and 1984 in various
mining areas in Canada and South Africa (Robock et al., 1984;
Selles et al., 1984) using scanning electron microscopy with energy
dispersive X-ray analysis (Asbestos International Association,
1984). Total inorganic fibre and asbestos fibre concentrations,
using the counting criteria used in the Membrane Filter Method
(> 5 µm in length; < 3 µm in diameter; aspect ratio > 3:1) and
evaluated in the same laboratory, are shown in Table 10.

Levels of asbestos in the vicinity of industrial sources in
Austria have also been reported (Felbermayer & Ussar, 1980).
Applying the counting criteria described above, levels in samples
taken in the vicinity of an asbestos deposit in Rechnitz averaged
0.2 fibres/litre (range 0 - 0.5 fibres/litre). In the vicinity of
an asbestos-cement plant (Vöcklabruck), the mean concentration was
0.5 fibres/litre (range 0 - 2.2 fibres/litre).

Table 10. Fibre concentrations in mining areas of Canada
and South Africa^a,b
------------------------------------------------------------
Area Locations Concentration (fibres/litre,
longer than 5 µm)
Total inorganic Asbestos
------------------------------------------------------------
Canada (Quebec area)
Residential (1) 3.2 1.8
areas near (2) 3.1 0.9
asbestos mines (3) 0.9 0.2

South Africa
Downwind mill (1) 600.0 600.0^c
(2) 81.6 80.3
(3) 8.6 8.6
(4) 300.0 300.0^d
(5) 10.6 9.3
(6) 4.9 2.4

Residences of (1) 6.3 6.0
asbestos mine (2) 7.4 7.1
workers (3) 2.7 2.0
(4) 11.0 11.0
(5) 3.2 3.2
(6) 8.1 7.3
------------------------------------------------------------

Table 10 (contd.)
------------------------------------------------------------
Area Locations Concentration (fibres/litre,
longer than 5 µm)
Total inorganic Asbestos
------------------------------------------------------------
Residential (1) 1.0 0.8
areas near (2) 0.6 0.3
asbestos mines (3) 1.1 0.7
(4) 0.4 0.2
(5) 0.8 0.2
(6) 0.8 0.5

Near a magnesium 1.5 0.1
mine

Near an iron 1.5 0.3
ore mine
------------------------------------------------------------
^a From: Robock et al. (1984) and Selles et al. (1984).
^b Practical limits of error, 95% (Poisson's distribution),
for the calculated concentrations of fibres/litre depend
on the number of fibres found in 1 mm² of the total
filter surface; for 0.1 fibre/litre, the range is
0.002 - 0.6 fibres/litre; for 1 fibre/litre, the range
is 0.5 - 1.8 fibres/litre).
^c Unprotected tailing dump.
^d Truck loaded with soil.

In general, the data indicate that levels of airborne asbestos
fibres (> 5 µm in length) in residential areas in the vicinity of
industrial sources are within the range of those in urban locations
(up to 10 fibres/litre) or, in some cases, slightly higher.

5.1.3 Ambient air

Available data on asbestos levels in ambient air, determined
by a variety of sampling, instrumental, and counting techniques,
were reviewed by Lanting & den Boeft (1979). Levels were
significantly lower than those in the occupational environment.

More recent data on levels of asbestos in outdoor air,
determined by currently-accepted techniques, are presented in Table
11. Only levels measured as fibre count concentrations are
presented as these are relevant to health effects. On the basis of
these data, it can be concluded that levels of asbestos fibres
(length > 5 µm) at remote locations are generally less than 1
fibre/litre. Levels in urban air generally range from < 1 up to
10 fibres/litre (occasionally, levels exceed this value). Mean
concentrations of other inorganic fibres of the same dimensions are
generally up to an order of magnitude higher, or occasionally more.

Recently, there has been concern about potential exposure to
asbestos in the air of public buildings with friable surfaces of
sprayed asbestos-containing insulation. Sprayed asbestos was used
extensively between the 1940s and 1970s on structural surfaces (to

retard collapse during fire) and on ceilings (for purposes of
acoustic and thermal insulation and decoration). The results of
available studies on asbestos levels in indoor air are presented in
Table 12. These values are usually within the range of those found
in ambient air (i.e., generally do not exceed 1 fibre/litre, but
may be higher, up to 10 fibres/litre).

5.2 Levels in Other Media

Asbestos is introduced into water by the dissolution of
asbestos-containing minerals and ores, from industrial effluents,
atmospheric pollution, and asbestos-cement piping. The presence of
asbestos fibres in drinking-water was first reported in Canada in
1971 (Cunningham & Pontefract, 1971) since when surveys of asbestos
concentrations in various public water supplies have been conducted
in Canada (Canada, Environmental Health Directorate, 1979), the
Federal Republic of Germany (Meyer, 1984), the United Kingdom
(Commins, 1979), and the USA (Millette et al., 1980).

On the basis of a compendium of published and unpublished
surveys in which 1500 water samples from 406 cities in the USA were
analysed (using various sample-preparation techniques), it was
concluded that the majority of the population consumes drinking-
water containing asbestos fibre levels of less than 1 x 10⁶/litre^a
(Millette et al., 1980). In some areas, however, levels of between
1 and 100 x 10⁶ fibres/litre were recorded and levels as high as
600 x 10⁶ fibres/litre were reported for one water supply
contaminated with amphibole fibres from the processing of iron ore.

A nation-wide survey of asbestos levels in drinking-water from
71 locations across Canada (serving 55% of the population) was the
basis for an estimation that 5% of the population receives water
containing levels higher than 10 x 10⁶ fibres/litre, about 0.6%
receives water having more than 100 x 10⁶ fibres/litre (Canada,
Environmental Health Directorate, 1979). Levels as high as 100 x
10⁶ fibres/litre in some areas were attributable to erosion from
natural sources. Levels in drinking-water supplies in the United
Kingdom have been reported to range up to 2.2 x 10⁶ fibres/litre
(Commins, 1979).

The size distribution of asbestos fibres in water supplies
differs from that of airborne asbestos. In general, fibre lengths
are much shorter; median values of 0.5 - 0.8 µm have been reported
(Canada, Environmental Health Directorate, 1979). Available data
also indicate that the release of fibres from asbestos-cement
piping is related to the aggresivity of the water (Canada,
Environmental Health Directorate, 1979; Meyer, 1984), and that
conventional treatment processes involving chemical coagulation
followed by filtration effectively reduce levels in drinking-water
supplies.

Table 11. Fibre concentrations in outdoor air
---------------------------------------------------------------------------------------------------------
Area Concentration (fibres/litre)^a Counting criteria Reference
Total Asbestos
inorganic Total > 5 µm
---------------------------------------------------------------------------------------------------------
AUSTRIA
Leoben
(heavy traffic) 7.0 4.6 length: > 5 µm Felbermayer (1983)
diameter: 0.2 - 3 µm
(SEM)

Schalchham
(low traffic) 1.7 0.1 length: > 5 µm Felbermayer (1983)
diameter: 0.2 - 3 µm
(SEM)

Village with 4.6 < 0.1 length: > 5 µm Felbermayer (1983)
asbestos-cement diameter: 0.2 - 3 µm
roofing (SEM)

Village without 4.3 < 0.1 length: > 5 µm Felbermayer (1983)
asbestos-cement diameter: 0.2 - 3 µm
roofing (SEM)

Remote rural 1.4 < 0.1 length: > 5 µm Felbermayer (1983)
areas diameter: 0.2 - 3 µm
(SEM)
---------------------------------------------------------------------------------------------------------

Southern < 2 - 4 length: > 5 µm Chatfield (1983)
Ontario diameter: all
(TEM)

Toronto 0 - 13^b length: > 5 µm Chatfield (1983)
(busy diameter: all
intersection) (TEM)

Mississauga 0 - 11^b length: > 5 µm Chatfield (1983)
diameter: all
(TEM)

Oakville 0 - 8^b length: > 5 µm Chatfield (1983)
diameter: all
(TEM)

Bracebridge 0 - 2^b length: > 5 µm Chatfield (1983)
(remote rural diameter: all
location) (TEM)

Peterborough 0 - 4^b length: > 5 µm Chatfield (1983)
diameter: all
(TEM)

Quebec

Sherbrooke 0.7 length: > 5 µm Lebel (1984)
diameter: all
(TEM)
---------------------------------------------------------------------------------------------------------

Soweto
(asbestos-cement 1.4 0.2 length: > 5 µm Selles et al. (1984)
application) diameter: 0.2 - 3 µm
(SEM)

Frankfort
(rural) 0.2 < 0.1 length: > 5 µm Selles et al. (1984)
diameter: 0.2 - 3 µm
(SEM)

at Cape Point
(reference) < 0.1 < 0.1 length: > 5 µm Selles et al. (1984)
diameter: 0.2 - 3 µm
(SEM)

USA
California

Upwind of < 0.2 - 11 length: all John et al.
an asbestos diameter: all (1976)
plant
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^a Practical limits of error, 95% (Poisson's distribution), for the calculated concentrations of
fibres/litre depend on the number of fibres found in 1 mm² of the total filter surface; for 0.1
fibre/litre, the range is 0.002 - 0.6 fibres/litre; for 1 fibre/litre, the range is 0.5 - 1.8
fibres/litre.
^b 95% confidence limits.

Table 12. Levels of asbestos fibre concentrations in indoor air
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Area Number of Concentration^a Counting criteria Reference
samples (fibres/litre)
---------------------------------------------------------------------------------------------------------
Canada

In 3 public buildings not < 2^b length: > 5 µm Chatfield (1983)
with amosite- applicable diameter: all
containing insulation

In 7 public buildings not < 4 to < 9^b length: > 5 µm Chatfield (1983)
with chrysotile- applicable diameter: all
containing insulation

In 19 public buildings 14 0 to 0.3 length: > 5 µm Pinchin (1982)
with asbestos- diameter: all
containing insulation

Germany, Federal Republic of

Sporting halls 45 0.1 to 1.1 length: > 5 µm Institute for Applied
(sprayed diameter: 0.2 - 3 µm Fibrous Dust Research
crocidolite (1984)

Schools (sprayed 5 0.1 to 11.0 length: > 5 µm Institute for Applied
crocidolite) diameter: 0.2 - 3 µm Fibrous Dust Research
(1984)
Public buildings 5 0.1 to 0.2 length: > 5 µm Institute for Applied
(asbestos-cement diameter: 0.2 - 3 µm Fibrous Dust Research
air ducts) (1984)

Public buildings 3 0.1 to 0.2 length: > 5 µm Institute for Applied
(asbestos-cement diameter: 0.2 - 3 µm Fibrous Dust Research
sheets) (1984)

Public buildings 1.0 to 10.0 length: > 5 µm Lohrer (1983)
(sprayed asbestos) diameter: 0.2 - 3 µm

Homes (electrical 0.1 to 6.0 length: > 5 µm Lohrer (1983)
storage heaters) diameter: 0.2 - 3 µm
---------------------------------------------------------------------------------------------------------
^a Practical limits of error, 95% (Poisson's distribution), for the calculated concentrations of
fibres/litre depend on the number of fibres found in 1 mm² of the total filter surface and for 0.1
fibre/litre (range 0.002 - 0.6 fibres/litre) and for 1 fibre/litre (range 0.5 - 1.8 fibres/litre).
^b 95% confidence limits.
The extent of asbestos contamination of solid foodstuffs has
not been well studied because a simple, reliable analytical method
is lacking. Foods that contain soil particles, dust, or dirt
almost certainly contain asbestos fibres. Foodstuffs may also
contain asbestos from water or from impure talc, which is used in
coated rice, and as an antisticking agent for moulded foods
(Eisenberg, 1974). Asbestos may also be introduced into foods from
impure mineral silicates, such as talc, soapstone, or pyrophyllite,
used as carriers for spray pesticides (Kay, 1974).

Asbestos fibres have been detected in beverages.
Concentrations of 0.151 x 10⁶ fibres/litre have been found in some
English beers (Biles & Emerson, 1968), and concentrations of 4.3 -
6.6 x 10⁶ fibres/litre have been recorded in Canadian beers
(Cunningham & Pontefract, 1971); levels between 1.7 and 12.2 x 10⁶
fibres/litre have been found in soft drinks. It has been suggested
that asbestos filters used for the clarification of beverages and
other liquids may have contributed to the asbestos content.
However, the presence of asbestos in the water used to constitute
these beverages has complicated interpretation of the data.

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^a Unless otherwise specified, levels in drinking-water are all
fibres visible by TEM.

6. DEPOSITION, TRANSLOCATION, AND CLEARANCE

Although most of the data concerning the deposition,
translocation, and clearance of fibres have been obtained in
studies with asbestos, it is likely that other natural mineral
fibres behave in a similar manner.

6.1 Inhalation

In 1966, the ICRP Task Group on Lung Dynamics (1966) published
a lung model that subdivided the respiratory tract into three
compartments: the nasopharynx, the tracheobronchial, and the
pulmonary or alveolar region. The deposition, clearance, and
translocation of particles in each of these three compartments was
described. This scheme of pathways was modified for fibres by
Bignon et al. (1978) as shown in Fig. 5.

6.1.1 Asbestos

6.1.1.1 Fibre deposition

(a) Models

There are five mechanisms of deposition of particles in the
respiratory tract (i.e., inertial impaction, sedimentation,
interception, diffusion, and electrostatic precipitation).

Sedimentation is determined principally by the aerodynamic
diameter of particles.

The geometric diameter and density of a fibre largely determine
the aerodynamic diameter with fibre length being of secondary
importance. It has been es