INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY
ENVIRONMENTAL HEALTH CRITERIA 138
2-NITROPROPANE
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.
First draft prepared by Dr. R.B. Williams,
United States Environmental Protection Agency
Published under the joint sponsorship of
the United Nations Environment Programme,
the International Labour Organisation,
and the World Health Organization
World Health Organization
Geneva, 1992
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WHO Library Cataloguing in Publication Data
2-Nitropropane.
(Environmental health criteria; 138)
1. Environmental exposure 2. Propane - analogs & derivatives
3. Propane - toxicity I. Series
ISBN 92 4 157138 1 (NLM Classification QV 633)
ISSN 0250-863X
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CONTENTS
ENVIRONMENTAL HEALTH CRITERIA FOR 2-NITROPROPANE
1. SUMMARY
1.1. Properties and analytical methods
1.2. Uses and sources of exposure
1.2.1. Production
1.2.2. Uses and loss to the environment
1.3. Environmental transport and distribution
1.4. Environmental levels and human exposure
1.5. Kinetics and metabolism
1.6. Effects on laboratory mammals and in vitro
systems
1.7. Effects on humans
1.8. Effects on other organisms in the laboratory
and field
2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, AND
ANALYTICAL METHODS
2.1. Identity
2.2. Physical and chemical properties
2.3. Conversion factors
2.4. Analytical methods
3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE
3.1. Natural occurrence
3.2. Anthropogenic sources
3.2.1. Production levels and processes
3.2.2. Uses
3.3. Release into the environment
4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION, AND TRANSFORMATION
4.1. Transport in the environment
4.2. Biotic and abiotic transformation
5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE
5.1. Environmental levels and general population
exposure
5.2. Potential occupational exposure
6. KINETICS AND METABOLISM
6.1. Absorption
6.2. Distribution
6.3. Metabolic transformation
6.4. Elimination and excretion
6.5. Retention and turnover
7. EFFECTS ON LABORATORY MAMMALS AND IN VITRO TEST SYSTEMS
7.1. Single exposure
7.2. Short-term and long-term repeated exposure
7.3. Reproduction, embryotoxicity, and teratogenicity
7.4. Mutagenicity and related end-points
7.4.1. Prokaryotes and yeast
7.4.2. Eukaryotes
7.5. Carcinogenicity
7.6. Pharmacological effects
8. EFFECTS ON HUMANS
8.1. General population exposure
8.2. Occupational exposure
8.2.1. Acute toxicity
8.2.2. Effects of long-term exposure
9. EFFECTS ON OTHER ORGANISMS IN THE LABORATORY AND FIELD
10. EVALUATION OF HUMAN HEALTH RISKS AND EFFECTS ON
THE ENVIRONMENT
10.1. Human health risks
10.2. Effects on the environment
11. RECOMMENDATIONS FOR PROTECTION OF HUMAN HEALTH
12. FURTHER RESEARCH
12.1. Environment
12.2. Epidemiology
12.3. Toxicokinetics
12.4. Carcinogenesis
13. PREVIOUS EVALUATIONS BY INTERNATIONAL BODIES
REFERENCES
RESUME
RESUMEN
WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR 2-NITROPROPANE
Members
Dr D. Anderson, British Industrial Biological Research
Association, Carshalton, Surrey, United Kingdom
Dr U. Andrae, Institute for Toxicology, Research Centre for
Environment and Health, Neuherberg, Munich, Germany
(Vice-chairman)
Dr B. Baranski, Hofer Institute of Occupational Medicine,
Lodz, Poland
Dr S. Dobson, Institute of Terrestral Ecology, Monks Wood
Experimental Station, Abbots Ripton, Huntingdon, United
Kingdom
Dr E.S. Fiala, Naylor Dana Institute for Disease Prevention,
American Health Foundation, Valhalla, New York, USA
(Chairman)
Dr P. Lundberg, Department of Toxicology, National Institute
of Occupational Health, Solna, Sweden
Dr M.H. Noweir, Industrial Engineering Department, College of
Engineering, King Abdul Aziz University, Jeddah, Saudi
Arabia
Dr C.N. Ong, Department of Community, Occupational and
Family Medicine, National University of Singapore, Singapore
(Joint Rapporteur)
Dr R.B. Williams, Exploratory Research, US Environmental
Protection Agency, Washington DC, USA (Joint Rapporteur)
Secretariat
Dr B.H. Chen, International Programme on Chemical Safety,
World Health Organization, Geneva, Switzerland
Dr P.G. Jenkins, International Programme on Chemical Safety,
World Health Organization, Geneva, Switzerland
Mr J. Wilbourn, International Agency for Research on Cancer,
Lyon, France
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 Director of the
International Programme on Chemical Safety, World Health
Organization, Geneva, Switzerland, in order that they may be
included in corrigenda.
* * *
A detailed data profile and a legal file can be obtained from
the International Register of Potentially Toxic Chemicals, Palais
des Nations, 1211 Geneva 10, Switzerland (Telephone No. 7988400 or
7985850).
ENVIRONMENTAL HEALTH CRITERIA FOR 2-NITROPROPANE
A WHO Task Group on Environmental Health Criteria for
2-Nitropropane met in Geneva from 4 to 8 November 1991. Dr B.H.
Chen, IPCS, welcomed the participants on behalf of the Director,
IPCS, and the three IPCS cooperating organizations (UNEP/ILO/WHO).
The Task Group reviewed and revised the draft criteria document and
made an evaluation of the risks for human health and the environment
from exposure to 2-nitro-propane.
The first draft of this monograph was prepared by Dr R.B.
Williams of the US Environmental Protection Agency. The second draft
was also prepared by Dr R.B. Williams incorporating comments
received following the circulation of the first draft to the IPCS
Contact Points for Environmental Health Criteria documents.
Dr B.H. Chen and Dr P.G. Jenkins, both members of the IPCS
Central Unit, were responsible for the overall scientific content
and technical editing, respectively.
The efforts of all who helped in the preparation and
finalization of the document are gratefully acknowledged.
ABBREVIATIONS
DDT dichlorodiphenyltrichloroethane
DMSO dimethyl sulfoxide
SCE sister chromatid exchange
SGPT serum glutamic pyruvic transaminase
STEL short-term exposure limit
TWA time-weighted average
SUMMARY
1.1 Properties and analytical methods
2-Nitropropane (2-NP) is a colourless, oily liquid with a mild
odour. It is flammable, only moderately volatile, and stable under
ordinary conditions. It is only slightly soluble in water but
miscible with many organic liquids, and it is an excellent solvent
for many types of organic compounds. Adequate analytical methods
exist for the identification and measurement of 2-NP at
environmental concentrations. Current methods use gas chromatography
and a flame ionization or electron capture detector or,
alternatively, high-performance liquid chromatography with an
ultraviolet detector. For measurement in air, 2-NP must first be
trapped and concentrated in a solid sorbent.
1.2 Uses and sources of exposure
1.2.1 Production
Current world production figures are not available. In 1977
production in the USA was approximately 13 600 tonnes. 2-NP is
currently manufactured by two USA companies and one French company.
It is produced naturally in trace amounts in the combustion of
tobacco and other nitrate-rich organic matter, but there is no
evidence that it is produced by any biological processes.
1.2.2 Uses and loss to the environment
2-NP is used as a solvent, principally in blends, and has many
industrial applications as a solvent for printing inks, paints,
varnishes, adhesives and other coatings such as beverage container
linings. It has also been used as a solvent to separate closely
related substances such as fatty acids, as an intermediate in
chemical syntheses, and as a fuel additive. Losses to the
environment are mainly to the air and are due principally to solvent
evaporation from coated surfaces.
1.3 Environmental transport and distribution
2-NP appears to be highly mobile in the natural environment.
Since it is slightly water soluble, slightly adsorbed by sediment,
slightly bioaccumulated, and evaporates readily into the atmosphere,
it will be distributed in both air and water and not accumulated in
any individual environmental compartment. Ultraviolet
photoabsorption by 2-NP is within the range of wavelengths occurring
naturally in the environment, and it is thus likely that 2-NP
undergoes slow photolysis in the atmosphere. Slow biological
conversion of 2-NP to less toxic compounds also appears likely in
both aquatic and terrestrial environments.
1.4 Environmental levels and human exposure
General population exposure to 2-NP appears to be very low and
is derived from cigarette smoke (1.1 to 1.2 µg/cigarette), from
residues in coatings such as beverage can coatings, adhesives and
print, and from vegetable oils fractionated with 2-NP. Industrial
exposure worldwide is unknown, but in the USA appears to be limited
to 0.02-0.19% of the workforce. Significant exposure (exposure to
9.1 mg/m3 (2.5 ppm) or more) in the USA may be limited to about
4000 workers (approximately 0.005% of the workforce). Occupational
exposure limits in the air vary among different countries and range
from 3.6 mg/m3 (1 ppm) (TWA) to 146 mg/m3 (40 ppm) (STEL).
Manufacture of 2-NP is an enclosed process and usually involves
little employee exposure, but some workers in industries such as
painting, printing, and solvent extraction have in the past been
exposed to levels much greater than occupational exposure limits.
Concentrations as high as 6 g/m3 (1640 ppm) in air were recorded
in a drum-filling operation.
1.5 Kinetics and metabolism
Human uptake of 2-NP occurs mainly through the lungs. In
experimental animals, 2-NP has been shown to be rapidly absorbed not
only via the lungs but also from the peritoneal cavity and the
gastrointestinal tract. There is no satisfactory information on
absorption via the skin. Information on distribution in rats is
somewhat contradictory. 2-NP is rapidly metabolized, mainly to
acetone and nitrite. Some isopropyl alcohol may also be formed.
Following intraperitoneal injection, 2-NP and its carbon-containing
metabolites are concentrated initially in fat and subsequently in
bone marrow as well as in the adrenal glands and other internal
organs. Following inhalation, 2-NP and its carbon-containing
metabolites are concentrated in the liver and kidney, with
relatively little in fat. Several different enzyme systems may be
involved and there are species differences concerning rates and
pathways. 2-NP and its carbon-containing metabolites are rapidly
lost from the body by metabolic transformation, exhalation, and
excretion in the urine and faeces. Satisfactory information on the
distribution and excretion of nitro moiety metabolites is lacking.
1.6 Effects on laboratory mammals and in vitro systems
2-NP has moderate acute toxicity for mammals. Males are more
sensitive than females, at least among rats, and sensitivity differs
widely among the species that have been tested. The LC50
(concentration causing 50% mortality within 14 days) for rats
following a 6-h exposure was 1.5 g/m3 (400 ppm) for males and
2.6 g/m3 (720 ppm) for females. Lethality appeared to be
associated mainly with the narcotic effects, but mammals exposed to
concentrations of at least 8.4 g/m3 (2300 ppm) for one hour or
longer displayed severe pathological changes including
hepatocellular damage, pulmonary oedema, and haemorrhage.
There is clear evidence that 2-NP is carcinogenic in rats.
Long-term inhalation exposure of rats to 0.36 g/m3 (100 ppm) for
18 months (7 h/day, 5 days/week) induced destructive changes in the
liver, including hepatocellular carcinomas in some males. A
concentration of 0.75 g/m3 (207 ppm) induced more severe damage,
including a high incidence of hepatocellular carcinomas, more
quickly. Moderate-chronic oral dosage also induced excess
hepatocellular carcinomas in rats. However, long-term inhalation
exposure of rats to 91 or 98.3 mg/m3 (25 or 27 ppm) produced no
detectable injury. Exposure of mice and rabbits to concentrations of
2-NP that induced hepatocellular carcinomas in rats had little or no
effect, but these studies were too limited to completely rule out
2-NP carcinogenicity in these two species. 2-NP slightly retarded
fetal development of rats, but there is a paucity of data on
embryotoxicity, teratogenicity, and reproductive toxicity. 2-NP was
found to be strongly genotoxic in rat hepatocytes both in vitro
and in vivo, but no significant genotoxicity was observed in other
organs of the rat or in cell lines of extrahepatic origin without
exogenous metabolic activation. 2NP has been shown to be mutagenic
in bacteria both in the presence and absence of exogenous metabolic
activation.
1.7 Effects on humans
Human exposure to high concentrations of 2-NP is largely or
entirely occupationally related. High concentrations (actual values
are unknown but in one case they were estimated to be 2184 mg/m3
(600 ppm)) are acutely toxic and have produced industrial
fatalities. Initial symptoms included headache, nausea, drowsiness,
vomiting, diarrhoea, and pain. Victims often showed temporary
improvement, but in some cases death occurred 4 to 26 days after
exposure. Hepatic failure was the primary cause of death, and lung
oedema, gastrointestinal bleeding, and respiratory and kidney
failure were contributing factors. Occupational exposure to
estimated levels of 73 to 164 mg/m3 (20 to 45 ppm) induced nausea
and loss of appetite, which persisted for several hours after
leaving the workplace, whereas occupational exposure to estimated
levels of 36.4 to 109 mg/m3 (10 to 30 ppm) (< 4 h/day for
< 3 days/week) produced no noticeable ill effects.
Although available data are inadequate, there is no indication
that chronic occupational exposure to 2-NP at concentrations usually
encountered in the workplace induces hepatic or other neoplasms, or
other long-term adverse effects.
1.8 Effects on other organisms in the laboratory and field
The few studies performed on microorganisms, invertebrates, and
fish indicate low toxicity of 2-NP for non-mammalian organisms.
2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, AND ANALYTICAL METHODS
2.1 Identity
Chemical structure: NO2
|
H3C - C - CH3
|
H
Empirical formula: C3H7NO2
Synonyms: Dimethylnitromethane, isonitropropane,
nitroisopropane, 2-NP
Trade names: NiPar S-20 (solvent), NiPar S-30 (solvent, a
mixture of 1- and 2-nitro-propane)
CAS registry number: 79-46-9
RTECS number: TZ 5250000
Relative molecular
mass: 89.09
2.2 Physical and chemical properties
2-Nitropropane (2-NP) is an important synthetic organic
chemical. Its physical properties have been described by Angus
Chemical Co. (1985), Baker & Bollmeier, (1981), Woo et al. (1985),
and Weast (1986) and are summarized in Table 1. It is a colourless,
oily liquid with a mild odour and remains liquid over a relatively
broad temperature range, i.e. -93 to 120 °C. 2-NP is flammable, and
although only moderately volatile, its vapour forms flammable or
explosive mixtures with air. It is stable under ordinary
circumstances, but may undergo explosive decomposition under
conditions of extreme shock combined with heavy confinement and
elevated temperature. 2-NP is only slightly soluble in water (17
ml/litre at 20 °C) and water is even less soluble in 2-NP (5 ml/l at
20 °C). With increasing temperature, solubility of both 2-NP in
water and water in 2-NP increases, and an azeotrope containing 29.4%
water ultimately is formed. Its boiling point is 88.6 °C. 2-NP is,
however, miscible with many organic compounds including chloroform,
aromatic hydrocarbons, alcohols, esters, ketones, ethers, and higher
aliphatic carboxylic acids. Alkanes and cycloalkanes have more
limited solubility in 2-NP (Baker & Bollmeier, 1981). Azeotropes are
formed with some organic liquids.
Table 1. Physical properties of 2-nitropropane
Reference
Appearance colourless, oily Stokinger (1982)
liquid
Relative molecular mass 89.09 Stokinger (1982)
Weast (1986)
Windholz (1983)
Specific gravity 0.988 Baker & Bollmeier (1981)
(liquid density) Stokinger (1982)
(at 20 °/4 °C)
Vapour density (air = 1.00) 3.06 Stokinger (1982)
Vapour pressure (20 °C) 1.72 MPa Stokinger (1982)
(12.9 torr)
Boiling point 120.3 °C Baker & Bollmeier (1981)
Stokinger (1982)
Windholz (1983)
Melting point -93 °C Weast (1986)
Water solubility (20 °C) 17 ml/l Baker & Bollmeier (1981)
Stokinger (1982)
Windholz (1983)
Refractive index (20 °C) 1.3944 Baker & Bollmeier (1981)
Weast (1986)
Flash point (open cup) 38 °C Stokinger (1982)
Lower inflammability limit 2.6 volume % National Fire Protection
in air Association (1968)
Partition coefficients
water/air 128 Filser & Baumann (1988)
olive oil/air 710 Filser & Baumann (1988)
in vivo whole body 175 Filser & Baumann (1988)
(rat)/air
2-NP, like other nitroparaffins, undergoes a variety of chemical
reactions. The chemistry of nitroparaffins has been the subject of a
number of reviews and symposia and has been summarized by Baker &
Bollmeier (1981), Goldwhite (1965), Stokinger (1982), Woo et al.
(1985), and others. 2-NP is an acidic substance. The nitro form,
which is mildly acidic, exists in equilibrium with its more strongly
acidic "aci" tautomer and with the anionic form (nitronate) of the
latter (Fig. 1). The aci tautomer is referred to as a nitronic acid
and forms metal salts. It can be dissolved and neutralized by strong
bases and gives a characteristic colour reaction with ferric
chloride. Prolonged action of bases leads to decomposition. Aqueous
acids hydrolyse 2-NP first to a hydroxamic acid and ultimately to
carboxylic acids and hydroxylammonium salts. 2-NP reacts with
nitrous acid to form a pseudonitrole which is colourless in
crystalline form but blue when melted or in solution. The carbon
atom bearing the nitro group is easily halogenated in the presence
of a base. Photochemical chlorination, however, yields reaction
products in which chlorine atoms are attached to the terminal
carbons. In the presence of a base, 2-NP condenses with carbonyl
compounds to yield a ß-nitroalcohol, which may dehydrate
spontaneously to a nitro-olefin. Nitro-olefins thus formed, and a
variety of other unsaturated compounds, undergo Michael addition
reactions with 2-NP in the presence of a catalytic amount of base.
2-NP will condense with formaldehyde and a secondary amine (the
Mannich reaction). Mild reduction of 2-NP yields
isopropylhydroxylamine, and strong reduction produces
isopropylamine. Auto-oxidation catalysed by cuprous chloride yields
2-hydroperoxy-2-nitropropane (Fieser & Fieser, 1972, 1974).
Taste and odour are subjective biological properties derived
from chemical and physical properties. The odour has been described
as "sweet-solventy, rubbery, and alcohol-like" (letter from S.E.
Ellis of Arthur D. Little, Inc. to G. Crawford of Occusafe Inc.,
1982). There also is some uncertainty concerning odour threshold.
Treon & Dutra (1952) stated that the odour of 2-NP was detectable at
1070 mg/m3 (294 ppm) but not at 302 mg/m3 (83 ppm), without
describing the methodology by which these values were determined;
their values have nevertheless been incorporated into various
guidelines (Crawford et al., 1984). Two recent studies redetermined
the odour threshold for 2-NP. In one the ED50 (minimum concentration
detected by 50% of the population) was estimated to be 18.2 mg/m3
(5.0 ppm) with 95% confidence limits from 11.3 mg/m3 (3.1 ppm) to
28.76 mg/m3 (7.9 ppm), and, in the other, all of a four-member
test panel detected 2-NP at 11.3 mg/m3 (Crawford et al., 1984).
There was no consensus as to the taste of a 6.4 g/litre (0.072
mol/litre) solution of 2-NP in water (Marcstrom, 1967). The most
frequent response was bitter, but other tasters found it (1) sweet,
(2) bitter and sour, (3) bitter, cool, and anaesthetizing, (4)
burning, (5) burning and cool, or (6) burning, sweet, bitter, and
sour. Wilks & Gilbert (1972a) reported a taste detection threshold
for 2-NP in water of 12.5 mg/litre.
2.3 Conversion factors
1 ppm 2-NP in air = 3.64 mg/m3
1 mg/m3 = 0.27 ppm 2-NP in air
2.4 Analytical methods
Analytical methods for 2-NP appear limited to the analysis of
air, water, blood plasma, coatings, and cigarette smoke (Table 2).
Since the colorimetric methods are less sensitive or are cumbersome,
the method of choice is probably gas chromatography with either a
flame ionization or an election capture detection. Charcoal is not a
satisfactory adsorbent for 2-NP since recovery is poor (Andersson et
al., 1983) and there may be decomposition (Glaser & Woodfin, 1981).
In addition to Chromosorb 106, Amberlite XAD-7 appears satisfactory
as a solid sorbent for quantitatively collecting 2-NP from the air
(Andersson et al., 1983), although its collection efficiency is
markedly reduced in humid air (Andersson et al., 1984). Use of a
collection tube with two sections, however, can compensate for the
reduced efficiency (Andersson et al., 1984). The high-performance
liquid chromatography method developed for blood (Derks et al.,
1988) could probably be adapted to other biological materials.
Table 2. Analytical techniques for determining 2-nitropropanea
Methods Detection limit Comments Reference
Air
Trapping in ethanol; 0.1 mg/ml ethanol absorption is linear between 0.1 and Treon & Dutra (1952)
concentration determined 2.0 mg/ml; ethanol was especially
spectrophotometrically at purified and redistilled
277.5 nm
Trapping in concentrated ca. 1 µg/ml sulfuric absorption is linear between 1 and 5 µg/ml Jones & Riddick (1952);
sulfuric acid; resulting acid sulfuric acid; no interference from primary Jones (1963)
nitrous acid combined nitroparaffins, but all other secondary,
with resorcinol to form a some tertiary and some halogenated
red-blue colour; measured nitroparaffins interfere
spectro-photometrically
at 560 nm
Trapping in solid sorbent tube 3.6 mg/m3 working range is 3.6 to 36 mg/m3; 2-NP Glaser & Woodfin (1981)
(Chromosorb 106, 60/80 mesh); stable on absorbent for at least 28 days
desorption: ethyl acetate;
separation-detection: GC-FID
Methodology similar to above 3.1 mg/m3 Method is a modification of that proposed US NIOSH (1987a)
by Glaser & Woodfin (1981); range: 3.1 to
28.3 mg/m3; no interference from methyl
butyl ketone, heptane, 1-nitropropane,
toluene, and xylene
Table 2 (contd).
Methods Detection limit Comments Reference
Water
Sample (40 µl) collected ca. 0.5 mg/m3 water elutes quickly extinguishing flame Wilks & Gilbert (1972a)
with a syringe and injected in FID; flame can be reignited before 2-NP
directly into GC; emerges
detection: FID
Blood
Blood collected in chilled, 1 ng UV absorption linear from 0 to 250 ng; Derks et al. (1988)
screw-capped vial with heparin; uses 0.3 ml blood sample; samples are
centrifuged; deproteinized with unstable and must be analysed promptly
acetonitrile; Tris buffer added;
separation: HPLC; detection:
UV at 224 nm
Coating (beverage can)
Redissolve coating in solvent not given reference provides few details on Wilks & Gilbert (1972b)
suitably distinct from 2-NP; methodology
acetone suitable for vinyl
co-polymer coatings;
separation: GC
Table 2 (contd).
Methods Detection limit Comments Reference
Coating (beverage can)
Can (empty) simultaneously not given method captures about 90% of Wilks & Gilbert (1972b)
perforated and fitted with a residual 2-NP
diaphragm; hypodermic needle
inserted through diaphragm and
fitted with stopcock; can heated
(150 °C, 15 min); headspace
sampled with heated syringe;
separation: GC
Cigarette smoke
Steam distillation of smoke 0.8 µg/cigarette may be adapted for air and water Hoffmann & Rathkamp
condensate on filters, analysis (1968)
extraction re-extraction in
NaOH, in ethyl ether,
neutralization with H2SO4 and
re-extraction in ethyl ether,
concentration of extract and
injection in GC equipped with
FID or ECD detectors
a Abbreviations: ECD = electron capture detector; FID = flame ionization detector; GC = gas chromatograph;
HPLC = high-performance liquid chromatograph; UV = ultraviolet
3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE
3.1 Natural occurrence
There is no evidence that 2-NP and other nitroaliphatic
compounds are produced by biological processes, although a related
organic compound, ß-nitropropionic acid, has been isolated from
plants and microorganisms (Goldwhite, 1965). However, nitroaliphatic
compounds are produced in low concentrations by combustion of
organic matter and have been detected in tobacco smoke. Hoffmann &
Rathkamp (1968) reported 1.1-1.2 µg 2-NP in the smoke from single
85-mm USA blended non-filter cigarettes, and ascribed the production
of this and other nitroaliphatics to interactions in the combustion
zone between hydrocarbons and nitrogen dioxide generated by
decomposition of nitrates.
3.2 Anthropogenic sources
3.2.1 Production levels and processes
Although 2-NP is an important industrial chemical, current world
production figures are not available. In 1977 production by the sole
USA manufacturer was estimated to be 13 600 tonnes, of which 5400
tonnes were sold in the USA and 8200 tonnes were either exported or
used internally (Finklea, 1977). 2-NP currently is produced by two
USA manufacturers, Angus Chemical Co., Sterlington, Louisiana, and
W. R. Grace Co., Deer Park, Texas (SRI International, 1988, 1990;
USITC, 1990), and by one European manufacturer, Société Chimique de
la Grande Paroisse, France (Anon., 1976, 1982; IARC, 1982). In the
USA, 2-NP, together with nitromethane, nitroethane and
1-nitropropane, is manufactured by a vapour phase reaction of nitric
acid with an excess of propane at high temperature and pressure
(370-450 °C, 0.8-1.2 MPa (8-12 atm.)) (Baker & Bollmeier, 1981). The
proportions of the four nitroaliphatic compounds in the reaction
product are a function of the reaction temperature. In Europe,
propane is reacted with nitrogen peroxide (N2O4) and an excess
of oxygen at 150-330 °C and 0.9-1.2 MPa (9-12 atm.), yielding the
same nitroaliphatic compounds in slightly different proportions as
produced by the USA process (Anon., 1976). Reaction products are
condensed, washed, and separated by fractional distillation. There
is no evidence that 2-NP is produced through human EHC 138:
2-Nitropropane activities except by combustion and deliberate
manufacture, although nitromethane has been detected in vehicle
exhaust (Seizinger & Dimitrades, 1972).
3.2.2 Uses
The importance of 2-NP as an industrial chemical stems mainly
from its desirable and occasionally unique characteristics as a
solvent (Purcell, 1967; Anon., 1976; Fishbein, 1981; Baker &
Bollmeier, 1981; IARC, 1982; ACGIH, 1986). It is an excellent
solvent or cosolvent for a variety of fats, waxes, gums, resins,
dyes and other organic compounds, including vinyl, acrylic,
polyamide and epoxy resins, chlorinated rubbers, and organic
cellulose esters. The ability of 2-NP to form an azeotrope with
water and the associated large heat of absorption permit it to
displace monomolecular layers of water molecules and secure a better
bond between pigments and the surfaces to which they are applied.
Its major use is as a solvent for inks, paints, varnishes, adhesives
and other coatings such as beverage container linings. It is used
principally in blends with other solvents to impart desirable
characteristics, such as greater solvency, better flow
characteristics and film integrity, greater pigment dispersion,
increased wetting ability, improved electrostatic spraying
properties, or reduced drying time. 2-NP is also used industrially
as a processing solvent for separating closely related substances in
natural products or reaction mixtures. These have included, for
example, separation of oleic acid from polyunsaturated fatty acids
and cetyl from oleyl alcohols.
In addition to the above, 2-NP has a number of minor uses
(Anon., 1976; Baker & Bollmeier, 1981). These include a medium for
chemical reactions, an intermediate for the manufacture of
2-nitro-2-methyl-1-propanol, 2,2-dinitropropane, 2-amino-2-
methyl-1-propanol and other propane derivatives, and a component of
explosives, propellants, and fuels for internal combustion engines.
The latter usage appears limited to model engines used by hobbyists
and to racing cars. Although the addition of 2-NP to fuel improves
diesel engine performance, it is not used commercially as a diesel
fuel additive since superior alternatives are available (Banes,
1989)a. In the USA, mixed isomers of nitropropane are used to
denature ethanol (US FDA, 1987). The addition of 2-NP to hydrocarbon
mixtures has been shown to inhibit corrosion of tin-plated steel
aerosol cans (Flanner, 1972).
3.3 Release into the environment
There is some quantitative data on releases of 2-NP into the
environment. The US Environmental Protection Agency has supported a
thorough, though largely speculative, analysis of the problem (US
EPA, 1980). Releases of 2-NP occur mainly into the atmosphere and
can result from spillage, from venting of gases and fugitive
emissions during manufacture, transfer and use, and from solvent
evaporation from coated surfaces. The US EPA document estimated that
of the 14 000 tonnes of 2-NP produced in the USA in 1979, 5714
tonnes (41%) was released into the air, and 1 tonne into water. Only
230 tonnes (1.6%) was destroyed by incineration or waste treatment.
The major contributor to this release estimate was evaporation of
2-NP used as a solvent in printing ink and surface coatings (4450
a Personal communication from the US Environmental Protection
Agency, Ann Arbor, Minneapolis
tonnes, 78% of releases). Manufacture of 2-NP is a largely enclosed
process and in 1979 it accounted for only 21 tonnes (0.3%) of the
amount released into the environment. A more recent examination of
this problem (National Library of Medicine, 1989) reported a similar
situation concerning environmental releases of 2-NP in the USA. Out
of a yearly total of 299 tonnes, 205 tonnes (69%) was released into
the air with 123 tonnes coming from point (large, easily identified)
sources and 82 tonnes from non-point (small, not easily identified)
sources. Only 2 tonnes (< 1%) was released directly into water, and
1 tonne into municipal sewage treatment plants. The remainder was
buried in closed containers (76 tonnes, 25%) or was disposed of in
unspecified ways (15 tonnes, 5%).
4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION, AND TRANSFORMATION
4.1 Transport in the environment
2-NP appears to be highly mobile in the natural environment.
Cupitt (1980) considered physical removal of 2-NP from the
atmosphere unlikely because it was not soluble enough to be rapidly
washed out and had a vapour pressure too great for strong adsorption
on particles. The partition of 2-NP between air and water at
equilibrium was estimated by the method of Swann et al. (1983) to be
about 0.5% in air and 99.5% in water (US EPA, 1985). These values
indicate rapid and easy exchange between air and water. The soil
sorption coefficient (ratio of soil concentration to water
concentration) and the bioconcentration factor were estimated by the
methods of Kenaga (1980) to be 20 and 2.5, respectively (US EPA,
1985). Measured values for absorption and bioaccumulation were
somewhat greater than these estimates. Freitag et al. (1982, 1985)
obtained concentration factors over water for activated sludge,
unicellular algae (Chlorella fusca), and fish (golden ide) of 70,
20, and < 10, respectively. These values indicate that 2-NP is not
strongly bioaccumulated and is readily desorbed from sediment
particles and leached from soil. Thus, in summary, since 2-NP is
slightly water soluble, slightly adsorbed by sediment, slightly
bioaccumulated, and evaporates readily into the atmosphere, it will
be distributed in both air and water and not accumulated in any
individual environmental compartment.
4.2 Biotic and abiotic transformation
Data concerning the destruction of 2-NP by biotic and abiotic
processes are limited. 2-NP has significant photoabsorption in the
environmentally relevant range of > 290 nm (Sadtler, 1961) and is
likely to undergo photolysis (Cupitt, 1980; US EPA, 1985). On the
basis of physical and chemical properties Cupitt (1980) hypothesized
that it would be rapidly removed from the atmosphere by photolysis
and estimated a reduction in concentration of 1/e (0.369) in 0.2
days. This is equivalent to a half-life of 0.14 days (3.36 h).
However, measurements of photochemical reactivity do not support
such a rapid destruction of 2-NP. Studies aimed at defining the
relationship between organic solvents and photochemical smog
production ranked 2-NP as low to moderate in terms of its
interactions with oxidants and its ability to produce formaldehyde
and other lachrymators (Levy, 1973). Laboratory measurements also
suggested slow decomposition (Freitag et al., 1985). The methodology
employed (Korte et al., 1978; Lotz et al., 1979; Freitag et al.,
1982), i.e. irradiation of the solvent adsorbed on silica gel by
light from a high pressure mercury lamp filtered through pyrex, was
too different from natural conditions to permit quantitative
extrapolation from laboratory results to rates of photolysis in the
atmosphere. The study provided comparative photodecomposition rates
for a large number of solvents. The rate of photodecomposition for
2-NP was roughly half that of dichlorodiphenyltrichloroethane (DDT),
similar to the rates for dodecane and 2,4-dichlorobenzoic acid, and
roughly twice those for kepone and dieldrin. Paszyc (1971) reported
that the major decomposition products for both gaseous and liquid
2-NP under laboratory conditions were nitrogen dioxide, acetone,
isopropyl nitrite, isopropanol, methylcyanide, water, and propane,
regardless of whether irradiation was monochromatic 253.7 nm light
or the full spectrum of light produced by a high pressure mercury
lamp. Cupitt (1980) speculated that the major products of
photodecomposition in nature would be formaldehyde and acetaldehyde.
Biological decomposition of 2-NP appears likely, but is probably
rather slow in nature. Enzymes capable of oxidizing or initiating
non-enzymatic oxidation of 2-NP have been identified in horseradish
(De Rycker & Halliwell, 1978; Porter & Bright, 1983; Indig &
Cilento, 1987), pea seedlings (Little, 1957), and a variety of
microorganisms including bacteria, yeasts, and fungi (Little, 1951;
Kido et al., 1975; Soda et al., 1977; Dhawale & Hornemann, 1979;
Patel et al., 1982). In in vitro preparations of horseradish
(Dhawale & Hornemann, 1979), pea seedlings (Little, 1957), a fungus
(Streptomyces achromogenes) (Dhawale & Hornemann, 1979), and a
yeast (Hansenula mrakii) (Kido et al., 1975), 2-NP was converted
to a less toxic compound, acetone, and a moderately toxic compound,
nitrite. In addition to nitrite, some nitrate may also be formed
(Indig & Cilento, 1987). In the yeast, nitrite was subsequently
reduced to ammonia. The importance of these processes in nature is
unknown. Kido et al. (1975), however, reported that only 4 out of 14
species of microorganisms tested would grow in a medium containing
5 g 2-NP/litre. The only study of 2-NP decomposition by a population
of microorganisms (Freitag et al., 1985) utilized activated sludge
grown at 25 °C. Solvent concentrations in these experiments were low
(50 µg/litre) to prevent adaptation to the substances tested (Korte
et al., 1978). In 5 days only 0.4% of the 2-NP was converted to
carbon dioxide. In summary, these data suggest that in both
terrestrial and aquatic communities 2-NP is biologically decomposed
and that the rate may be slow, but they offer no definite
information on the problem.
5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE
5.1 Environmental levels and general population exposure
General population exposure to 2-NP appears to be very low.
There seem to be no records of its occurrence in water or in outdoor
air away from areas of manufacture and use. The only information on
intake exists in the form of a memorandum from Modderman (1983)a.
The daily intake per person in the USA was estimated to be 50 to
100 mg. The residuum from its use as a solvent for beverage can
coatings, film laminating adhesives and printing inks for flexible
food packaging may account for as much as 37 ng/day and from
vegetable oils fractionated with 2-NP, 30 ng/day. 2-NP residues of
0.077 mg/litre (77 ppb) to 0.24 mg/litre (204 ppb) have been found
in such oils. In a further report on the evaluation of
2-nitropropane as a food processing solvent, it was assumed that
residues of less than 10 µg/kg would occur in oils, giving rise to
estimated daily intakes of 10 ng/day. The use of 2-NP as a food
processing solvent was not recommended (FAO/WHO, 1990a,b). As
mentioned in section 3.1, smokers are exposed regularly to low
concentrations of 2-NP. Hoffmann & Rathkamp (1968) reported 1.1 to
1.2 µg in the smoke of a single cigarette. 2-NP was reported to
occur in the expired air of 11.1% (5 of 54 individuals) of a sample
of healthy adult urban dwellers (Krotoszynski et al., 1979). The
geometric mean was 0.406 ng/litre with one-sigma limits of
0.119 ng/litre and 1.38 ng/litre, (at 25 °C, 98 MPa (760 mmHg)). The
sample was entirely of non-smokers who had avoided medication and
prolonged exposure to perfume, paint, glue, aerosols, dust, tobacco
smoke and areas polluted with industrial wastes during, and for at
least 7 days prior to, the sampling period, and also avoided
cosmetics, spices, seasonings, and alcoholic beverages during and
immediately prior to sampling. The origin of the exhaled 2-NP is
unclear. Exposure to 2-NP may be further reduced in the future since
a number of regulations have been enacted and recommendations made
to declare it a harmful, carcinogenic substance and a toxic waste,
and to discourage its use (IARC, 1982; IRPTC, 1986; US FDA, 1987; US
NIOSH, 1988).
a Memorandum from Modderman, J.P. to Shibko, S., Associate
Director for Regulatory Evaluation, Department of Health & Human
Services, USA, 8 pp. "Exposure estimates for chemicals to be
included in the NTP annual report on carcinogens".
5.2 Potential occupational exposure
The number of workers in the USA who handle 2-NP or mixtures
containing 2-NP has been variously estimated as 15 000 (US EPA,
1977), 38 600 (Occupational Health Services, Inc., 1982), 100 000
(Finklea, 1977), and 185 000 (Beall et al., 1980). Based on
employment values of the US Bureau of the Census (1987) these
estimates of exposed workers represent from 0.02% to 0.19% of the
civilian workforce in the USA. The low estimate of 15 000, although
quoted in a US Environmental Protection Agency report, originated
with a manufacturer of 2-NP. The estimate generated by Occupational
Health Services, Inc., carried out under contract with a
manufacturer of 2-NP, was based on a detailed survey of
distributors, manufacturers, and users, and thus may represent a
reasonable approximation. This report considered 38 600 to be the
best estimate of the total number of exposed workers in the USA and
set 126 600 workers as an upper boundary. It further estimated that
significant exposure (defined as exposure to at least 10% of the US
OSHA exposure limit or 9.1 mg/m3 (2.5 ppm)) ranged from 4000 (best
estimate) to 10 600 (upper boundary) workers. Sources of worker
exposure identified in a survey conducted in the USA by the National
Institute for Occupational Safety and Health (Finklea, 1977)
included rotogravure and flexographic inks used in printing, and
coatings and adhesives used in industrial construction and
maintenance, highway marking, ship building and maintenance,
furniture manufacture, and food packaging. A US NIOSH survey
estimated that 9815 workers in the USA were exposed to 2-NP or to
trade-name products containing 2-NP (US NIOSH, 1983).
Occupational exposure limits are summarized in Table 3.
There is little data on actual occupational exposure, although
the limited information on conditions in the USA summarized in
Table 4 suggests that it is highly variable. Manufacture of 2-NP, an
enclosed process, appears to involve little employee exposure much
of the time, but spills and operations such as filling drums can
briefly expose a few workers to high concentrations. In general, low
exposures may be typical in some painting operations and in the
manufacture of tyres, but other painting and manufacturing
operations appear, at least in the past, to have exposed workers to
dangerously high concentrations. Workers were exposed to
concentrations of 2-NP up to at least 2744 mg/m3 (754 pm) in a
pigment production facility and up to at least 265 mg/m3 (73 ppm)
in a solvent extraction plant. Evidence exists that concentrations
of 2-NP at the solvent extraction plant prior to the investigation
were at times substantially greater than the values measured during
the investigation (Crawford et al., 1985). Exposure levels mentioned
in the report by Occupational Health Services, Inc. (1982) were
mainly in the vicinity of 3.6 mg/m3 (1 ppm), although one printing
Table 3. Occupational exposure limits for 2-nitropropane in aira
Exposure limit
Country mg/m3 ppm Category of limit
Australia 36 10 TWA
Belgium 36 10 TWA
Brazil 70 20 AL
Canada 90 25 CLV
Denmark 36 10 TWA
Finland 18 5 TWA
150 40 STEL
Germany 18 5 1-year TWA(TRK)
Hungary 10 3 CLV
Netherlands 3.6 1 8-h TWA
7.3 2 STEL
Poland 30 8 TWA
70 20 STEL
Romania 47 13 TWA
70 20 STEL
Sweden 18 5 TWA
36 10 CLV
Switzerland 18 5 TWA
United Kingdom 90 25 8-h TWA
USA 90 25 8-h TWA (OSHA)
36 10 8-h TWA (ACGIH)
Yugoslavia 90 25 TWA
a From: IRPTC (1986)
ACGIH = American Conference of Governmental Industrial Hygienists
AL = Acceptable or tolerable limit
CLV = Ceiling value
MAK = Maximum worksite concentration
OSHA = Occupational Safety and Health Administration
STEL = Short-term exposure limit
TLV = Threshold limit value
TWA = Time-weighted average (MAK in Switzerland)
TRK = Technical guiding concentration
plant (Table 4) reported a peak concentration of 237 mg/m3
(65 ppm) which lasted 30 min, and a time-weighted average of
36-44 mg/m3 (10-12 ppm). In addition to inhalation, it is likely
that workers using 2-NP as a solvent will have at least occasional
contact with the liquid. 2-NP also has been reported to be a minor
component of both fresh and used machine cutting fluid emulsion
(Yasuhara et al., 1986). The importance of exposure to 2-NP as a
contaminant of 1-nitropropane is unknown. In an investigation
involving 1-nitropropane-sensitized ammonium nitrate blasting agents
(Cocalis, 1982), 2-NP was below the detectable limit. Occupational
exposure may be reduced in the future since strong recommendations
have been issued on minimizing all worker contact with 2-NP and its
fumes and on substituting other less toxic solvents where possible
due to the carcinogenicity of 2-NP (IRPTC, 1986; US EPA, 1986; US
NIOSH 1988).
Table 4. Air concentration of 2-NP in the work place
Concentration
Activity mg/m3 ppm Reference
Manufacture of 2-NP 3.64 approx 1 Brown & Dobbin (1977)
Manufacture of 2-NP 0.7-364; 0.2-100; Miller & Temple (1979)
98% of 98% of
samples samples
were < 36.4 were < 10
Vulcanizing tyres 0-0.18 < 0.05 Hollett & Schloemer
(1978)
Painting (bus 0.11 < 0.03a Love & Kern (1981)
maintenance)
Painting (railway 1.46 < 0.4 Hartle (1980)
cars)
Solvent extraction 167.4 0-100 Crawford et al. (1985)
(average
46)
Laboratory (1958) 14.6 0-21 Angus Chemical Co. &
(average 4) Occusafe, Inc. (1986)
2-NP storage & 2111-6000 580-1640 Angus Chemical Co. &
transfer area (1962); Occusafe, Inc. (1986)
drum filling
operation
Pigment production 109-2745 30-754 Angus Chemical Co. &
facility (1970) Occusafe, Inc. (1986)
Painting (battery 36.4-109 10-30 Skinner (1947)
cases)
Manufacturing 72.8-164 20-45 Skinner (1947)
(coating forms)
Printing approx 40 1-65 Occupational Health
(approx 11) Services, Inc. (1982)
a below limit of detection
6. KINETICS AND METABOLISM
6.1 Absorption
2-NP is absorbed via the lungs, the peritoneal cavity, the
gastrointestinal tract, and possibly, to a lesser extent, via the
skin. Absorption via the lungs, peritoneal cavity, and
gastrointestinal tract have been used in experimental studies and
have been investigated using rats and 14C-labelled 2-NP. Pulmonary
absorption was examined by Nolan et al. (1982), Müller et al.
(1983), and Filser & Baumann (1988). Nolan et al. (1982) considered,
on the basis of respiratory rate and tidal volume of the rat and the
accumulation of 2-NP during the 6-h period of exposure, that a
minimum of 40% of the inhaled 2-NP was absorbed. This value is
minimal since it does not include 2-NP metabolized and eliminated
during exposure. The data of Müller et al. (1983) suggested that
immediately following exposure to 728 mg/m3 (200 ppm) for 3 h,
plasma contained approximately 0.4% as 2-NP and 7.2% as metabolites
of the 2-NP inhaled. The metabolites were mainly acetone but also
included a small amount of isopropanol. These percentages were
estimated from the results of Müller et al. (1983) and normative
data for the laboratory rat (Baker et al., 1979), and support rapid
pulmonary uptake of 2-NP since 2-NP and its metabolites sequestered
elsewhere in the body and loss of metabolized 2-NP during exposure
were not considered. Filser & Baumann (1988) reported that uptake of
gaseous 2-NP was rapid, the clearance rate being equal to the
ventilation rate. The value they cite for the latter, 32
litres.h-1.kg-1), seems large in comparison with normative data
for the laboratory rat (Baker et al., 1979).
The rate of 2-NP uptake by rats from intraperitoneal injection
was examined by Müller et al. (1983). Ten minutes after an injection
of 25 mg/kg, the blood plasma contained 3.3% of the dose as 2-NP and
1.9% as metabolites, acetone, and isopropanol, indicating an uptake
of greater than 5.2% since presumably some of the dose was already
lost from the body and to other tissues in the body during this
initial period. These percentages were estimated from the data of
Müller et al. (1983) and normative data for the laboratory rat
(Baker et al., 1979). A dose of 50 mg/kg yielded partially
dissimilar results. The 10-min average value was low and also had a
very large standard deviation. This may have reflected large
differences among the rats in their initial uptake rates for 2-NP or
an experimental problem such as injection into the gastrointestinal
tract rather than the peritoneal cavity. Blood plasma contained, 10
min after injection, only 1.4% of the dose as 2-NP and 1.3% as
acetone and isopropanol. These data indicate that uptake from the
peritoneal cavity is fairly rapid, and suggest that the relatively
slower uptake of the 50-mg/kg dose may have reflected saturation of
the uptake mechanisms, since initial concentrations in the plasma
did not exceed those following the 25-mg/kg dose. Intraperitoneal
injections were used by Andrae et al. (1988), Guo et
al. (1990), Hussain et al. (1990), and Conaway et al. (1991) to
demonstrate that 2-NP induced nucleic acid damage in the livers of
Wistar, F-344 and Sprague-Dawley rats.
Absorption of 2-NP via the gastrointestinal tract was
investigated with male Wistar rats by Derks et al. (1989). They
found that the systemic availability of orally administered 2-NP
from a water solution was very high (90%) and absorption was rapid,
maximum plasma values being reached within 15 min after dosage.
Absorption of 2-NP given in olive oil was much slower and
availability was only 34% during the initial 3 h following dosage.
The authors suggested that absorption from oil was incomplete at 3 h
and might ultimately be much higher, since the olive oil was
absorbed and the 2-NP redistributed between the oil and aqueous
phases.
Although workers are cautioned against dermal contact with 2-NP
(Beall et al., 1980; US EPA, 1986), there appear to be no
quantitative data on dermal absorption. The solubility of 2-NP in
both polar and non-polar solvents, together with its small molecular
size, suggests that it should be absorbed readily through the skin
(Malkinson & Gehlmann, 1977). Dermal application of 2 g 2-NP/kg to
rabbits produced no obvious symptoms (Wilbur & Parekh, 1982);
however, as noted below, the rabbit is relatively resistant to the
toxicity of 2-NP.
6.2 Distribution
The distribution of 2-NP and its carbon-containing metabolites
among the organs and tissues in Sprague-Dawley rats was examined by
Nolan et al. (1982) via inhalation, and by Müller et al. (1983) via
intraperitoneal injection. Both utilized 14C-labelled 2-NP and
thus their data do not reveal the distribution of nitrite and other
nitrogen-containing metabolites generated from the nitro portion of
the 2-NP molecule. One hour after intraperitoneal injection,
radioactivity was concentrated in fat; there were intermediate
amounts in the blood, liver, and kidney, and lower amounts in other
organs and tissues (Müller et al., 1983). By 40 h, the highest
concentrations were in bone marrow and adrenal tissue, intermediate
amounts being found in the kidney, liver, spleen, lungs, and omental
fat, and by 8 days only the concentration of 14C in adrenal tissue
was noticeably greater than elsewhere in the body. However, Nolan et
al. (1982), found, both immediately and 48 h after a 6-h period of
2-NP inhalation, that highest concentrations of carbon from 2-NP
were in the liver and kidney and relatively little in the fat.
Differences in methodology limit intercomparison of these
studies. Their major consistency is the presence of high
concentrations of 2-NP and its labelled carbon in the liver and
kidney, organs (as discussed below) actively involved in the
metabolism of 2-NP and excretion of its metabolites.
The relevance of these studies on tissue distribution of 2-NP
and its carbon-containing metabolites to the toxicity of 2-NP is
unclear, since (as discussed below) most of the dose is rapidly
metabolized initially to acetone and nitrite. The 14C label used
in these studies thus traced mainly the distribution of acetone and
its metabolites in measurements made more than a few hours after
dosing.
Dequidt et al. (1972) provided limited data on the distribution
of nitrite among body organs of the rat following inhalation and
intraperitoneal injection of 2-NP. The data suggest a fairly uniform
distribution among the heart, lungs, kidney, spleen, and,
sometimes, the liver. In the majority of experiments, however, no
nitrite was detected in the liver. No explanation is offered for the
latter observation, and data in the paper are so erratic as to
suggest the possibility of analytical problems.
6.3 Metabolic transformation
Starting with a report by Scott (1943), there have been numerous
studies on the metabolic transformation of 2-NP by mammals,
mammalian cells, microorganisms, and isolated enzymes. These studies
have shown that the major pathway for metabolic transformation of
2-NP involves oxidation to nitrite and acetone. Evidence for the
formation both of nitrite and acetone was reported from studies on
liver microsomes from rats pretreated with phenobarbital or
3-methylcholanthrene (Ullrich et al., 1978), cultured hepatocytes
from untreated rats (Haas-Jobelius et al., 1991), liver microsomes
from untreated mice (Marker & Kulkarni 1986a,b; Dayal et al., 1991),
V79 Chinese hamster cells (Haas-Jobelius et al., 1991), and a yeast
(Kido et al., 1975). Nitrite was reported to be a major metabolite
of 2-NP in rabbits (Scott, 1943), rats (Dequidt et al., 1972), and
liver microsomes from rats (Sakurai et al., 1980) and mice (Marker &
Kulkarni, 1985). Acetone was identified as a major metabolite of
2-NP in rats and chimpanzees (Muller et al., 1983).
Enzymatic oxidation of the nitronate form of 2-NP to nitrite and
acetone by horseradish peroxidase (Porter & Bright, 1983), a
dioxygenase from the yeast Hansenula mrakii (Kido et al., 1984),
and mouse liver microsomes (Dayal et al., 1991) was several times
more rapid than that of 2-NP under identical conditions. In addition
to acetone, a smaller amount of isopropanol is produced at least in
rats and chimpanzees (Muller et al., 1983). The source of the
isopropanol was not specified in this study, but since reduction of
acetone in the body is negligible (De Bruin, 1976), isopropanol may
be formed directly by oxidation of 2-NP. The formation of a
hydroxyisopropyl radical during the oxidation of 2-NP was suggested
by Kuo & Fridovich (1986).
The metabolic fates of these metabolites of 2-NP are well known.
Acetone is produced by a minor metabolic pathway in the mammalian
body (Smith et al., 1983) and has been detected in small amounts in
the blood, urine, and expired air of normal humans (Mabuchi, 1979;
Conkle et al., 1975). It may be excreted directly via expired air,
urine, and loss through the skin, or may enter into the general
metabolism either via cleavage to a 2-carbon acetyl fragment and a
1-carbon formyl fragment or via oxidation to pyruvic acid (De Bruin,
1976). The proportion excreted unchanged increases with increasing
dosage, suggesting an easily saturable metabolic pathway.
Isopropanol is oxidized to acetone (De Bruin, 1976).
Nitrite may exist as a minor constituent of the mammalian body.
It is constantly replenished by ingestion and synthesis, and
constantly removed by oxidation to nitrate. Nitrite and nitrate in
the blood stream are rapidly and homogeneously distributed
throughout the body (Parks et al., 1981). Nitrite rapidly oxidizes
divalent ferrous haemoglobin to trivalent ferric methaemoglobin
(Burrows, 1979). Little is transported to the tissues or excreted,
at least in dogs, sheep, and ponies (Schneider & Yeary, 1975).
Dequidt et al. (1972), however, reported substantial urinary
excretion of nitrite by rats following inhalation or intravenous
injection of 2-NP. Methaemoglobin is incapable of transporting
oxygen and, during enzymatic repair of this defect, nitrite is
reoxidized to nitrate. Parks et al. (1981) reported that 10 min
after intratracheal instillation of labelled nitrite into mice, 70%
of the label in plasma was in nitrate, 3% in nonionic compounds, and
only 27% remained as nitrite. Similar results were obtained with
rabbits. Nitrate is slowly excreted through the kidneys (Schneider &
Yeary, 1975) and also into saliva where it is reduced back to
nitrite by bacteria and reabsorbed into the body via the
gastrointestinal tract (Friedman et al., 1972). Small amounts of
nitrite in the stomach may react with secondary amines and other
amino substrates to form N-nitroso compounds which might be absorbed
(Sander & Schweinsberg, 1972; Fine et al., 1982).
The enzymatic system oxidizing 2-NP to acetone and nitrite was
identified through in vitro experiments using microsomes isolated
from mammalian liver. Ullrich & Schnabel (1973) determined that
cytochrome P-450, in liver microsomes from phenobarbital-pretreated
rats, bound 2-NP. Ullrich et al. (1978) subsequently reported that
liver microsomes from rats pretreated with phenobarbital or
3-methylcholanthrene rapidly catalysed the oxidation of 2-NP to
acetone and nitrite. The latter were produced in roughly equal
quantities. Surprisingly, however, the rate of this reaction was not
diminished under conditions of reduced oxygen pressure. The activity
of preparations from untreated control rats was generally very low.
Sakurai et al. (1980) demonstrated that this enzyme system in rats
was active in metabolizing other aliphatic nitro compounds. Marker &
Kulkarni (1985, 1986a, 1986b), working with mice, obtained somewhat
different results. They reported rapid denitrification of
2-NP to nitrite and acetone by liver microsomes from untreated mice,
and an acetone production at least twice the nitrite release. These
authors suggested that multiple forms of cytochrome P-450 are
involved, and claimed that nitrite is sequestered in the reaction
mixture and that denitrification of 2-NP may involve a reductive or
at least non-oxidative pathway as well as an oxidative pathway. They
also noted large differences in the rates of hepatic microsomal
enzymatic nitrite release among the five strains of mice tested.
Jonsson et al. (1977) demonstrated that hepatic microsomes from
uninduced rabbits could denitrify a compound related to 2-NP,
2-nitro-1-phenylpropane.
In addition to oxidative denitrification, a reductive pathway
has been shown to occur in cultured hepatocytes from Wistar rats and
in V79 Chinese hamster cells. Nitroreduction was indicated by the
fact that the cells formed acetone oxime, the tautomeric form of
nitrosopropane (Haas-Jobelius et al., 1991).
Evidence for the involvement of more than one pathway for the
metabolism of 2-NP in the rat was also obtained by Denk et al.
(1989). Their experiments on the pharmacokinetics of 2-NP in rats
exposed by inhalation suggested that there are two different
pathways both in male and female animals, a saturable one of low
capacity and high affinity according to Michaelis-Menten kinetics
and a non-saturable one following first-order kinetics. First-order
kinetics was similar in the two sexes, but striking differences
between sexes were observed in the kinetics of the saturable
pathway. The authors showed that in females more 2-NP was
metabolized by the non-saturable pathway at concentrations above 655
mg/m3 (180 ppm), and in males at concentrations above 218 mg/m3
(60 ppm), and linked their observations to the reported higher
susceptibility to liver damage of males as compared to females
(Griffin & Coulston, 1983). Denk et al. (1989) suggested that it is
the first-order metabolic process which results in the formation of
toxic products whereas the saturable pathway was suggested to lead
to less toxic metabolites.
These observations on the hepatic metabolism of 2-NP and related
compounds by rats, mice, and rabbits indicate differences among
species and even strains. It is probable that more than one enzyme
system is involved. In mice as well as rats hepatic cytochrome P-450
may be important in the metabolism of this xenobiotic.
Observations by Ivanetich et al. (1978) suggested an additional
detoxifying role for hepatic microsomal cytochrome P-450. They
demonstrated that under aerobic conditions in vitro 2-NP could
degrade the haem moiety of cytochrome P-450 in phenobarbital-induced
rats and speculated that this provided an additional mechanism for
trapping reactive metabolites before these could damage essential
cellular constituents.
In addition to the hepatic enzymatic systems examined in rats
and mice, Mochizuki et al. (1988), as mentioned above, described a
2-NP denitrifying system in adrenal microsomes of uninduced
guinea-pigs. They identified this cytochrome-P-450-dependent
monooxygenase as benzo[ a]pyrene hydroxylase.
6.4 Elimination and excretion
Elimination of 2-NP and its metabolites has been examined mainly
in rats and, to a much lesser extent, in chimpanzees in studies
which utilized measurements of radioactivity from 14C-labelled
2-NP as well as measurements of 2-NP and its metabolites. Dosage by
inhalation, intravenous injection, and intraperitoneal injection all
yielded fairly similar results. During a 48-h period after a 6-h
exposure of rats to 73 mg/m3 (20 ppm) and to 560.6 mg/m3
(154 ppm) of 14C-labelled 2-NP, about 50% of the radioactivity in
the absorbed dose was excreted via the lungs as carbon dioxide
(Nolan et al., 1982). The proportion of the absorbed dose excreted
via the lungs as unchanged 2-NP was 4% at the low dose level and 22%
at the high level. Still less of the labelled carbon was eliminated
via faeces and urine, i.e. 11% and 8%, respectively, at the low dose
level, and 5% and 11%, respectively, at the high level.
Disappearance of 2-NP from the blood after exposure at the high dose
level followed a first-order relationship and yielded a half-life of
48 min. Limited data in Müller et al. (1983) yielded a half-life for
rats of approximately 80 min for 2-NP in plasma following a 3-h
exposure to a concentration of 728 mg/m3 (200 ppm). Nolan et al.
(1982), however, found that disappearance of the 14C label of the
2-NP from the plasma was markedly slower and biphasic. During the
first 12 h following exposure to 560.6 mg/m3 (154 ppm), the
half-life for plasma radioactivity was 172 min and, following
exposure to 73 mg/m3 (20 ppm), 354 min. After 12 h, loss of
radioactivity from plasma was much slower, the half-life being
approximtely 35-36 h for both doses. These data on loss of 2-NP and
its 14C label indicate that 2-NP is rapidly eliminated from the
body mainly by metabolic transformation and to a lesser degree by
pulmonary excretion of the unchanged compound. The major
carbon-containing metabolites of 2-NP, acetone and isopropanol,
presumably enter into the general metabolism of the body and are
eliminated via the intermediary metabolism as part of a much larger
carbon pool.
Pulmonary excretion of 2-NP, like loss of the 14C label from
plasma, is dose dependent, biphasic, and follows first-order
kinetics (Nolan et al., 1982). Fifty times more 2-NP was exhaled
during the first hour following exposure to 560.6 mg/m3 (154 ppm)
than during the first hour following exposure to 73 mg/m3
(20 ppm). Following exposure to 73 mg/m3, 2-NP was excreted for
the first 7 h at a rate which decreased by one half every 64 min,
and subsequently decreased by one half every 16 h, whereas following
exposure to 560.6 mg/m3, the half-times of excretion were 71 min
for the first 12 h, and 16 h for the subsequent period. Changes in
the rates of pulmonary excretion of 14C-labelled carbon dioxide
were similar for 48 h following exposure to 73 and 560.6 mg/m3.
Eighty seven per cent of the total was eliminated during the first
12 h after exposure; loss was somewhat less rapid thereafter. The
dose-dependent nature of pulmonary excretion of 2-NP suggests that
greater concentrations of 2-NP in the blood markedly increase
exhalation of the unchanged compound and reduce the percentage
metabolized. Thus exposure of the tissues to 2-NP and its
metabolites may not be a linear function of the inhaled dose.
Derks et al. (1989) found that the plasma half-life of
intravenous doses of 0.01-0.05 g/kg in rats was 45 min during the
first 4 h. Loss was linear over this dose range and could be
described by an open single-compartment model. They suggested that
the measured loss from plasma may be due in part to spontaneous
conversion of 2-NP to its anionic form, 2-NP nitronate.
Elimination of 2-NP by Sprague-Dawley rats following
intraperitoneal injection of 14C-labelled 2-NP was studied by
Müller et al. (1983) and was generally similar to the elimination of
2-NP following inhalation. The concentration of 2-NP in plasma
declined exponentially with time, with half-lives of 70 and 125 min
during at least the initial 6 h following injections of 25 and
50 mg/kg, respectively. Metabolites of 2-NP, acetone and
isopropanol, reached maxima 2-4 h after injection of 25 mg/kg, and
at least 4-6 h after injection of 50 mg/kg. The concentration of
isopropanol ranged from 1/16 to 1/34 the concentration of acetone.
During the initial 40 h after injection of 50 mg/kg, 4.5% of the
dose was exhaled as 2-NP, 10.4% as acetone, and 38.1% as carbon
dioxide. Losses via urine (5.9%) and faeces (0.7%) were small in
comparison with the 53% loss via exhalation. Müller et al. (1983),
however, reported that only 12% of the dose was recovered from the
carcasses, leaving a large amount (28.4%) unaccounted for and thus
casting some doubt on their values.
With one exception, results similar to the above were obtained
following intravenous injection of 14C-labelled 2-NP (10 mg/kg)
into male chimpanzees (Müller et al., 1983). The concentration of
2-NP in plasma declined exponentially with time, the half-life being
92 min. The concentration of acetone reached a maximum 6 h after
injection and remained high for at least 48 h. The concentration of
isopropanol peaked 3 h after injection when it approached that of
acetone, but otherwise was a third to a quarter that of acetone. The
concentration of 2-NP and its carbon-containing metabolites (i.e.
the concentration of 14C) in plasma declined exponentially with a
half-life of 5.5 h for the initial 10 h, and a half-life of 48 h
thereafter. As in rats, exhalation was the major means of
elimination of 2-NP and its carbon-containing metabolites. During
the first 3 days after injection, only 5-6% of the 14C in the dose
was recovered in urine and only 0.4-0.5% in faeces. Acetone,
isopropanol, and 2-NP were mainly eliminated via renal excretion;
urine collected during 6 to 24 h after injection contained 3.1 mg
acetone/litre, 7.2 mg isopropanol/litre, and 1.8 mg 2-NP/litre.
Isopropanol may thus be maintained at low concentrations in the
plasma both by oxidation to acetone and by rapid excretion. In
addition to acetone, isopropanol, and 2-NP, 14C was excreted as an
unidentified polar metabolite which by 24 h after injection
contained 90% of the radioactivity in the urine. The one striking
difference between results with rats and with chimpanzees, i.e. the
relatively much higher plasma concentrations of isopropanol in
comparison to acetone, might indicate interspecific variation in the
excretion of 2-NP and its metabolites.
Dequidt et al. (1972) provided limited information on the
excretion of nitrite following inhalation and intraperitoneal
injection of 2-NP. Rats weighing approximately 250 g each were given
a daily injection of 0.11 g/kg. Urinary excretion of nitrite was 10
to 35 µg/animal during the first day following the initial
injection, and reached a daily rate of 11 mg/animal by the fourth
injection. The latter rate of excretion represents three quarters of
the nitrogen injected daily as 2-NP, and stands in sharp contrast to
the results of Schneider & Yeary (1975), who reported that little
intravenously injected nitrite was excreted by dogs, sheep or
ponies. Following exposure of rats to a 2-NP concentration of
2766 mg/m3 (760 ppm) for 8 h on each of two successive days, daily
elimination of nitrite was approximately 30 mg/animal. This was
equivalent to about 20% of the 2-NP inhaled daily and possibly as
much as 50% of the absorbed daily dose. The amount of 2-NP inhaled
was estimated from normative data for the rat (Baker et al., 1979),
and absorption was assumed to be 40% of the amount inhaled (Müller
et al., 1983). No nitrite was detected in urine during exposure of
rats to 291 mg/m3 (80 ppm) for 8 h per day on 5 successive days.
6.5 Retention and turnover
There is no evidence that 2-NP is retained for more than a few
hours in the body. It is rapidly lost by exhalation and metabolic
transformation. The known carbon-containing metabolites, acetone and
isopropanol, are excreted rapidly and are also transformed into
compounds which are normal to the body and enter into its general
intermediary metabolism. There is less information on nitrite, the
major metabolite of the nitro moiety. In rats much of the nitrite is
excreted as such in the urine. There is no evidence for excessive
accumulation of 2-NP or its metabolites in any organ or tissue.
Information is also lacking on possible N-nitroso and other toxic
compounds synthesized from nitrite or the nitro moiety.
7. EFFECTS ON LABORATORY MAMMALS AND IN VITRO TEST SYSTEMS
7.1 Single exposure
Data on single exposures to experimental animals, summarized in
Table 5, indicate substantial differences in sensitivity among the
species tested. The route of administration was mainly via
inhalation, but other routes were also used. A quantitative
comparison of the results from these studies is difficult due to a
lack of information on both strain and sex of the animals used, to
differences in the route of administration, and to large variations
in dosage. The LC50 for mortality within 14 days following a 6-h
exposure was estimated to be 1456 mg/m3 (400 ppm) for male rats
and 2621 mg/m3 (720 ppm) for females (Baldwin & Williams, 1977).
Exposure of rats (of unspecified sex) via inhalation to 14 058
mg/m3 (3862 ppm) for 1 h or to 9584 mg/m3 (2633 ppm) for 2.25 h
killed some of the animals within days. Exposure of rats to a much
higher concentration of 2-NP, i.e. 53 508 mg/m3 (14 700 ppm),
killed all animals within 4 h. An inhalation exposure of 4994
mg/m3 (1372 ppm) or less for 2.25 h was not lethal to rats. Unlike
rats, LC50 values were similar, i.e. 2031 and 2038 mg/m3
(558 and 560 ppm), respectively, for male and female mice following
a 6-h exposure (Baldwin & Williams, 1977). Cats appeared more
sensitive to acute exposure to 2-NP than rats. Exposure to
8565 mg/m3 (2353 ppm) for 1 h (or lower concentrations for
proportionately longer) was lethal to some cats. Rabbits and
guinea-pigs, on the other hand, appeared far less sensitive to 2-NP
than rats. Rabbits survived a 2.25-h exposure to 9584 mg/m3
(2633 ppm) and guinea-pigs a 2.25-h exposure to 15 699 mg/m3
(4313 ppm). The sequence in acute sensitivity of animals to inhaled
2-NP (from most to least) was cat, rat and mouse, rabbit, and
guinea-pig. The cat was almost an order of magnitude more sensitive
than the guinea-pig.
There are limited data on lethality via routes of administration
other than inhalation. Large intraperitoneal doses were found to be
promptly lethal to rats; 1.7 and 1.1 g/kg killed animals within 2 h
and 4 h, respectively (Dequidt et al., 1972). The 14-day oral LD50
for mice was 0.40 g/kg. The minimal oral lethal dose for the rabbit,
0.50-0.75 g/kg, was much larger than the estimated minimum lethal
dose via inhalation, 0.24 g/kg. Dermal application of 2 g/kg to
rabbits produced no obvious local or systemic effects.
Table 5. Effects of single exposure to 2-nitropropane in mammalsa
a) Lethality
Species Sex Concentration Estimated total Effects/results Reference
doseb (g/kg)
g/m3 ppm
Oral administration
Rabbitcd lethal dose estimated as 0.50- Machle et al. (1940)
0.75 g/kg
Mouse M/F 14-day LD50: 0.40 g/kg Hite & Skeggs (1979)
Inhalation
Ratc (Wistar) 53.5 14 700 1.51 all animals died within 4 h Dequidt et al (1972)
Ratcd 5.5-14.1 1513-3865 0.10-0.17 death of some animalsc Treon & Dutra (1952)
2.6-8-57d 714-2353 0.06-0.08 no deathsd Treon & Dutra (1952)
Ratcd (Wistar) 2.77 760 0.16 death within 48 h; Dequidtet al. (1972)
Rate (CD) M 2.93 805 0.16 8/10 died within 14 days Baldwin & Williams (1977)f
F 2.19 602 0.13 no deaths within 14 dayse Baldwin & Williams (1977)f
M 2.10 574 0.14 8/8 died within 14 days Baldwin & Williams (1977)f
M 1.68 461 0.10 7/8 died within 14 days Baldwin & Williams (1977)f
Table 5 (contd)
Species Sex Concentration Estimated total Effects/results Reference
doseb (g/kg)
g/m3 ppm
Inhalation (contd)
Rate (CD) M 1.47 405 0.08 5/8 died within 14 days Baldwin & Williams (1977)f
1.34 367 0.08 no deaths within 14 days Baldwin & Williams (1977)f
Mousee (ICR) F 2.70 740 0.47 14/14 died within 14 days Baldwin & Williams (1977)f
2.33 640 0.41 11/14 died within 14 days Baldwin & Williams (1977)f
1.8 495 0.32 2/14 died within 14 days Baldwin & Williams (1977)f
Mouse (ICR) M 2.69 738 0.41 9/14 died within 14 days Baldwin & Williams (1977)f
2.08 558 0.28 7/14 died within 14 days Baldwin & Williams (1977)f
1.65 454 0.23 no deaths within 14 days Baldwin & Williams (1977)f
Catcd 2.6-8.56 714-2353 0.07-0.19 death of some animals Treon & Dutra (1952)
1.19-2.87 328-787 0.02 no deaths Treon & Dutra (1952)
Guinea-pigcd 16.8-35.0 4622-9607 0.53-0.63 death of some animals Treon & Dutra (1952)
8.67-34.7 2381-9523 0.23-0.32 no deaths Treon & Dutra (1952)
Rabbitc 8.67-34.7 2381-9523 0.24-0.27 death of some animals Treon & Dutra (1952)
5.1-14.1 1401-3865 0.10-0.16 no deaths Treon & Dutra (1952)
Table 5 (contd)
Species Sex Concentration Estimated total Effects/results Reference
doseb (g/kg)
g/m3 ppm
Intraperitoneal administration
Ratcf 1.7 death within 2 h Dequidt et al. (1972)
1.1 death within 4 h Dequidt et al. (1972)
Intraperitoneal administration (contd)
Mousec M 0.80 LD50 Friedman et al. (1976)
(Swiss ICR/HQ)
a Values recalculated as necessary to ppm and g/kg
b Estimated dose calculated by the formula: tidal volume x respiration frequency x exposure time x 2-NP conc. x alveolar
retention/animal weight. Tidal volume and respiration frequency from Kaplan et al. (1983) for mice (0.15 ml, 163/min);
from Baker et al. (1979) for rats (0.86 ml, 85.5/min); from Hoar (1976) for guinea-pigs (1.68 ml, 84/min); from Kozma
et al. 81974) for rabbits (15.8 ml, 45/min); from Reece (1984) and Breazile (1971) for cats (42 ml, 31/min); alveolar
retention in rat (0.40) from Nolan et al. (1982), used for all species; where not stated animal weights assumed to be
average values, 20 g for mice, 250 g for rats, 500 g for guinea-pigs, 2.5 kg for rabbits, and 4.0 kg for cats
c Sex not specified
d Exposure time varied from 1 to 7 h
e Exposure time was 6 h
f Baldwin & Williams (1977) also exposed female rats for 6 h to concentrations of 2-NP lower than 2.2 g/m3 (602 ppm); these
concentrations, i.e. 1.15, 1.35 and 1.69 g/m3 (316, 370 and 464 ppm), like 2.2 g/m3, produced no deaths within 14 days
Table 5. Effects of single exposure to 2-nitropropane
b) other effects
Route of Species Sex Estimated Effects/results Reference
administration total doseb
g/kg)
Intraperitoneal rat M 0.15 maximum hepatic injury achieved Filser & Daumann (1988)
(Sprague-Dawley) with this dose
Intraperitoneal rat M 0.05 lipid accumulation, centrilobular Zitting et al. (1981)
(Wistar) necrosis, mitochondrial
abnormalities, and changes in
endoplasmic reticulum and
glutathione content in liver
within 24 h, as well as changes
in enzyme activity in liver and
brain
Dermal rabbit M & F 2 no toxic effects observed Wilbur & Parekh (1982)
b See footnote b in Table 5a.
The effects of acute exposure to 2-NP, in addition to lethality,
are characterized primarily by hepatotoxicity and, at high exposure
levels, methaemoglobin formation and depression of the central
nervous system. Machle et al. (1940), in a description of symptoms
resulting from exposure of laboratory animals to simple
nitroparaffins, listed the following progression for guinea-pigs and
rabbits after a latent period of 20 to 40 min: progressive weakness,
unsteadiness, and incoordination ending in complete ataxia. The rate
of respiration at first slowed and later became increasingly rapid.
Most of these symptoms appear to reflect the narcotic effects
normally associated with inhalation of volatile hydrocarbons, but
the more rapid breathing may reflect an attempt by the body to
compensate for the formation of methaemoglobin and loss of
oxygen-carrying capacity of the red blood cells. Treon & Dutra
(1952) noted similar progression from exposure to high
concentrations of 2-NP vapour, i.e. lethargy and weakness, dyspnoea,
cyanosis, prostration, and ultimately coma and death, but did not
report more rapid breathing following a depression in rate of
respiration. They also noted lacrimation, salivation, and gastric
regurgitation in cats. In addition, the authors observed that, even
with animals which died promptly (within 9.5 h) following a single
exposure, there was a loss in body weight averaging 2.8%. Animals
exposed to 8.56 g/m3 (2353 ppm) or higher concentrations of 2-NP
displayed pathological changes including hepatocellular damage,
pulmonary oedema and haemorrhage, some disintegration of neurones in
the brain, and widespread damage to the endothelium.
Methaemoglobin and Heinz bodies (masses of denatured haemoglobin
within erythrocytes) were found in the blood of animals following
single exposures to high concentrations of 2-NP. In the case of
cats, 60 to 80% of the haemoglobin was converted to methaemoglobin
by exposure to 15.9 to 33.6 g/m3 (4360 to 9230 ppm) for 1 to 2 h,
whereas much longer exposure of rabbits to these concentrations
converted only 4 to 8% of the haemoglobin (Treon & Dutra, 1952).
Dequidt et al. (1972) reported high levels of methaemoglobin in
rats, i.e. 84% and 89%, following 1-h inhalation exposure to
53.5 g/m3 (14 700 ppm) and intraperitoneal injection of 1.7 g/kg,
respectively. Much lower methaemoglobin concentrations, 0.2 to 8.6%,
resulted from a 1-h exposure to 2.8 g/m3 (760 ppm). One day after
exposure to 15.4 g/m3 (4230 ppm) for 4.5 h or 8.5 g/m3
(2335 ppm) for 2.25 h, rabbits had Heinz bodies in 45 to 80% of
their red cells. These bodies disappeared gradually over 9 to 16
days (Treon & Dutra, 1952). Exposure of rabbits to 13.8 g/m3
(3790 ppm) for 1 h or 9.4 g/m3 (2580 ppm) for 2.25 h resulted in
the formation of Heinz bodies in only 0 to 2% of the red cells.
Formation of Heinz bodies in the cat may have reflected its high
sensitivity to 2-NP. A 1-h exposure to 13.8 g/m3 (3790 ppm) and a
20 min exposure to 16.4 g/m3 (4505 ppm) resulted in the appearance
of Heinz bodies in 27% and 16% of the erythrocytes, respectively.
Observations by Zitting et al. (1981) indicated that a single
intraperitoneal injection of 0.05 g 2-NP/kg to rats can produce
significant changes in the fine structure of the liver and in the
physiology of both liver and brain. 2-NP produced a visible
accumulation of lipid in hepatocytes, especially in periportal areas
after 4 h, and the lipid level continued to increase for the next
20 h. Within 4 h after injection there was also degranulation of the
rough endoplasmic reticulum in hepatocytes and proliferation of the
smooth endoplasmic reticulum. Within 24 h the former had almost
disappeared and the latter was vacuolated or compacted. In addition,
some hepatocytes had abnormal mitochondria and there was necrosis of
hepatocytes around the central vein. The latter was reflected in a
concurrent fourfold increase of serum alanine aminotransferase.
Other enzymatic parameters in the liver were also markedly affected
within 24 h. Cytochrome P-450 was markedly depressed,
7-ethoxycoumarin O-deethylase and 7-ethoxyresorufin O-deethylase
were diminished in activity, and microsomal epoxide hydratase,
UDP-glucuronosyltransferase and glutathione peroxidase were
increased in activity. In addition, the liver concentration of
glutathione nearly doubled.
The major observed neurochemical effect was a significant
increase in acetylcholine esterase activity in the cerebrum and in
isolated synaptosomes. There was little or no change in RNA,
2',3'-cyclic nucleotide 3'-phosphohydrolase, or acid proteinase in
the brain.
Zitting et al. (1981) noted that these histopathological and
enzymatic changes induced by 2-NP are nearly identical to the
effects of carbon tetrachloride on the rat and are thus indicative
of lipid peroxidation.
Hepatotoxicity following exposure to 2-NP has also been
observed in mice (Dayal et al., 1989). Intraperitoneal doses of
0.8 g/kg (9 mmol/kg) in male mice and 0.6 g/kg (6.7 mmol/kg) in
female mice significantly increased plasma activities of enzymes
indicative of hepatic damage (sorbitol dehydrogenase, alanine
aminotransferase, and aspartate aminotransferase) 48, 72, and 96 h
after injection. These enzyme activities were not elevated 24 h
after this dosage nor after small doses of 2-NP.
7.2 Short-term and long-term repeated exposure
Data for repeated exposure, like that for single exposure,
indicate that the cat is more sensitive to 2-NP than the other
species tested (Table 6). Rats, rabbits, guinea-pigs, and monkeys
survived 1.2 g/m3 (328 ppm) (7 h/day, 5 days/week) throughout
approximately 6 months of exposure (Treon & Dutra, 1952). However,
cats exposed to the same concentrations of 2-NP began dying after
the third day of exposure and were all dead by the end of the 17th
day (Treon & Dutra, 1952). Rats and guinea-pigs survived 5 days of
Table 6. Short-term and long-term toxicity of 2-nitropropane in mammalsa
Species Sex Dose and/or concentrationb Effects/results Reference
Oral studies
Rat (Wistar) M & F 0.25 g/kg, 5/week for 4 mortality of males (4/10); decreased growth in Wester et al. (1989)
weeks first week; increased urine, ALAT, ASAT, and
gamma-GT (males only), anaemia, thrombocyte
and leucocyte count, liver, spleen and heart
weight, and haemosiderin content of spleen;
cellular and nuclear polymorphism, single cell
necrosis, and proliferation of oval cells
and/or bile ducts in liver
Rat M 0.089 g/kg (1 mmol), some deaths by 16 week; throughout study body Fiala et al. (1987b)
(Sprague-Dawley) 3/week for 16 weeks, weights significantly lower than controls; all
maintained but not dosed rats exposed 16 weeks or longer developed
for next 61 weeks massive hepatocellular carcinomas; metastases
to the lungs in 4 animals
Rat M & F 0.05 g/kg, 5/week for 4 increased anaemia, thrombocyte conc., and Wester et al (1989)
(Wistar) weeks heart weight
Rat (Wistar) M & F 0.002 and 0.01 g/kg, 5 increased water intake by males dosed with Wester et al. (1989)
week for 4 weeks 0.002 g/kg
Inhalation studies
Ratd 2.77 g/m3 (760 ppm), 8 animals dead within 2 h after end of second Dequidt et al. (1972)
(Wistar) h/day for 2 days; inhalation session; 2-NP conc. in liver =
estimated dose over 8 h, 180 ppm; methaemoglobin = 2.4%
0.16 g/kg
Table 6 (contd)
Species Sex Dose and/or concentrationb Effects/results Reference
Inhalation studies (contd)
Rate 2.45 g/m3 (672 ppm), 7 no deaths Treon & Dutra (1952)
h/day for 5 days;
estimated dose over
7 h, 0.12 g/kg
1.20 g/m3 (328 ppm), 7 no deaths Treon & Dutra (1952)
h/day, 5 days/week for
130 days over 199 days;
estimated dose over 7 h,
0.06 g/kg
Rat M 0.75 g/m3 (207 ppm), 7 body weight and haematological parameters Lewis et al. (1979)
(Sprague-Dawley) h/day, 5 days/week for up unaffected; some pulmonary oedema and some
to 24 weeks; estimated pulmonary lesions within 3 months; liver
dose over 7 h, 0.04 g/kg weight elevated; hepatocellular hypertrophy,
hyperplasia and liver necrosis in all rats
within 3 months; liver neoplasms in all rats
within 6 months; these hepatocellular
carcinomas appeared to be growing rapidly and
deforming surrounding tissues
Rate 0.73 g/m3 (200 ppm), 7 growth slightly reduced and SGPT elevated in Griffin et al. (1978)
h/day, 5 days/week for 6 male rats; liver weight increased in both sexes;
months; estimated dose morphological changes in liver more pronounced
over 7 h, 0.03 g/kg in males; these included fatty metamorphosis
and hepatic nodules consisting mainly of
hyperplastic areas with distortion of lobular
architecture, necrosis and peripheral
compression
Table 6 (contd)
Species Sex Dose and/or concentrationb Effects/results Reference
Inhalation studies (contd)
Ratd 0.73 g/m3 (200 ppm), 7 no effect on body or organ weight to end of Griffin et al. (1986)
(Sprague-Dawley) h/day for 5 days experiment (94 week) aside from a brief
decrease in weight gain immediately following
exposure; no significant effects on mortality
or pathology
Ratc M 0.73 g/m3 (200 ppm), 7 severe liver damage with vacolar degeneration Coulston (1982)
h/day, 5 days/week for in exposed rats after 3 months up
to 7 months
Rate 0.36 g/m3 (100 ppm), 7 male rats had lower body weight, increased Griffin & Coulston (1983);
h/day, 5 days/week for up renal calcification, elevated SGPT, and Coulston et al. (1985)
to 18 months; estimated enlarged livers with necrosis, vacuolar
dose over 7 h, 0.013 g/kg degeneration and probable hepatocellular
carcinomas; female rats had increased renal
calcification and occasional hepatic masses
and nodules showing hyperplasia and vacuolar
degeneration
Rate 0.29 g/m3 (80 ppm), 8 no deaths; no trace of 2-NP in organs at end of Dequidt et al. (1972)
h/day for 5 days; estimated experiment; methaemoglobin = 0; nitrite = 0-10
dose over 8 h, 0.016 g/kg ppm in tissues, but no nitrite in urine
Rat M 0.1 g/m3 (27 ppm), 7 no gross or microscopic alteration of any Lewis et al. (1979)
(Sprague-Dawley) h/day, 5 days/week for up tissue, haematological parameter or serum
to 24 weeks; estimated biochemistry
dose over 7 h, 0.005 g/kg
Table 6 (contd)
Species Sex Dose and/or concentrationb Effects/results Reference
Inhalation studies (contd)
Rate 91 mg/m3 (25 ppm), 7 no changes in behaviour, appearance, rate of Griffin et al.
h/day, 5 days/week for up weight gain, final weight, serum chemistry or (1980, 1981)
to 22 months; estimated haematology; no significant increase in tumours
dose over 7 h, 0.003 g/kg and lesions associated with exposure; no
evidence of methaemoglobinaemia
Moused 0.73 g/m3 (200 ppm), 7 depression in body weight during first 3 months Griffin et al. (1984)
(ICR) h/day, 5 days/week for in females and throughout experiment in males;
48 weeks increased liver weight and elevation of liver
transaminases in females; toxic hyperplasia of
liver predominantly in females
Mousee 0.36 g/m3 (100 ppm), 7 slight depression of body weight during first 8 Coulston et al. (1986);
h/day, 5 days/week for months in males; no effects on organ weight; no Griffin et al. (1987)
18 months evidence of hepatocellular carcinoma; some
indications of liver toxicity (nodular
hyperplasia in females)
Cate 2.6 g/m3 (714 ppm), 4.5 deaths starting with first exposure, but Treon & Dutra (1952)
h/day for 4 days; some animals survived 4 exposures
estimated dose over 7 h,
0.10 g/kg
1.2 g/m3 (328 ppm), 7 deaths starting with third exposure; all Treon & Dutra (1952)
h/day, 5 days/week for 17 animals dead by end of 17th exposure
exposures; estimated dose
over 7 h, 0.07 g/kg
Table 6 (contd)
Species Sex Dose and/or concentrationb Effects/results Reference
Inhalation studies (contd)
Cate 1.15 g/m3 (317 ppm), 7 no deaths Treon & Dutra (1952)
h/day for 2 days;
estimated dose over 7 h,
0.07 g/kg
0.3 g/m3 (83 ppm), 7 no deaths Treon & Dutra (1952)
h/day, for 130 out of 191
days; estimated dose over
7 h, 0.02 g/kg
Rabbite 1.2 g/m3 (328 ppm), 7 no deaths Treon & Dutra (1952)
h/day, ca. 5 days/wk for
up to 130 out of 199 days;
estimated dose over 7 h,
0.06 g/kg
Rabbit M 0.75 g/m3 (207 ppm), 7 no gross or microscopic alterations to Lewis et al. (1979)
(white, NZ) h/day, 5 days/week for 24 tissues
weeks; estimated dose
over 7 h, 0.03 g/kg
Rabbite 0.3 g/m3 (83 ppm), 7 h/day no deaths Treon & Dutra (1952)
for 130 out of 191 days;
estimated dose over 7 h,
0.014 g/kg
Rabbite 0.1 mg/m3 (27 ppm), 7 no gross or microscopic alterations to Lewis et al (1979)
h/day, 5 days/week for 6 tissues
months; estimated dose
over 7 h, 0.005 g/kg
Table 6 (contd)
Species Sex Dose and/or concentrationb Effects/results Reference
Inhalation studies (contd)
Guinea-pige 2.45 g/m3 (672 ppm), 7 no deaths Treon & Dutra (1952)
h/day for 5 days;
estimated dose over
7 h, 0.12 g/kg
Guinea-pige 1.2 g/m3 (328 ppm), 7 no deaths Treon & Dutra (1952)
h/day, ca. 5 days/week
for 95-130 days out of
up to 199 days; estimated
dose over 7 h, 0.06 g/kg
Monkeye 1.2 g/m3 (328 ppm), 7 no deaths Treon & Dutra (1952)
h/day, ca. 5 days/week
for 100 days exposure
0.3 g/m3 (83 ppm), no deaths Treon & Dutra (1952)
7 h/day for 130 out of
191 days
Intraperitoneal studies
Ratd 0.11 g/kg, 1/day for apparently no deaths prior to sacrifice Dequidt et al. (1972)
(Wistar) 7 days 3 days after the last injection;
methaemoglobin = 4.3%; nitrite = 0.29-1.15 ppm
in organs (heart, lungs, kidneys, spleen)
Ratd 0.11 g/kg, 1/day for 15 apparently no deaths prior to sacrifice 36 h
(Wistar) days after the last injection; methaemoglobin = 0;
nitrite = 0-10.8 ppm in organs (heart, lungs,
kidneys, spleen)
Table 6 (contd)
Species Sex Dose and/or concentrationb Effects/results Reference
Intraperitoneal studies
Rat F 0.001 g/kg, 5/week for no significant effects on kidney function Bernard et al. (1989)
(Sprague-Dawley) 2 weeks
Dermal study
Rabbite 1 application/day on no skin irritation, illness, systemic effects Machle et al. (1940)
clipped anterior abdomen or deaths
for 5 days; dose not
stated
a values from the literature recalculated as necessary to ppm or g/kg
b estimated dose calculated by the formula: tidal volume x respiration frequency x 2-NP conc. x alveolar retention/animal weight.
Tidal volume and respiration frequency from Baker et al. (1979) for rats (0.086 ml, 85.5 /min); from Hoar (1976) for guinea-pigs
(1.68 ml, 84/min); from Kozma et al. (1974) for rabbits (15.8 ml, 45/min); from Reece (1984) and Breazile (1971) for cats
(42 ml, 31/min); alveolar retention in rat (0.40) from Nolan et al. (1982), used for all species; where not stated in reference,
animal weights assumed to be average values, 250 g for rats, 500 g for guinea-pigs, 2.5 kg for rabbits, and 4.0 kg for cats
c strain not specified
d sex not specified
e neither strain nor sex specified
exposure (7 h/day) to 2.46 g/m3 (672 ppm), but death occurred in
rats exposed for 2 days (8 h/day) to 2.77 g/m3 (760 ppm). Rats
were reported to survive 15 days of daily intra-peritoneal
injections of 0.11 g/kg (Dequidt et al., 1972). The doses used in
these repeated exposures were found to produce no more than trace
levels of methaemoglobin (maximum = 4.3%) and low concentrations
(0 to 11 mg/kg) of nitrite in the tissues.
Non-lethal chronic doses of 2-NP have been shown to produce a
number of harmful effects in rats (Table 6). Exposure of rats to
0.75 g/kg (207 ppm) for up to 24 weeks (7 h/day, 5 days/week)
initially induced pulmonary lesions and oedema, hepatocellular
hypertrophy, hyperplasia, and necrosis of the liver (Lewis et al.,
1979). By the end of 24 weeks all the rats developed rapidly growing
hepatocellular carcinomas. A similar exposure to a slightly lower
concentration of 2-NP, 0.73 g/m3 (200 ppm), induced hepatic
nodules and other destru