For information about other occupational safety and health problems, call I -8OO-3 5 -NIOSH DHHS (NIOSH) Publication No. 93-102 PREFACE A memorandum of understanding has been signed by two government agencies in the# A B B R E V I A T I O N S

# ACGIH

# I N T R O D U C T I O N
Chlorobenzene is one of twelve possible chemical species in the group of chlorinated benzenes (36). At room temperature, the substance is a colorless volatile liquid with an odor that has been described as "not unpleasant" (63), like that of "mothballs or benzene" (25) and "almondlike" (1, 32). The compound has been used extensively in industry for several years and its main use is as a solvent and intermediate in the production of other chemicals (19, 25, 32). In occupational settings, the main exposure is from inhalation of the volatile compound.
The present document summarizes and evaluates information that has been considered most relevant for the assessment of the potential adverse health effects from occupational exposure to chlorobenzene. To achieve this objective, a literature search was performed in different biomedical and toxicological databases (e.g., in Medline; Cancerlit; Toxline; Excerpta Medica; National Technical Information Service; Healthline and Chemical Safety Newsbase) before the assessment was initiated (July, 1991).
The U.S. Environmental Protection Agency (EPA) has recently prepared a health effects criteria document (final draft) for chlorobenzene (32) as well as an updated health effects assessment (33). A similar document has also been prepared in Germany (19). These, and other reviews (e.g., 2, 6, 25, 68, 92), have been included among the references in the present document.
The various health criteria documents mentioned above have included information from several unpublished toxicity studies, mainly performed by, or on behalf of, various manufac turers of chlorobenzene. Some unpublished investigations are also cited in the present document, although the primary sources of information often were unavailable for critical examination. In such a case, this has been indicated with a short remark, provided with the citation.

# PHYSICAL AND CHEMICAL PROPERTIES
If not stated otherwise, the data on the physical and chemical properties of chlorobenzene were obtained from various reference books and review articles (1, 6, 7, 15, 19, 25, 26, 32, 63).
Although not always declared, it is assumed that the figures given refer to chlorobenzene of analytical quality. Only scarce information on amounts and identities of potential impurities was available. In a teratogenicity study on rats and rabbits (48) using >99.9% pure chloroben zene, it was stated that incidental impurities found consisted of benzene (<0.005 %), bromobenzene (0.018%) and water (0.0077%). Chlorobenzene of technical quality from one of the German manufacturers is at least 99.8% pure, containing at most 0.06% dichlorobenzenes and 0.08% benzene, as the major impurities (19). -55C
Vapor The information on human thresholds for the detection of chlorobenzene is not uniform. In air, the recognition odor has been reported to vary between 0.21 ppm (7) and 0.68 ppm (3,92) (i.e., between 1 and 3.1 mg/m3). However, in another source of information (91) it is stated that the almondlike odor of chlorobenzene is barely perceptible at 60 ppm (276 mg/m3). The air-dilution threshold given by Amoore and Hautala (3), 0.68 ppm, represents the geometric average of all available literature data, omitting extreme points and duplicate quotations.
The substance is practically insoluble in water, but the two liquids form an azeotrope that boils at 90C (32). On surface water, chlorobenzene is believed to evaporate rapidly to the air. However, due to its greater density, chlorobenzene may also sink to the bottom of still volumes of water (32, 39). The reported taste/odor thresholds in water varies between 0.45-1.5 |jg/l (this interval is based on the work by Tarkhova from 1965, cited in references 6 and 32) and 10-20 pg/1 (based on the work by Varshavskaya from 1967, cited in references 6 and 32), respectively. However, these figures were considered difficult to interpret since none of the citations described the experimental conditions employed (32). In another, more recent work, the odor threshold for chlorobenzene in water was reported to be 50 jig/1 (3). This so-called water-dilution odor threshold value was calculated from the concentration of the substance in water that would generate the air odor threshold (estimated to 0.68 ppm) in the headspace of a stoppered flask.

# USES AND OCCURRENCE
The figures given below for production volumes, ambient air levels, etc., have mainly been obtained from secondary sources of information (6,19,32,36,49). Consequently, the original sources, often unpublished information, have generally not been evaluated in detail.

# Production and U ses
Like other chlorinated benzenes, monochiorobenzene is commercially produced by the chlorination of benzene at an elevated temperature. This is done in the presence of a chlorination catalyst such as ferric chloride (36,39,63). Chlorobenzene may also be produced by treating phenol with aqueous sodium hydroxide under high pressure and in the presence of chloride (39).
Chlorobenzene is one of the most widely used chlorinated benzenes and it has been the dominant commercial isomer for at least 50 years. The compound has been utilized in numerous processes. Previously, its main uses were as a chemical intermediate in the synthesis of DDT and other organochlorine pesticides, and in the production of phenol and aniline (36, 92). During the first world war, it was also used in large quantities in the production of picric acid, which was utilized as an explosive (92). Its principal use today is as a chemical intermediate in the production of chemicals such as nitrochlorobenzenes and diphenyl oxide (19, 32, 36). In 1989, 76% of the total amount of chlorobenzene manufactured in the Federal Republic of Germany was processed into nitrochlorobenzenes (19). These compounds are subsequently used as starting products for crop protection agents, dyestuffs and rubber chemicals (19). Chlorobenzene is also used as a solvent in degreasing processes (e.g., in metal cleaning operations) and in the dry cleaning industry. It serves as a solvent for paints, adhesives, waxes and polishes and has also been used as a heat transfer medium (6, 63, 92) and in the manufacture of resins, dyes, perfumes and pesticides (92).
Although the annual production rates for chlorobenzene in the United States , show a decreasing trend, it has been estimated that the consumption of chlorobenzene would grow at an average annual rate of 1-2% in the United States (37). Large manufacturers of chlorobenzene in the United States are Monsanto Co., PPG Industries, and Standard Chlorine Chemical Co. (19). In the late 1970s, approximately 500 tons of chlorobenzene was imported into the United States (32).
In 1989, a total of 60,000 to 70,000 tons of chlorobenzene was produced in the Federal Republic of Germany by two different manufacturers, Bayer AG and Hoechst AG (19). In 1985, the total production in Western Europe was estimated to be 82,000 tons (19). In Eastern Europe, the total production of chlorobenzene in 1988 was calculated to be 200,000-250,000 tons (19). The production volume in Japan was 28,300 tons in 1988 (19).
According to the Products Register at the National Chemicals Inspectorate , monochlorobenzene occurred in ten different chemical products in Sweden in 1990. The estimated annual use of the compound that year in Sweden was 11 to 64 tons.
There is no production of chlorobenzene in Sweden.

# Occupational Exposure and Ambient Air Levels
The amount of data available with regard to the potential exposure to chlorobenzene in various types of occupational settings is limited. In Sweden, for example, the National Board of Occupational Health and Safety (an authority responsible for protecting workers who handle chemicals in the workplace from ill-health and accidents) had no information available on the present exposure levels of monochlorobenzene at Swedish workplaces.
A monitoring program of air levels in chlorobenzene-and nitrochlorobenzene-producing plants in the United States, which was performed 1978/79, showed that the chlorobenzene concentrations varied from not detectable to 18.7 mg/m3 (19). There are no natural sources for chlorobenzene and most releases result from its use as a solvent (6). The substance is delivered into the environment mainly with exhaust air and waste water from production plants, processing industries and from its use as a solvent. In the atmosphere, chlorobenzene is anticipated to degrade slowly by free radical oxidation. Due to its high volatility, chlorobenzene is expected to evaporate rapidly into air when released to surface water, but when released to the ground it has been assumed to first bind to the soil and then migrate slowly to the ground water (6). In January 1987, there was an accidental release of approximately 450 tons of monochlorobenzene to the Baltic Sea outside Kotka, Finland (39). Because the sea was calm and covered with ice, it was believed that most of the chlorobenzene sank to the bottom of the sea. The environmental consequences of this release are not known.
Chlorobenzene is resistant to biodegradation as well as to chemical and physical degeneration (6, 32). In accordance with its relatively high lipid solubility, it has been shown to bioaccumulate in, for example, fish and algae (6). In 1978 it was estimated that almost a total of 80,000 tons of chlorobenzene were released to the atmosphere each year in the United States (32). Apart from an occupational exposure, humans may be exposed to chlorobenzene from drinking water, food, ambient air and consumers' products.
Based on various national surveys, the U.S. EPA has estimated the concentrations of chlorobenzene to be less than 1-5 pg/1 in groundwater, and less than 1 jug/1 in surface water (32). The magnitude of the potential dietary intake of chlorobenzene was not estimated since the available data were considered insufficient (32). The median concentration of chloroben zene in ambient air of urban and suburban areas has been calculated at 1.5 (ig/m3 (32). Various measurements performed in Germany and The Netherlands showed that the average outdoor air levels of chlorobenzene varied between 0.3 and 1.5 fig/m3 (19).
Like other volatile halogenated hydrocarbons, chlorobenzene may very well be present in the indoor air of, for example, household settings in amounts exceeding those of the ambient air.
When the indoor air concentrations of chlorobenzene were measured in the Bavarian city, Hof, Germany, these were found to vary between 0.1 and 4 |4g/m3, with a geometric mean of 0.5 |Jg/m3 (19). Somewhat higher indoor air concentrations were found in various cities in the USA, ranging between not detectable and 72.2 fig/ m3, with an average of 16.5 (Jg/m3 (19).
Chlorobenzene may be formed during the biotransformation of other compounds. It has, for example, been shown that chlorobenzene is a major metabolite of hexachlorocyclohexane, better known as the insecticide Lindane, at least when Lindane is incubated with rat liver microsomes under anaerobic conditions (14).
To summarize, although chlorobenzene may be present in ambient air, the levels are generally considerably lower than those that can be found in industries manufacturing or processing chlorobenzene.

# Analytical Methods for Air Monitoring
NIOSH manual of analytical methods (69) describes a standardized method for sampling and analysis for chlorobenzene in ambient air. The method was revised in 1987 (69b). First a known volume of air is drawn through a charcoal tube to trap the organic vapors present. Then the charcoal in the tube is transferred to a stoppered sample container where the amount of chlorobenzene adsorbed to the charcoal is eluated with carbon disulfide. An aliquot of the desorbed sample is then injected into a gas chromatograph with a flame ionization detector (GC-FID). The amount of chlorobenzene present in the sample is determined by measuring and comparing the areas under the resulting peaks from the sample and those obtained from the injection of standards.
Sampling can be done either actively with adsorption tubes or passively through personal air sampling using passive diffusion techniques. It seems as if most investigators have preferred personal air sampling using a passive organic solvent sampler in the breathing zone, when they measured the occupational exposure to chlorobenzene (56, 71, 96). Using personal air sampling and GC analysis, the detection limit has been reported to be 0.05 ppm (0.23 mg/m3) for an exposure time of 8 hr in an industrial setting (56). Alternatives to the GC-FID technique have also been used for the analysis of ambient air levels of chlorobenzene; for example, high pressure liquid chromatography (96). To confirm the identity of the compound, GC can be combined with mass spectrometry (71).
A similar technique is used when the amount of chlorobenzene is determined in water samples. The procedure used is the so-called purge-and-trap gas chromatographic procedure, a standard method for the determination of volatile organohalides in drinking water (6). An inert gas is bubbled through the sample so that chlorobenzene is trapped on an adsorbent material. The adsorbent is then heated to drive chlorobenzene onto a GC column.

# Present Occupational Standards
In 1989, ACGIH adopted a TLV-TWA of 10 ppm (46 mg/m3) for the occupational exposure to chlorobenzene in the United States (2).

# Distribution
Various experimental studies have shown that, after being absorbed, chlorobenzene is dis tributed rapidly to various organs.

# Animals:
The toxicokinetics of inhaled chlorobenzene has been studied by Sullivan and coworkers and reported in two different papers (86,87). The study has also been briefly reviewed in a short notice (4). Male Sprague-Dawley rats were exposed to 14C-chlorobenzene (uniformly labelled) at 100, 400 or 700 ppm (460, 1,840 or 3,220 mg/m3) for 8 hr/day; either one day only or for five consecutive days. Each group consisted of six animals. Immediately after the last exposure, three rats from each group were sacrificed for determination of chlorobenzene-associated radioactivity in liver, kidneys, lungs, adipose tissue and blood. The remaining rats were kept in metabolism cages for 48 hr before they were sacrificed. The vapor concentrations of chlorobenzene were monitored with an infrared gas analyzer at 9.25 pm.
Adipose tissue was found to accumulate the largest amounts of radioactivity. The percentage of chlorobenzene-associated radioactivity in fat, presumably representing unchanged sub stance, was found to increase at higher exposure levels. In the other tissues investigated, the 14C-levels w'ere increased in proportion to the exposure concentration; liver and kidneys being the dominant organs. Lung and blood contained 25-50% and 10-30%, respectively, of the amounts found in the liver. When the exposure concentration was increased from 100 to 400 ppm (from 460 to 1,840 mg/m3), there was an increase of over ten-fold of the exhaled amount of radioactivity, presumably representing unchanged substance. A further increase to 700 ppm (3,220 mg/m3) caused another seven-fold increase of the exhaled amounts. The data showed that the metabolic clearance from the blood became saturated at an exposure concentration of 400 ppm for 8 hr. At this exposure, there was also a reduced predominance of the excreted amount of mercapturic acid (the only urinary metabolite investigated) in relation to the total amount of radioactivity excreted in the urine. Consequently, the observed dose-related changes in various pharmacokinetic parameters in rats suggests that the metabolic elimination of chlorobenzene becomes saturated at high dose levels.
Maximum liver concentrations of chlorobenzene-associated radioactivity in male Sprague-Dawley rats given 14C-chlorobenzene as a single i.p. injection was seen 24 hr after the administration ( 22). The radioactivity represented both the parent compound and its metabo lites.
The distribution and fate of nonvolatile radioactivity from uniformly labelled 14C-chlorobenzene has also been studied in female C57BL mice, using whole-body autoradiography (16). Six mice were given a single i.v. injection of the labelled compound diluted with unlabelled substance (1.2 mg/kg b.wt.; 7 (jCi in DMSO). The survival times were 1 and 5 min; 1, 4, and 24 hr; and 4 days, respectively. Two other mice were injected i.p. and killed after 4 and 24 hr, respectively. Whole-body autoradiograms from heated tissue sections showed a selective localization of nonvolatile metabolites in the mucosa of the entire respiratory system 1 min after an i.v. injection. The labelling of the mucosa of the respiratory tract was persistent and still present 4 days after the injection. Microautoradiography showed that the chlorobenzeneassociated radioactivity was bound to the epithelium of the tracheo-bronchial mucosa. Uptake of nonvolatile radioactivity was also observed in other tissues 1 and 5 min after the i.v. injection, although not to the same extent as in the respiratory tract. Relatively high amounts of nonvolatile metabolites of chlorobenzene were also observed in the liver, the cortex of the kidney, the mucosa of the tongue, cheeks and esophagus and in the inner zone of the adrenal cortex.
Humans: Due to its high lipid solubility, chlorobenzene can be anticipated to accumulate in human fat, and possibly in milk. However, none of the recognized studies on chlorobenzene levels in human fat and breast milk samples from the general population included monochlorobenzene among the various isomers measured (23,47,64). The chlorobenzenes analyzed in the monitoring programs generally included various isomers of dichlorinated and trichlorinated benzenes and pentachlorobenzene and hexachlorobenzene.

# Biotransformation
Like other monosubstituted halogenated benzenes, chlorobenzene is oxidized by the micro somal cytochrome P-450 system to reactive epoxide intermediates (also known as arene oxides). These have not actually been isolated and identified, but their presence has been deduced from the various metabolic end-products of chlorobenzene that have been isolated and identified, both in vitro and in vivo. Covalent binding of chlorobenzene-related epoxides to various tissue constituents has provided a convenient explanation for the cytotoxic effects observed in various organs after the administration of the otherwise unreactive chlorobenzene (for further discussion see p. 18). The epoxides are converted either nonenzymatically to various chlorophenols or enzymatically to the corresponding glutathione (GSH) conjugates and dihydrodiol derivatives. The GSH conjugates are either eliminated as such, or transformed to even more water-soluble products and excreted in the urine as mercapturic acids. The dihydrodiol derivatives are converted to catechols and excreted as such in the urine. In a study where the in vitro hepatic microsomal formation of halophenols from chlorobenzene and bromobenzene was investigated in both human and mouse liver microsomes (50), important differences were observed between the metabolic pathways suggesting that humans may be more susceptible than mice to halobenzene-induced hepatotoxicity. Mouse liver microsomes were prepared from untreated male B6C3F1 mice (livers from 35 mice were pooled). Human liver microsomes were made from transplants obtained from three different donors who suffered acute head injuries in accidents. Mixtures containing microsomal proteins and various co-factors were incubated with either chlorobenzene or bromobenzene. The formation of halophenols was studied using a selective HPLC method with electrochemical detection (HPLC/ECD-technique).
The metabolism of chlorobenzene to ortho-and p-chlorophenol followed the same pattern as that of bromobenzene, both in human and mouse liver microsomes, indicating that both compounds were metabolized by the same cytochrome P450/P448-isozymes. Microsomes from the mouse liver contained approximately five times more cytochrome P450 than those taken from the livers of the three donors, but the production of p-halophenols was only two times greater in the mouse liver enzymes. When the production of p-halophenols (i.e., the metabolic pathway that has been associated with the hepatotoxicity of chlorobenzene and bromobenzene) was expressed relative to the cytochrome P450 content (i.e., nmol of halophenol produced/min/nmol of cytochrome P450), the human liver microsomes were twice as efficient as the mouse liver microsomes. Moreover, in comparison to the mouse liver microsomes, human cytochrome P450-isozymes produced less of the nonhepatotoxic o-halophenols. Whereas the ratio of para-to ortho-halophenol production was 1.3 for bromobenzene and 1.4 for chlorobenzene in the mouse microsomes, the average ratio was 4.8 for both compounds in the human microsomes. The human liver microsomes also had a slightly greater affinity for chlorobenzene and bromobenzene than the mouse microsomes. Taken together, these in vitro results indicate that the main metabolic pathway of chlorobenzene in human liver microsomes is through the hepatotoxic 3,4-epoxide pathway.
Studies on the metabolism of chlorobenzene have mainly been restricted to the liver. However, experiments performed in vitro with tissue slices prepared from pieces of nasal mucosa, lung, and liver taken from female C57BL mice, showed that chlorobenzene can also be transformed to nonextractable metabolites in extrahepatic organs (16). In these experiments, tissue slices were incubated with 14C-labelled chlorobenzene (5 |iM; 0.3 juCi) in a phosphate buffer containing glucose, and in the presence of oxygen, for 15, 30 or 60 min. Incubation mixtures with tissue slices heated for 10 min were used as controls. All three organs investigated were found to produce metabolites that could not be removed by extensive organic solvent treatment; nasal mucosa being the most efficient tissue. After 60 min of incubation, the nasal mucosa had produced approximately 0.8, the lung 0.4 and the liver 0.2 pmoles 14C-metaboIites/mg wet weight tissue. In a second series of experiments, the effect of various mixed function oxidase inhibitors on the extrahepatic metabolism of chlorobenzene, was investigated using tissue slices from nasal mucosa and lung (16). The formation of nonextractable metabolites in vitro was decreased by metyrapone, piperonylbutoxide and SKF-525A, clearly showing that the metabolism of chlorobenzene is also cytochrome P450-dependent in these organs.
The major metabolites o f chlorobenzene in man appear to be p-chlorophenol and 4-chlorocatechol (2). These are eliminated in the urine as sulphate and glucuronide conjugates in the urine. Apparently, the metabolic pathways in man differ somewhat from those in rabbits and other experimental animals. Para-chlorophenol is, for example, only a minor urinary metabolite of rabbits ( 12), and a major excretion product in rabbit urine, p-chlorophenyl mercapturic acid, is excreted only in min amounts in human urine.
The proposed metabolic pathways for chlorobenzene are shown in Figure 1

# Elimination
There are three potential routes of elimination for inhaled or ingested chlorobenzene, via the expired air, via the urine, and via feces. Although the eliminated amount of unchanged chlorobenzene in the expired air may be as high as 60% depending on the exposure conditions and species involved, urinary excretion of various chlorobenzene-associated metabolites is no doubt the dominant route of elimination for chlorobenzene. Excretion of unchanged substance via urine or feces is consequently unimportant. At the dose levels humans normally are exposed to, most of the chlorobenzene absorbed is believed to be metabolized and then excreted in the urine, predominantly as free and conjugated forms of 4-chlorocatechol and chlorophenols.
Animals: Experiments on three Chinchilla doe rabbits given a single oral dose of 500 mg chlorobenzene/kg b.wt., showed that the eliminated amount of unchanged chlorobenzene in the expired air was as high as 24-32% during the first 30 hr following the administration of the compound (11). Another experiment on Chinchilla rabbits given a single oral dose of 150 mg chlorobenzene/kg b.wt. (84), showed that 72 percent of the given dose was eliminated as various conjugates in the urine within two days after the administration.
Although the route of administration seems of minor importance for the elimination pattern of chlorobenzene, dose levels and dosing schedule may have some influence.
In the previously mentioned pharmacokinetic study of inhaled 14C-chlorobenzene (86,87), it was shown that multiple exposures of rats at doses saturating the metabolic pathways, versus a single exposure at a dose not saturating the biotransformation of chlorobenzene, resulted in  higher tissue levels of radioactivity (notably in adipose tissue), a lowered total excretion of chlorobenzene-associated radioactivity, a lesser percentage of the total amount excreted through respiration and a change in the rate of respiratory excretion. Consequently, rats exposed to 100 ppm (460 mg/m3) for 8 hr, excreted only 5% of the total dose via exhalation and 95 % in the urine. Repeated exposure to 700 ppm (3,220 mg/m3), 8 hr/day for 4-5 days, resulted in exhalation of 32% of the total dose, the urinary excretion being 68%.
In a study of the liver toxicity of chlorobenzene in male Sprague-Dawley rats given the compound as a single i.p. injection ( 22), it was found that the fraction of the total dose excreted in the urine within 24 hr decreased as the dosage of chlorobenzene increased. At the lowest dose tested, 2.0 mmol/kg b.wt. (225 mg/kg), 59% of the total dose was excreted in the urine, but at the highest dose, 14.7 mmol/kg (1,655 mg/kg), the corresponding figure was only 19% (all of the excreted products represented metabolites).
In an investigation on the potential differences between various species with regard to the elimination pattern of chlorobenzene, rats, mice, and rabbits were given an i. There was a dose-related increase of the excreted amounts of both metabolites in the urine from the rats, mice, and rabbits. However, whereas the mercapturic acid derivative was the dominant excretion product in the urine of the animals, it was only a fraction of the amount of 4-chlorocatechol collected in the urine from the chlorobenzene exposed humans.
In another study (55), chlorobenzene was diluted in com oil and given to ten male Wistar rats as a single i.p. injection (500 mg/kg b.wt.). Four rats were pretreated with 80 mg phenobarbital/kg b.wt., 54 hr before the chlorobenzene injection. Twenty-four-hr urinary samples were collected over a period of seven days and analyzed for the presence of p-chlorophenylmercapturic acid, various chlorophenols and guanine adducts using different chromatographic tech niques. The major urinary metabolite identified was p-chlorophenylmercapturic acid, the total amount excreted being 13.5 mg after six days. Most of the p-chlorophenylmercapturic acid was excreted during the first 24 hr (65% of the total amount). The pretreatment with phenobarbital did not significantly affect the elimination pattern of this particular metabolite. The excretion of para-, meta-and ortho-chlorophenol was significantly lower. The total amount of free chlorophenols was 1.1 mg after 6 days. The corresponding figure for free and conjugated chlorophenols was 2.55 mg. The ratio of free para-to meta-to ortho-chlorophenols was 4:3:1 and that for free and conjugated forms 3:2.3:1. Pretreatment with phenobarbital was found to have a significant effect on the elimination pattern of the various chlorophenols. The excretion of para-and meta-chlorophenol was twice as high in rats given phenobarbital before chlorobenzene as compared to the amounts excreted by those given chlorobenzene alone. In the case of o-chlorophenol, there was a fourfold increase of the excreted amount in the phenobarbital-induced rats. A DNA-adduct, probably identical with N7-phenylguanine, was also present in the urine 1 and 2 days after the injection, and between day 4 and 6 after the administration. The total amount of adduct excreted in the urine was low (29 |ig after 6 days) and was not affected by the pretreatment with phenobarbital.
Humans: As indicated above, the elimination pattern of chlorobenzene-associated metabolites in humans appears to differ from that observed in experimental animals (70). It was shown in a Japanese field study (96), for example, that 11 persons occupationally exposed to 1.7-5.8 ppm (7.8-26.7 mg/m3) chlorobenzene for 8 to 11 hr, excreted more than 75 % of the urinary metabolites as 4-chlorocatechol, and more than 20% as various chlorophenols (the dominant isomer being p-chlorophenol). A main urinary metabolite of chlorobenzene in rats and rabbits, 4-chlorophenylmercapturic acid, was present only in insignificant amounts (0.4% of the total amount of the chlorobenzene-related urinary metabolites). Chlorophenylmethylsulfides were not detected at all. A similar study from Belgium on 44 chlorobenzene-exposed workers (56) showed that more than 80% of the excreted 4-chlorocatechol and p-chlorophenol in the urine was eliminated within 16 hr after the end of exposure (i.e., end of shift). Both studies are described in more detail under the section "Biological Exposure Indicators."
In a controlled exposure chamber study (71), five male volunteers were exposed for 7 hr to either 12 or 60 ppm (55 or 276 mg/m3) monochlorobenzene. Elimination curves for major urinary metabolites were calculated using pharmacokinetic models. In the calculations, the exposure was standardized to 1 ppm chlorobenzene and it was assumed that the absorption rate for chlorobenzene in the lung was 100%. Two-compartment models gave the following estimated half-lives for 4-chlorocatechol: 2.2 hr (phase I; fast) and 17.3 hr (phase II; slow). The corresponding half-lives for p-chlorophenol were 3.0 and 12 hr, respectively. When the data were fitted to a one-compartment model, the biological half-lives of 4-chlorocatechol and p-chlorophenol were estimated to be 2.9 and 7 hr, respectively.

# Biological Exposure Indicators
Measurements of chlorobenzene in blood, and possibly also exhaled air, can be used for monitoring purposes (2,71). However, the best biological exposure indicator for chlorobenzene in humans is, as previously discussed, the presence of 4-chlorocatechol and p-chlorophenol in the urine.
The urinary concentrations of 4-chlorocatechol and p-chlorophenol are determined by HPLC (56,70,96). Various protocols have been used, but one way is to treat the urine with perchloric acid at 95C, and then extract the metabolites with diisopropyl ether. The ether fraction is evaporated and the residue is dissolved in acetonitrile and water. An aliquot of this fraction is then separated on a column packed with for example chrompack C18, using acetonitrile/water/hexasulphonic acid as the mobile phase (56). The detection limit using the described procedure, a flow rate of 0 . 8 ml/min and peak detection at 282 nm, has been reported to 0.2 mg/ml (56). Recently, Ogata et al. (71) described a slightly modified procedure eliminating the ether extraction step. In the revised protocol, the urinary samples are first treated with various enzymes, and then the enzymatic hydrolysates are applied directly on a column for HPLC.
One of the first studies showing a good correlation between air concentrations of chlorobenzene and urinary concentrations of metabolites was the above-mentioned field study by Yoshida et al. (96). The chlorobenzene concentrations were measured in the air of two different chemical factories using personal air sampling. The total number of subjects was eleven, and the exposure time per shift varied between 8 and 1 1 hr. The estimated air concentrations ranged between 1.7 and 5.8 ppm, with a geometric mean of 3.15 ppm (14.5 mg/m3). The previously mentioned field study from Belgium (56), involving 44 male workers in a diphenylmethane-4 -4 'diisocyanate-producing plant, confirmed the good correlation between air concentrations of chlorobenzene and urinary levels of 4-chlorocatechol and 4-chlorophenol at the end of shift.
The time-weighted average exposure values in the latter study were log-normally distributed and varied from 0.05 to 106 ppm, with a median value of 1.2 ppm (5.5 mg/m3). From extrapolations performed on the data, it was calculated that 8 hr exposure to 50 ppm chlorobenzene (230 mg/m3), without any simultaneous skin contact to the compound, would give an average urinary concentration of 33 mg total chlorocatechol/g creatinine and 9 mg total p-chlorophenol/g creatinine at the end of a working day.
ACGIH recently recommended that measurements of the total amounts of 4-chlorocatechol and p-chlorophenol (i.e., both free and conjugated forms) should be used for monitoring occupational exposure (2). Based on data from studies cited above (70,71,96) and an unpublished simulation study by Droz (cited in reference 2), ACGIH recommended the following biological threshold limits (biological exposure indices; BEIs): 150 mg total 4-chlorocatechol/g of creatinine (= 116 mmol/mol of creatinine) and 25 mg total p-chlorophenol/g creatinine (= 2 2 mmol/mol creatinine) at the end of shift.
Even if 4-chlorocatechol and p-chlorophenol are assumed to be absent in the urine taken from a general population, it may be worthwhile to note that the presence of them is not exclusively linked to an occupational exposure to monochlorobenzene. Both metabolites may, for example, also be found in the urine from persons exposed to dichlorobenzenes or p-chlorophenol (2 ).

# GENERAL TOXICITY
In this section, information from various types of general toxicity tests (i.e., tests for acute, subchronic, and chronic toxicity) on experimental animals has been gathered together with the scarce amount of data available regarding human chlorobenzene exposure. Information from various types of experimental tests measuring specific toxicological endpoints such as immunotoxicity, genotoxicity, carcinogenicity, reproductive toxicity and teratogenicity are treated separately (pp. 38-54). Moreover, following the general outline of traditional NIOH criteria documents, information on specific organ effects has been gathered in a separate section, starting on p. 28. The section "General Toxicity" begins with a discussion on toxicological mechanisms.

# Suggested Toxicological M echanisms
As previously discussed, chlorobenzene undergoes oxidative metabolic bioactivation to form epoxides. It is generally assumed that the toxicity of chlorobenzene is mediated by covalent binding of reactive metabolic intermediates to critical cell structures. However, the exact molecular mechanisms of action behind the various toxic effects of chlorobenzene remain unknown. Different mechanisms may be involved in the various organs that are associated with chlorobenzene-induced toxicity.
The reactive electrophilic metabolites formed in the liver are detoxified mainly by conjugation with reduced glutathione, GSH. Liver damage following exposure to chlorobenzene and other monosubstituted halogenated aromatic monocyclics has therefore been attributed to the depletion of hepatic glutathione, leaving the reactive metabolites free to bind covalently to proteins and other cellular macro-molecules (17,76). It has been suggested that the hepatotoxic effects of chlorobenzene are mainly mediated by the 3,4-epoxide that subsequently rearranges to p-chlorophenol (50, 54). Since the hepatotoxic effects of chlorobenzene are mediated by one or several reactive metabolites, it should be possible to modulate the toxicity by affecting the enzyme systems involved. Consequently, experiments in rats have shown that the liverdamaging effect of chlorobenzene is potentiated when the cytochrome P450 enzyme system is induced with phenobarbital (17).
Impairment of the main detoxifying enzyme system, i.e., mainly the GSH conjugation pathway, could possibly also affect the hepatotoxicity of chlorobenzene. If the detoxification system is handicapped (e.g., by administration of large doses of chlorobenzene) the amount of reactive metabolites available for toxic insults would then theoretically be increased. Initial depletion of hepatic glutathione levels has been shown in both rats (22,95) and mice ( 83) given chlorobenzene intraperitoneally. However, this seems to be a transient phenomenon without any obvious dose-response relationship (22,83). It may also be pointed out that since chlorobenzene appears to lower the cytochrome P450 levels, at least in the livers of rodents given the compound orally or intraperitoneally (9, 22), exposure to chlorobenzene seems associated with a lowered capacity of both bioactivating and detoxifying enzyme systems. The cited studies are described in more detail on pp. 29-32. Koizumi et al. (54) showed that the bromobenzene-induced hepatotoxicity in male Wistar rats could be modified if chlorobenzene was given simultaneously. Groups of rats ( 6 animals/group) were given an i.p. injection of bromobenzene, alone ( 2 mmole/kg b.wt) or in combination with chlorobenzene (4 mmole/kg). The rats were killed after 12, 24, 48 or 72 hr. Hepatotoxicity was assessed both biochemically and histopathologically. The injection of a mixture of bromobenzene and chlorobenzene initially suppressed the hepatotoxic effects of bromoben zene alone (24 hr after the injection). However, at a later stage there was a dramatic potentiation of the toxicity, the maximum response being observed 48 hr after the injection. This was true both with regard to the bromobenzene-induced ALAT elevation and the centrilobular necrosis.
Whereas the suppression in the early phase was believed to be a result of metabolic inhibition of the 3,4-epoxidation pathway, the subsequent potentiation was most likely a result of a delayed recovery in the glutathione levels.
The causal role of protein binding to the chlorobenzene-induced hepatotoxicity has been questioned. In the previously mentioned study of male Sprague-Dawley rats given a single i.p. injection of chlorobenzene (2 2 ), little correlation was found between the histopathological and functional damages of the liver and the metabolism of the substance. A poor correlation was also found between the extent of liver damage and the degree of protein binding. The dose that produced the most extensive liver necrosis (14.7 mmol/kg b.wt., i.e., 1,655 mg/kg) gave the same degree of protein binding as the dose producing only a minimal necrosis (4.9 mmol/kg; 552 mg/kg).
Oxidative stress is one alternative mechanism of action that has been proposed to explain the hepatotoxic effects of chlorobenzene and other aryl halides (21). Evidence for this alternative, or complementary, mechanism of action was obtained from experiments on cultured rat hepatocytes. In this particular in vitro system it was shown that the toxicity of chlorobenzene, bromobenzene and iodobenzene could be manipulated in ways that modified the sensitivity of the cells to oxidative stress. Primary cultures of hepatocytes were prepared from livers taken from Sprague-Dawley rats pretreated with phenobarbital for three days (21). Chlorobenzene and the two other aryl halides were diluted in DMSO and added to the cultures for 2 hr of exposure, with and without addition of l,3-bis(2chloroethyl)-l-nitrosourea (BCNU). The latter compound inhibits glutathione reductase, an enzyme that plays an important role in the glutathione redox cycle and is responsible for the reduction of GSSG to GSH. The concentrations of chlorobenzene varied between 0.25 and 2 mM. Cell viability was determined after 4 hr. The cultured hepatocytes were not as sensitive to the toxicity of chlorobenzene as that of bromobenzene or iodobenzene. However, all compounds induced the same type of effects, although at different concentra tions. At 1 mM of chlorobenzene alone, there was a 30% cell killing, but when BCNU was added to the cultures, the cell killing increased to 90%. BCNU was without toxic effects of its own. The enhanced cell killing in the presence of BCNU could be completely prevented by SKF-525A, an inhibitor of the mixed function oxidase system. The changes of cell killing in the presence of BCNU occurred without parallel changes in the metabolism or covalent binding of  bromobenzene (not investigated parameters in the case of chlorobenzene and iodobenzene).
It has also been suggested that the hepatotoxic effects of chlorobenzene are mediated through an alpha-adrenergic system (83). When phentolamine, an alpha-adrenergic antagonist, was given to male B6C3F1 mice after an i.p. injection of chlorobenzene, the chlorobenzeneinduced hepatotoxicity was significantly reduced.
The kidneys are also main targets for chlorobenzene-induced toxicity (75). However, here it is the 2,3-epoxide, subsequently rearranging to o-chlorophenol, which has been suggested to be the responsible reactive species (50, 51). Pretreatment of mice and rats with piperonyl butoxide, an inhibitor of microsomal enzymes, blocks the renal toxicity of chlorobenzene. However, in contrast to the liver, pretreatment with phenobarbital did not enhance the kidney toxicity of chlorobenzene to a significant extent (75).
Covalent binding of chlorobenzene-associated metabolites has not only been observed in proteins, but also to RNA and DNA taken from various organs of mice and rats given 1 4 C-labelled chlorobenzene (20,40,73). The reported binding of 1 4 C-chlorobenzeneassociated radioactivity to nucleic acids should be interpreted with some cautiousness, because of the conceivable problem of protein contamination. Reid (75), for example, stated, without giving any figures, that nucleic acids isolated from liver and kidneys of mice and rats given 1 4 C-chlorobenzene did not contain any "significant amounts" of covalently bound radioactivity. To examine the reported ability of chlorobenzene to interact directly with DNA in more detail is of great interest, especially when one considers that chlorobenzene has been reported genotoxic in some short-term tests for mutagenicity and/or genotoxicity (see pp. 39-47).
The third major target for chlorobenzene-induced toxicity is the central nervous system.
No studies were available with regard to the mechanism of action of the narcotic and CNS depressant effects of chlorobenzene. This is not surprising when one considers that, so far, nobody knows exactly how conventional inhalational general anesthetics act on a molecular basis. Several theories have been proposed. One theory is that general anes thetics act by inducing reversible binding directly to a particularly sensitive protein in the neuronal membrane, thereby inhibiting its normal function (37), possibly by competing with endogenous ligands (38). The current hypothesis seems to be that general anesthesia at a molecular level, either follow from changes in lipid thermotropic behavior or malfunction of neuronal proteins, or a combination of both processes (83). Since inhala tion anesthetics have diverse structures and act by forming reversible bonds to the critical structure, possibly of Van der Waals type rather than irreversible covalent-ionic bonds (83), it seems likely that it is chlorobenzene itself that induces the CNS-depressant effects. Additional support for this assumption comes from the fact that the intact compound has higher lipophilicity than any of the metabolites formed. High lipophilicity seems to be a prerequisite for CNS-depressant agents.
To summarize, although the different toxic effects observed after administration of chlorobenzene usually are induced by one or more of the various metabolites formed, it cannot be excluded that the compound itself also may produce adverse effects, especially in the CNS. Exactly how chlorobenzene and/or its metabolites cause the toxic effects observed is not known in detail-not even in the liver, the organ most thoroughly studied for chlorobenzene-induced toxicity. Several possible toxicological mechanisms may be involved. Conse quently, whereas the hepatotoxic and nephrotoxic action of chlorobenzene most likely are due either directly to the covalent binding of reactive metabolites to critical structures in the cells, and/or indirectly to oxidative stress, the CNS-depressant effect is probably mediated by other toxicological mechanisms, most likely provoked by the unmetabolized substance itself.

# Acute Toxicity
The acute toxicity of chlorobenzene in experimental animals is relatively low, after oral administration, inhalation, and dermal exposure. Consistently observed chlorobenzene-induced signs of acute intoxication in various species of experimental animals include hyperemia of the visible mucous membranes, increased salivation and lacrimation, initial excitation fol lowed by drowsiness, adynamia, ataxia, paraparesis, paraplegia, and dyspnea (i.e., mainly signs of disturbance of the central nervous system). Changes observed at gross necropsy include hypertrophy and necrosis of the liver and submucosal hemorrhages in the stomach.
Histopathologically observed lesions include necrosis in the centrilobular region of the liver; the proximal convoluted tubules of the kidneys; and the bronchial epithelium of the lungs and stomach. Death is generally a result of respiratory paralysis.
Animals: There are many published and unpublished reports on the acute toxicity of chloro benzene after various routes of administration (see Table 1). The information given in the table was mainly obtained from secondary sources of information. The indicated primary sources of information are in many cases either unpublished reports, or written in a language not familiar to the evaluator. It has consequently not been possible to critically examine each individual study, and the information may appear fragmentary with regard to details on strains, dose levels, methods, and observations.
Apart from the studies listed in Table 1, there are also other acute toxicity studies available in the literature. When the acute toxicity of chlorobenzene was examined in male and female F344/N rats and B6C3F1 hybrid mice (51, 68), the mice were found to be more sensitive than the rats toward the lethal effects of the compound. However, the acute toxicity after a single oral dose of chlorobenzene was also low in both species in this study. The compound was given by gavage, diluted in com oil at the following doses: 250, 500, 1,000, 2,000 or 4,000 mg/kg b.wt. Each group consisted of 5 males and 5 females of each species. The animals were followed for 14 days and observed daily for mortality and morbidity. The animals were not subjected to necropsy and there were no records on possible effects on body weight gain. Whereas a dose of 1,000 mg/kg b.wt. was lethal to the male mice, the rats had to be given up to 4,000 mg/kg before mortality became evident. Most deaths occurred within a few days after the administration. Clinical signs of toxicity among the rats in the two highest dose groups  In the previously mentioned exposure chamber study involving five volunteers exposed to up to 60 ppm chlorobenzene (276 mg/m3) for 7 hr (71) it was shown that this exposure was associated with acute subjective symptoms such as drowsiness, headache, irritation of the eyes, and sore throat. There is also a recently published case report (13) describing severe liver cell necrosis in a 40-year-old man who had ingested 140 ml of a solution containing 90% chlorobenzene in a suicide attempt. The case-report is further discussed on p. 32.

# Subchronic Toxicity
Repeated administration of chlorobenzene to experimental animals for several weeks or months is mainly associated with liver and kidney damage. Typical evidence for the chlorobenzene-induced liver toxicity observed in the subchronic toxicity studies are increased activity of serum liver enzymes, increased liver weight, lipid accumulation, hepatic porphyria, and hepatocellular necrosis. The chlorobenzene-induced nephrotoxicity is mainly manifested as increased kidney weights, focal coagulative degeneration, and necrosis of the proximal tubules. Repeated administration of relatively large doses to experimental animals is also associated with lesions in the thymus, spleen, bone marrow, and lungs.
Although repeat-dose toxicity studies for 14 days do not fall within the category of subchronic toxicity studies, this section of the document begins with such a study (previously often referred to as "subacute" toxicity study) for practical reasons.
Animals: Male and female F344/N rats and B 6 C 3 F 1 mice were given chlorobenzene diluted in c o m oil by gavage for 14 days (51,68). Groups of 5 animals were given daily doses varying between 0 and 2,000 mg/kg b.wt. for the rats, and between 0 and 500 mg/kg for the mice. The animals were observed daily for mortality and morbidity, weighed at the beginning and the end of the study. They were also subjected to necropsy. The 2-week daily exposure to 1,000 and 2,000 mg/kg b.wt. resulted in 1 0 0 % mortality a m o n g the male and female rats. Most deaths occurred within the first few days of exposure. Clinical signs of toxicity a m o n g the rats in the highest dose groups included prostration and reduced response to stimuli. There were no toxic effects that could be related to the administration of chlorobenzene in the mice. Consequently, the N O E L for both male and female rats and mice in this 14-day study w as found to be 500 mg/kg b.wt./day.
In a regular subchronic toxicity study on rats and mice (51, 68), male and female F344/N and B 6 C 3 F 1 mice were given chlorobenzene by gavage, 5 days/week, for 13 weeks in the following doses: 0 (com oil; vehicle), 60, 125, 250, 500 or 750 mg/kg b.wt./day. Each group consisted of 10 animals of each sex and species. The animals were observed daily for mortality, morbidity and clinical signs of toxicity. Food consumption and body weights were measured weekly. Urine was collected during the last week of exposure, and at the end of the study. A blood sample was taken from the orbital venous plexus of each surviving animal and analyzed for various blood parameters. Clinical biochemistry determinations were performed on blood samples obtained from cardiac puncture, taken at the time of sacrifice. All animals were subjected to a complete gross examination, and a number of organs were taken for histopatho logic examination.
The mortality was increased in the two highest dose groups a m o n g the rats, and in the three highest dose groups a m o n g the mice. There were no clinical signs of toxicity reported. The body weight gain appeared to be reduced in the male rats and mice, starting from 250 mg/kg/day, and a m o n g the female rats and mice starting from 500 mg/kg/day. There were no consistent changes in the hematological parameters, and the only significant findings reported from the investigation on serum chemistry were some observations m a d e in surviving female rats of the two highest dose groups: slight to moderate increases in serum alkaline phosphatase and serum g a m m a glutamyl transpeptidase. Apart from increased urine volumes observed in s o m e of the high-dose animals, the urinalysis showed no abnormalities.
Liver weights were slightly increased in a dose-related manner in both male and female rats and mice. Histologic examination showed chlorobenzene-induced lesions in the liver, kidney, spleen, bone marrow and thymus of both rats and mice. In the liver there was a dose-dependent centrilobular hepatocellular degeneration and necrosis (LOEL: 250 mg/kg/day). In the kidneys the lesions were characterized by vacuolar degeneration and focal coagulative necrosis of the proximal tubules (lowest L O E L : 250 mg/kg/day). The renal lesions were judged to be mild to moderate. Chlorobenzene induced moderate to severe lymphoid necrosis of the thymus in male and female mice (lowest L O E L being obtained in males: 250 mg/kg/day). However, there was no perfect dose-response relationship with regard to this effect of chlorobenzene. Lymphoid depletion of the thymus, lymphoid and myeloid depletions of the spleen, and myeloid depletion of the bone marrow, all regarded as minimal to moderate, were only seen in animals in the highest dose groups. Taken together, the results suggested a N O E L of 125 mg/kg b.wt./day. Mice appeared to be more sensitive toward the toxic effects of chlorobenzene, and males were somewhat more sensitive than females.
The subchronic toxicity of inhaled chlorobenzene has been evaluated in male rats and rabbits (28). The animals were exposed to 0, 75, or 200 p p m (0, 345, or 920 m g / m 3) of chlorobenzene for 7 hr/day, 5 days/week, for up to 24 weeks. Groups of animals were killed after 5, 11, and 24 weeks and examined for hematology, clinical chemistry, and gross and histopathological changes. Chlorobenzene-related toxicity in the male rats included decreased food utilization, increased liver weights, lowered A S A T activity at all survival times, increased number of blood platelets after 11 weeks of exposure and microcytic anemia. Histopathological changes observed were occasional focal lesions in the adrenal cortex, tubular lesions in the kidneys, and congestion in both liver and kidneys. Reported toxic effects in the rabbits included increased lung weights and, after 11 weeks of exposure, decreased A S A T activity. The results were presented in a short meeting abstract that did not include any information on, for example, strains, number of animals, or nonobserved effect levels.
The subchronic toxicity of chlorobenzene has also been investigated in dogs, exposed either orally or by inhalation. The only published information from these studies available for evaluation, is a condensed meeting abstract by Kn a p p et al.  76-166, 1979; unpublished].
In the IBT-study, groups of beagle dogs (four males and four females in each group) were exposed to 0, 0.75, 1.5, or 2 mg/1 air (0, 750, 1,500 or 2,000 m g / m 3) of chlorobenzene vapors, 6 hr/day, 5 days/week for 90 days. S o m e of the animals in the two highest dose groups (HD: 5/8; M D : 2/8) became moribund and were sacrificed after approximately 30 days. According to the secondary source of information (32), the exposure to chlorobenzene resulted in lowered body weight gain ( H D dogs), lower leukocyte counts and elevated levels of alkaline phos phatase, A L A T and A S A T ( H D dogs), lower absolute liver weights ( H D females), lower absolute heart weights ( M D males only) and increased absolute pancreas weights ( M D and H D females). Histopathological changes included vacuolization of hepatocytes ( H D animals), aplastic bone marrow ( H D dogs), cytoplasmatic vacuolization of the epithelium of the collecting tubules in the kidneys (one male and three males in H D ) and bilateral atrophy of the seminiferous epithelium of the testes (two males in HD). The results of the IBT-study, as reported in the secondary source of information (32), should be interpreted with some cautiousness. It is not k n o w n if this particular IBT-study has been validated. Consequently, it is not k n o w n if the study should be judged invalid, pending, supplemental, or valid.
In the inhalation study from Hazleton, six adolescent dogs per sex and group (strain not given), were exposed to various levels of chlorobenzene vapors, 6 hr/day, 5 days/week, for 6 months.
The target levels of chlorobenzene were 0, 0.78, 1.57, or 2.08 mg/1 air (0, 780, 1,570, or 2,080 m g / m 3). Significant changes included decreased absolute adrenal weights in the male dogs of the two highest dose groups, increased relative liver weights in the female dogs of the two highest dose groups, a dose-related increased incidence of emesis in both males and females, and an increased frequency of abnormal stools in treated females. The N O A E L was determined to be 780 m g / m 3 (6, 32).
In one of the oral subchronic toxicity studies, male and female beagle dogs were given chlorobenzene by capsule at doses of 0, 27.25, 54.5, or 272.5 mg/kg b.wt./day, 5 days/week, for 13 weeks (93 days). Four of eight dogs in the highest dose group died within 3 weeks. At this dose level, chlorobenzene was found to produce a significant reduction of blood sugar, an increase in immature leukocytes, elevated serum A L A T and alkaline phosphatase levels and, in some dogs, increases in total bilirubin and total cholesterol (25, 32, 53). In the condensed meeting abstract it was stated that there were no consistent signs of chlorobenzeneinduced toxicity at the intermediate and low dose levels ( 53), but according to the unpublished report, cited by, for example, the U. According to the brief information given in the meeting abstract (53), groups of rats were also given chlorobenzene in the diet for 93 to 99 consecutive days (0, 12.5, 50, or 250 mg/kg b.wt./day). Reported effects of chlorobenzene were retarded growth (males in the highest dose group) and increased liver and kidney weights ("some rats at the high and intermediate levels"), resulting in a N O E L of 12.5 mg/kg b.wt./day.

# Chronic Toxicity
Animals: Apart from a cancer study, carried out as a part of the National Toxicology Program, where chlorobenzene was given orally to F344/N rats and B 6 C 3 F 1 mice, 5 days/week for 103 weeks (51,68), there were no other chronic toxicity studies available forevaluation. Since the N T P study, as other regular cancer bioassays, was concentrated on histopathological data and consequently devoid of clinical chemistry, hematological investigation, and urinalysis, etc., the study has been evaluated under the heading "Carcinogenicity" on page 48. However, it m a y be useful to note that the administration of up to 120 mg/kg b.wt./day (rats and female mice) or 60 mg/kg/day (male mice) of chlorobenzene for 2 years, failed to induce the type of toxic responses (e.g., damage to the liver, kidney, and hematopoietic system) that was observed in the previously cited subchronic toxicity study in rats and mice (51). There is also another Russian paper, by Lychagin et al. from 1976, reporting a higher incidence of w o m e n with i m munological shifts, disturbed phagocytic activity of the leukocytes, reduced absorption capacity of the neutrophils, dermal infections, occupational dermatitis, and chronic effects to respiratory organs in a glass insulating enameling department. Study design, number of workers and controls involved, exposure levels, duration of exposure, etc., were not given in the short citation ( 91), but the exposure situation appeared to have been complex, also involving exposure to, for example, acrolein, acetone, and glass fiber dust. Unpublished experimental data from the G e r m a n manufacturer Bayer A G (Suberg 1983a(Suberg , 1983b, cited in the G e r m a n B U A report ( 19), showed that chlorobenzene is moderately irritating to the skin. The same unpublished studies from Bayer A G also showed that chlorobenzene is a moderate irritant to the eyes (19). N o detailed information was given in the short citation of these studies, but both the dermal irritancy/corrosivity study, and the eye irritation test were performed on rabbits according to the O E C D guidelines for testing of chemicals (19).

# ORGAN EFFECTS

# Respiratory S y s t e m
Results obtained in some of the general toxicity tests show that chlorobenzene m a y be toxic to the lung. Necrotic lesions in the bronchial epithelium of the lungs are one of the chlorobenzene-induced histopathological changes that have been observed in acute toxicity tests after administration of large doses. A subchronic toxicity study of inhaled chlorobenzene in rabbits (28) showed increased lung weights after up to 24 weeks of exposure to 75 or 200 p p m chlorobenzene.
Apart from the fact that inhalation of chlorobenzene vapors is irritating to the membranes of the upper respiratory tract, no other h u m a n data have been found with regard to the chlorobenzene-induced adverse effects on the lungs.
The lungs are evidently not the major targets for the chlorobenzene-induced toxicity. The reported effects from animal experiments were observed only at relatively high exposure concentrations of the compound.

# Liver
Animals: A s discussed above in the section "General Toxicity," the liver is one of the main targets for chlorobenzene-induced toxicity. Studies on experimental animals have sh o w n that chlorobenzene produces various types of deleterious effects on the liver, both morphological and functional. Typical consequences of chlorobenzene exposure are increased liver weights, increased activities of serum liver enzymes, porphyria, and hepatocellular necrosis. This has, for example, been observed in male and female rats after both acute and repeated oral administration and inhalation (22,46,51,53,67,68), in male and female mice after acute and repeated oral exposure (51,53), in dogs given the comp o u n d orally or by inhalation for several weeks (6,25,32,53) and in pregnant rabbits after inhalation of chlorobenzene vapor during the period of gestation (48).
A carcinogenicity study on Fischer 344/N rats (51,68) showed that there was a slight increase in the frequencies of male rats with neoplastic nodules of the liver after two years of oral exposure to 120 m g chlorobenzene/kg b.wt./day. N o such changes were observed in male rats receiving a lower dose, female rats, or in male and female B 6 C 3 F 1 mice (see p. 48).
In a study of chlorobenzene-induced hepatotoxicity, male Sprague-Dawley rats were injected for three consecutive days with physiological saline or phenobarbital before they were given an i.p. injection with various doses of chlorobenzene diluted in sesame oil (17). The animals were killed 24 hr after the injection with chlorobenzene. The livers were removed and examined histopathologically. The pathological changes of the hepatocytes in the centrilobular region in the non-induced rats given 0.04 ml chlorobenzene varied from glycogen loss to minimal necrosis. However, the centrilobular necrosis in the phenobarbital-pretreated rats given the same amount of chlorobenzene was found to be extensive or massive.
In another single-dose experiment on male Sprague-Dawley rats ( 22), the relative liver weights were found to be increased about 1.5 times those of the controls 24 hr after an i.p. injection of 9.8 mmol/kg b.wt. At this time a mild but progressive development of a hepatic lesion was observed around the central veins. The damage was manifested as a cloudy swelling and hydropic changes of the centrilobular hepatocytes. Forty-eight hr after the injection, the signs of necrosis had become even more pronounced. Rats given 9.8 or 14.7 mmol/kg (1,100 or 1,655 mg/kg), showed extensive hydropic changes throughout the liver and clear evidence of necrosis. However, signs of mild morphological alterations (cloudy swelling and hydropic changes in centrilobular regions) were also present at the lowest dose level tested (2.0 mmol/kg; 225 mg/kg). N o evidence of fatty changes was observed at any dose level or survival time.
In a study on the relationship between the chemical structure of chlorinated benzenes and their effects on hepatic and serum lipid components, chlorobenzene was given to male Sprague-Dawley rats in the diet at a concentration of 500 p p m for two weeks (46). Whereas the body weight gain and kidney and spleen weights were unaffected, the liver weights were slightly increased. The level of lipid peroxide was reported to be increased in the livers of the chlorobenzene-exposed rats and this increase was accompanied by an elevated level of triglycerides and lowered levels of vitamin E and glutathione peroxidase.
In order to explore the relationship between chemical structure and liver toxicity, Ariyoshi et al. (9) gave various chlorinated benzenes suspended in 2 % tragacanth g u m solution orally to female Wistar rats for three consecutive days. Chlorobenzene was also given as a single oral dose of 125, 250, 500, and 1,000 mg/kg b.wt. A m o n g the various parameters investigated in controls and exposed rats (six animals/group) were the contents of microsomal proteins, including cytochrome P450, and phospholipids, the activities of the drug-metabolizing en zymes aminopyrine methylase and aniline hydroxylase, and the activity of a-aminolevulinic acid synthetase. Oral doses of 125-1,000 mg/kg b.wt./day for three days were found to increase the hepatic h e m e synthesis but to decrease the microsomal cytochrome P450 content as well as the activity of aminopyrine demethylase. The activity of a-aminolevulinic acid synthetase was markedly increased at all dose levels employed. Liver weights and the contents of fatty acids of phospholipids were increased in the chlorobenzene-exposed animals, but there were no compound-related effects on the contents of glycogen, triglycerides, or the total amount of microsomal proteins.
Obviously, chlorobenzene differs from m a n y polychlorinated aromatic hydrocarbons, in not being a general inducer of the microsomal metabolism (i.e., the c o m p o u n d does not stimulate the activity of the cytochrome P450/P448 enzyme system). This conclusion has subsequently been confirmed in experiments performed on male Sprague-Dawley rats given a single i.p. injection of chlorobenzene (22).
O n e w a y of studying liver toxicity is to monitor the serum alanine aminotransferase ( A L A T ) activity. This enzyme is regarded highly specific to the liver, and its concentration in the blood is regarded directly proportional to liver damage. Several investigations have sh o w n that chlorobenzene exposure is associated with increased serum A L A T activity. In one of these experiments ( 22), already mentioned above, chlorobenzene was given diluted in c o m oil as a single i.p. injection of 2, 4.9, 9.8, or 14.7 mmol/kg b.wt. (225, 550, 1,100 or 1,655 mg/kg) to male Sprague-Dawley rats. Controls received vehicle only. The effects of chlorobenzene were also investigated after various intervals (3 to 72 hr) following a dose corresponding to the estimated L D i o dose (1,100 mg/kg b.wt.). Each group of chlorobenzene-treated rats consisted of two to six animals. The A L A T activity was found to be significantly elevated at all intervals studied, the m a x i m u m increase being observed after 48 hr. T he dose-response experiment showed elevated A L A T activities at all doses tested, but the authors considered 1,100 mg/kg b.wt. to be the L O E L with regard to this specific experimental parameter. Chlorobenzene was also found to elevate the sulphobromophtalein (BSP) retention significantly at all intervals studied. The m a x i m u m effect was already obtained 3 hr after the injection. Consequently, functional evidence of hepatotoxicity can be detected early in the time course of events induced by chlorobenzene.
In another study (83) employing male B 6 C 3 F 1 mice, chlorobenzene w as given as an i.p. injection at doses of 0 (com oil) 0.01, 0.1, 0.25, 0.5, or 1 ml/kg b.wt. (higher doses than that resulted in 1 0 0 % lethality within 24 hr after the injection). Each group consisted of at least nine animals. A s in the above-mentioned study on the male Sprague-Dawley rats, the m a x i m u m increase of serum A L A T activity was obtained 48 hr after the injection. T he L O E L with regard to increased serum A L A T activity in the male mice was established at 0.5 ml/kg b.wt.
Another typical effect of chlorobenzene on the liver, is its influence on the glutathione levels.
Glutathione is a tripeptide that is involved in the detoxification of electrophilic substances.
The reaction between the nucleophilic groups in glutathione and electrophilic sites in reactive molecules often leads to the formation of mercapturic acids that are excreted in the bile or, as in the case of chlorobenzene, in the urine.
In one of the studies investigating the effect of chlorobenzene on the G S H levels in the liver (95), male Wistar rats were given an i.p. injection of chlorobenzene diluted in olive oil. The compou n d was given either as a single dose of 2 mmol/kg b.wt. (225 mg/kg), or repeatedly four times during a 48-hr period ( 4 x 2 mmol/kg b.wt.). In the single-dose experiment, the rats were sacrificed after 3,6,24, and 30 hr, and in the repeat dose experiment, they were sacrificed either 48 hr after the first injection or 48 hr after the last injection. Each group consisted of four to five animals. Controls received olive oil only. In the single-dose experiment, chloroben zene was found to induce a significant, but transient, decrease of the hepatic levels of total and oxidized glutathione. Six hr after the injection, the total amount of glutathione was only 2 4 % of that in the controls. However, 24 hr after the injection, there was already a significant increase of both the total and oxidized glutathione (188% and 1 7 0 % of controls, respectively), an effect that was accompanied by an increased glutathione synthesis (193% of controls) and an elevated glutathione reductase level (136% of controls). After 48 hr, all these levels were still increased, and at this survival time the liver weights, as well as the protein-and DNA-contents, were also found to be significantly increased. The repeat dose experiment confirmed the chlorobenzene-induced liver enlargement and accumulation of hepatic glutathione.
Initial depletion of glutathione levels shortly after an intraperitoneal injection of chlorobenzene to male Sprague-Dawley rats was also observed in the previously mentioned study by Dalich and Larson (22). Four hr after the injection, there was a significant depletion of G S H levels at all doses investigated (from 2.0 to 14.7 mmol/kg b.wt.). Apart from the lowest dose group, the G S H levels remained low 8 hr after the injection, the longest survival time employed for this study parameter. The experiments also included measurements on the potential effects of chlorobenzene on the microsomal cytochrome P450 levels in the liver. Four hr after the administration, these were found to be depressed between 3 0 -5 0 % at all dose levels tested. After 24 hr, the cytochrome P450 content was lowered to 5 0 -8 0 % of the control level. However, there was no obvious relationship between the dose and the observed effect on the cytochrome P450 content. With regard to the time course of the covalent binding of chlorobenzene-associated radioactivity to liver proteins, measurable amounts were al ready present 2 hr after the injection. The amount of binding increased steadily during the first 24 hr. Again there was a poor correlation between the dose and the magnitude of the covalent binding to the liver proteins, the m a x i m u m covalent binding being obtained at 4.9 mmol/kg (550 mg/kg).
Experiments on male B 6 C 3 F 1 mice showed temporal changes in hepatic glutathione con centrations following an i.p. injection of chlorobenzene (83). W h e n groups of animals (at least 8 animals/group) were killed after 2, 4, 8, or 24 hr after an i.p. injection of either c o m oil (controls) or 0.48 ml chlorobenzene/kg b.wt., the hepatic glutathione concentrations were found to be depleted maximally 4 hr after the injection of chlorobenzene (90% reduction).
After 24 hr, the glutathione levels recovered back to normal. In a dose-response experiment, groups of mice (at least eight animals in each group) were given 0, 0.01, 0.1, 0.25, or 1 ml chlorobenzene/kg b.wt. i.p. and sacrificed 3 hr later. It was reported that 0.1 ml/kg was the lowest effective dose that significantly exhausted the liver G S H . However, there w as no perfect dose-response relationship for this effect (0.1 ml/kg: 2 3 % reduction; 0.25 ml/kg: 7 8 % reduction; 0.1 ml/kg: 7 6 % reduction and 1.0 ml/kg: 7 1 % reduction).
H u m a n s : A recently published case report from France ( 13) described quite severe effects on the liver by chlorobenzene. Exposure occurred in a suicide attempt in which a 40-year-old m a n ingested approximately 140 ml of a 9 0 % chlorobenzene solution. After two hr, the patient became drowsy. At that time the serum activities of A S A T and A L A T were increased approximately three times. Three days after the ingestion of chlorobenzene, the serum A S A T and A L A T activities were 345 and 201 times the upper limits of normal, respectively. The liver was not enlarged, but the patient had a diffuse erythema covering the face. A liver specimen was taken by transjungular biopsy. The histopathological examination showed centrilobular and mediolobular necrosis, but no evidence of inflammatory infiltration, hepatocyte ballooning or fibrosis. Immunoglobulin M antibodies to hepatitis A virus and to hepatitis B core antigen, as well as hepatitis B surface antigen, were absent, and the serological test results for recent infection with herpes simplex viruses were negative. T he serum level of chlorobenzene was determined to be 500 jig/13 days after the suicide attempt, and 2 jag/1 after 15 days. Although the m a n was described as an alcoholic (the consumption of alcohol was estimated to 200 g per day), the authors concluded that the observed liver cell necrosis was directly linked to the acute intake of chlorobenzene (there was, for example, no history of chronic liver disease, which was also confirmed by the liver biopsy). However, it cannot be ruled out that the chronic ethanol consumption might have played a role in the severity of the observed lesions. After being treated with prostaglandin Ei for several days, the patient recovered.
Apart from the above-mentioned case report, no other data was found concerning hepatotoxic effects of chlorobenzene in humans. However, it m a y be worth noting that results obtained in vitro ( 50) suggest that humans m a y be more susceptible to the hepatotoxic effects of chlorobenzene than rodents. Liver microsomes taken from humans were reported to be more efficient in producing p-chlorophenol than mouse liver microsomes, showing that the main metabolic pathway of chlorobenzene in h u m a n livers is through the hepatotoxic 3,4-epoxide pathway (see p. 12).

# K i d n e y s
Animals: A s shown in the previously discussed general toxicity studies, but also in other toxicity tests (see p. 53), the kidney is another target for chlorobenzene-induced toxicity. This has also been shown in experiments designed to investigate the mechanisms behind the nephrotoxic action of chlorobenzene (75). Male Sprague-Dawley rats and male C57BL/6J mice given a single i.p. injection of unlabelled and/or 14C-labelled chlorobenzene, developed a renal tubular lesion within 48 hr. Extensive necrosis of the proximal convoluted renal tubules was, for example, observed among 8 0 % of the mice given 6.75 mmoles/kg b.wt.
(760 mg/kg). The rats were not as sensitive as the mice to the nephrotoxic action of chlorobenzene.
The development of renal necrosis was associated with covalent binding of chlorobenzeneassociated radioactivity to kidney proteins. After administration of 14C-chlorobenzene (1 mmol/kg b.wt.; 10-30 jiCi/animal), a considerable amount of chlorobenzene-associated radioactivity became covalently bound in the region with the necrotic lesions, i.e., in the proximal convoluted tubule cells. The nephrotoxic action of chlorobenzene could be reduced if the animals were pretreated with piperonyl butoxide, an inhibitor of microsomal enzymes. The pretreatment did not only block the renal toxicity, it also markedly reduced the binding of chlorobenzene-associated radioactivity to the kidney proteins. However, in contrast to the situation in the liver (see above), pretreatment with phenobarbital, an inducer of microsomal enzymes, did not significantly enhance the nephrotoxicity of chlorobenzene in the rats and mice.
H u m a n s : N o data was found concerning nephrotoxic effects of chlorobenzene in humans.

# P a n c r e a s
In a study on the effects on the pancreas of benzene and various halogenated analogues, including chlorobenzene, male Holzman rats were given an i. There is no h u m a n data available on chlorobenzene-induced effects in the pancreas.

# Gastrointestinal Tract
Acute toxicity studies on experimental animals have shown that exposures to high doses of chlorobenzene are associated with necrosis in the stomach as well as submucosal hemorrhages.
In an oral subchronic toxicity study in dogs ( 53), it was noted that the animals given the highest dose of chlorobenzene (272.5 mg/kg b.wt., 5 days/week for 13 weeks) developed histopathological changes in the gastrointestinal mucosa.
N o data was found concerning chlorobenzene-induced effects in the gastrointestinal tract of humans.

# Circulatory S y s t e m
Apart from an isolated case of intoxication where a 2-year-old boy was reported to suffer from vascular paralysis (an effect that could be due to CNS-depression) after having swallowed chlorobenzene (see p. 23), no other data were available on chlorobenzene-induced effects on the circulatory system.

# Hematological S y s t e m
Animals: It has been reported from various experiments in animals that exposure to chloroben zene is associated with some hematopoietic toxicity. Male and female Swiss mice were exposed to chlorobenzene vapor, either to 100 m g / m 3 (22 ppm), 7 hr/day for three months or to 2,500 m g / m 3 (544 ppm), 7 hr/day for 3 weeks (97). The number of animals in each group was ten (5 males and 5 females). During the experiment, and after its termination, blood was drawn from the tail vein and examined for the leukocyte counts and blood picture. C o m parisons were m a d e with controls and mice receiving either benzene or trichlorobenzene. Chlorobenzene-induced leukopenia (characterized by neutropenia, destruction of l ym phocytes and lymphocytosis) and a general bone marrow depression. Similar effects were observed in the benzene-exposed mice. However, in comparison to the latter compound, chlorobenzene was found not equally potent in inducing hematopoietic toxicity.
According to secondary sources of information (32, 92), Varhavskaya reported pathologic changes (inhibition of erythropoiesis, thrombocytosis and mitotic activity) in the bone marrow of male rats given oral doses of 0.01 or 0.1 m g chlorobenzene/day for 9 months. The results, which were presented in a paper originally published in Russian , were not available for a critical examination. However, the results appear unrealistic, at least if the indicated dosages are correct. There is no evidence from any of the other available toxicity studies on rats (or other species) that chlorobenzene would be such a potent toxin to the bone marrow.
In a previously mentioned inhalation study on male rats and rabbits exposed to 75 or 250 p p m (345 or 1,150 m g / m 3) chlorobenzene vapors for 11 weeks (28), both species showed un specified pathological changes in various red cell parameters.
A dose-related increase of the number of micronucleated polychromatic erythrocytes was observed in the bone marrow of male N M R I mice given i.p. injections of 225-900 m g chlorobenzene/kg b.wt. (65,66). N o information was given on the potential general bone marrow toxicity of the substance (see p. 43).
Minimal to moderate myeloid and/or lymphoid depletions were observed in the spleen and thymus in another previously mentioned subchronic toxicity study on rats and mice given chlorobenzene by gavage for 13 weeks (51, 68). Effects on the bone marrow were only seen in animals given the highest dose of chlorobenzene (750 mg/kg b.wt.). A n increased number of blood platelets and microcytic anemia were observed in male rats and rabbits exposed to up to 200 p p m chlorobenzene vapor (920 m g / m 3) for up to 24 weeks (28). Histopathological changes in hematopoietic tissues have also been reported in an oral subchronic toxicity study in dogs given 272 m g chlorobenzene/kg b.wt., 5 days/week for 13 weeks (53).
H u m a n s : There are no relevant data available in the literature on chlorobenzene-induced effects on the hematological system in humans.

# I m m u n o l o g i c a l S y s t e m
Subchronic toxicity studies in mice and rats showed that repeated exposure to relatively high doses of chlorobenzene produced minimal to moderate lymphoid depletion of the thymus, and lymphoid or myeloid depletions of the spleen and moderate to severe lymphoid necrosis of the mouse thymus (51, 68).
N o relevant h u m a n data was available on chlorobenzene-induced toxic effects on organs and tissues constituting the immunological system.

# Central N e r v o u s S y s t e m
Most organic solvents are to a greater or lesser extent able to induce CNS-depression when given at large doses. The neurotoxic effects of organic solvents m a y be divided into acute effects and chronic effects. Generally it is assumed that whereas the acute effects m a y be a result of the direct action of the solvent on the nerve cell m e m b r a n e and energy metabolism, the chronic effects are caused by the formation of reactive intermediates (80). In the case of chlorobenzene, no information was available with regard to possible chronic neurotoxic effects of the compound. However, its potential to induce acute neurotoxic effects is well documented.
It is kno w n that even a short duration of exposure to low concentrations of various solvents can induce moderate signs of toxicity such as mucous m e m b r a n e irritation, tearing, nasal irritation, headache and nausea. At higher exposure levels, the toxic effects become more pronounced and m a y include overt signs of intoxication, incoordination, exhilaration, sleepi ness, stupor, and beginning anesthesia (27). While the former group of symptoms combined can be viewed as prenarcotic effects, the latter symptoms are generally regarded as indicators of narcosis (27). However, it m a y be difficult to establish safe exposure limits with regard to the solvent-induced C N S effects following from short-term exposure. M a n y of the mild symptoms described above are subjective, tolerance is often developed, the estimated exposure levels are often uncertain and the recorded effects are in m a n y cases reversible. It has therefore been argued that it would be better to use various forms of neurobehavioural tests that measures basic psycho-physiological functions such as alertness, reaction time, memory, and sensorymotor performance as indicators of mild CNS-effects, instead of reported signs of mild intoxication (27).
Animals: A s previously discussed, acute symptoms of chlorobenzene-induced intoxication in various species of experimental animals include C N S effects such as excitation followed by drowsiness, adynamia, ataxia, paraparesis, paraplegia, and dyspnea (see p. 21).
A specific study on behavioral changes following short-term inhalation of chlorobenzene was performed in male Swiss O F 1 mice (24). T he animals were exposed for 4 hr to high concentrations of chlorobenzene vapor: 650,785, 875, or 1,000 p p m (i. e., 2,990, 3,610,4,025, or 4,600 m g / m 3), respectively. Controls were exposed to clean filtered air only. T he number of animals in each group was ten. Measurements were m a d e to see whether the acute exposure affected the immobility developed in a so-called "behavioral despair" s w i m m i n g test. T he test is based on the fact that rodents that are forced to s w i m in a limited space, after a while develop a characteristic immobile posture that can be timed. Chlorobenzene, as well as other solvents included in the study, was found to reduce the total duration of immobility over a 3-min period in a dose-related manner. The exposure that would give a 5 0 % decrease in immobility was estimated to be 804 p p m (i.e., 3,700 m g / m 3) for chlorobenzene. This level was considerably higher than, for example, that calculated for benzyl chloride (15 ppm) and styrene (549 ppm), but notably lower than that calculated for other solvents such as 1,2-dichloroethylene (1,983 ppm), methyl ethyl ketone (2,056 ppm) and 1,1,1-trichloroethane (2,729 ppm).
H u m a n s : A s previously discussed, isolated case reports of acute poisonings have shown that inhalation or ingestion of high doses of chlorobenzene is associated with CNS-effects such as drowsiness, incoordination, and unconsciousness (see p. 23). In the previously mentioned controlled exposure chamber study (71) where five male volunteers were exposed to chlorobenzene vapors for up to 7 hr, a significant decrease in flicker-fusion values, indicating a lowered perception, was observed after 3 hr of exposure to 60 p p m (275 m g / m 3). Subjective symptoms reported after 7 hr of exposure were drowsiness, headache, irritation of the eyes, and sore throat.

# Rep r o ductive O r g a n s
A two-generation reproductive toxicity study on rats (67) showed that chlorobenzene induced dose-related changes in the testes. These were manifested as an increased incidence of males with a degeneration of the testicular germinal epithelium in the highest dose group (450 p p m in the diet). Despite these lesions, there were no adverse effects on the reproductive perfor mance or fertility. The results of the reproductive toxicity study are described in more detail starting on p. 51.
Bilateral atrophy of seminiferous epithelium of the testes was also noted a m o n g s o m e of the male beagle dogs that were exposed to 273 p p m chlorobenzene vapor for 90 days in the previously discussed IBT-study (see pp. 25-26).

# Othe r O r g a n s
Apart from the organs mentioned above, chlorobenzene has also been found to affect the adrenals of male dogs and rats in subchronic inhalation toxicity tests (28, 53). In the dogs, the effect on the adrenals was manifested as decreased absolute adrenal weights in animals exposed to 1,570 or 2,080 m g / m 3 chlorobenzene vapor, 6 hr/day, 5 days/week for 6 months.
In the rats, the toxicity was manifested as occasional focal lesions in the adrenal cortex (the inhalation concentrations of chlorobenzene in the latter study were 345 or 920 m g / m 3,7 hr/day, 5 days/week up to 24 weeks).
T he significance of these findings is not known, and there are no other reports on chlorobenzene-induced adrenal toxicity in any of the other identified toxicity studies.

# IM M U N O T O X IC IT Y A N D A L L E R G Y
Animals: Aranyi et al. (8) investigated the effects of single and multiple 3-hr exposures to TLV-concentrations of various industrial compounds (75 p p m for chlorobenzene) in female C D 1 mice by monitoring their susceptibility to experimentally induced streptococcus aerosol infection and pulmonary bacterial activity to inhaled Klebsiella pneumoniae. The results of the study have also been presented in a short notice (5). Whereas, for example, methylene chloride, ethylene chloride and toluene affected both investigated experimental parameters, chlorobenzene apparently lacked significant effects on the murine lung host defenses.
The G erma n B U A report on chlorobenzene ( 19) cited an unpublished study from Bayer A G (Mihail 1984) reporting that chlorobenzene did not induce skin sensitization (i.e., did not induce allergic contact dermatitis) in the so-called maximization test using male guinea pigs. N o further information was given in the short citation.
H u m a n s : N o relevant reports were available with regard to immunotoxic or allergic effects of chlorobenzene in exposed humans.

# G E N O T O X IC IT Y
The results from the testing of the genotoxicity of chlorobenzene in various test systems are not consistent. The overall data seem to show "limited evidence of genotoxicity" since chlorobenzene was reported "positive" in at least three different test systems measuring mutagenicity, chromosomal anomalies and D N A damage/DNA-binding (most of the other test results were reported as "negative").
Most of the published data on the potential genotoxicity of chlorobenzene are summarized in Table 2. However, as indicated below, there are also some additional tests on the genotoxicity of chlorobenzene. Although cited, these are in general either unpublished studies performed by, or on behalf of, various chemical manufacturers, or reports written in a language not familiar to the evaluator. Consequently, it has not always been possible to judge the validity or significance of each individual result, as reported by others.
N o h u m a n data are available on possible genotoxic effects following from accidental, occupational or environmental exposure to chlorobenzene.

# G e n e Mutations
The ability of chlorobenzene to induce gene mutations (point mutations) has been investigated in various strains of Salmonella typhimurium (43,82), in one strain of Aspergillus nidulans (74), and in one m a m malian cell system, the L 5 1 7 8 Y mouse cell l y m p h o m a assay (62).
Chlorobenzene was found mutagenic in the mammalian test system, but without effects in the two reverse mutation test systems based on nonmammalian cells. The absence of a mutagenic effect in the various strains of S. typhimurium, and the presence of a mutagenic effect in the L 5 1 7 8 Y cells, was not affected w h e n a metabolic activation system was added to the test systems.
Reverse mutations in bacteria: O n e of the two recognized and published reverse mutation assays in Salmonella (82) was performed as a standard plate incorporation assay. The mutagenicity was tested both in the absence and presence of a liver microsomal fraction (the S9-fraction was prepared from livers from male Sprague-Dawley rats pretreated with a polychlorinated biphenyl). Five different strains of S. typhimurium were used: TA1537, TA1538, T A 9 8 (for the detection of frameshift mutations), T A 1 5 3 5 and T A 1 0 0 (for the detection of base pair substitutions). Chlorobenzene was diluted in D M S O and tested in a series of concentrations from 0.02 jal to 1.28 jal per plate (the highest concentration was clearly toxic in all strains), without being mutagenic. In the other Salmonella/mammalian microsome assay (43,68), a preincubation procedure was used instead of the standard plate protocol w h e n testing the potential mutagenicity of chlorobenzene (and 349 other coded chemicals). Four different strains of S. typhimurium were used: TA1535; TA1537; T A 9 8 and TA100. The potential mutagenicity was tested both with and without an exogenous metabolic activation system (liver S9-fractions from male Sprague-Dawley rats and Syrian hamsters induced with Aroclor 1254). Chlorobenzene was dissolved in D M S O and tested in concentrations ranging from 33.3 to 3,333.3 pg/plate. In contrast to a positive control, chlorobenzene did not increase the number of revertants in any of the strains tested.
The final draft of the health effect criteria document from E P A (32), and the B U A report ( 19), referred to other Salmonella!microsomal assays than those mentioned above. Since none of these appear to have been published , it has not been possible to evaluate them in the present document. All were reported negative, including an investigation on E. coli W P 2 . In the E P A document (32), it was noted that the statistical analysis of the data in these studies did not include information on the number of revertants per unit of survivors.
The ability of chlorobenzene to induce point mutations has apparently also been tested by Koshinova in an assay based on Actinomyces antibioticus 400. According to the brief details given in secondary sources of information (6, 92), chlorobenzene was reported to induce reverse mutations in the presence of an exogenous metabolic system. The original study (the information on where this article was published varies, but it appears to have been in Genetica 4 (1968) 121-125; presumably in Russian) was not available for evaluation and it has consequently not been possible to evaluate the significance of the reported "positive" effect in the indicated test system (not one of the most established short-term tests for genotoxicity).
Reverse mutations in moulds: A n auxotroph strain of Aspergillus nidulans requiring methionine and pyridoxine was used w h e n testing for the ability of chlorobenzene to induce reverse mutations (72). A suspension of freshly prepared spores was added to a 6 % diethyl ether solution of chlorobenzene. After 1 hr of exposure, the mixture of compound, vehicle and spores was diluted and a fraction was spread over pyridoxine and methionine-supplemented minimal m e d i u m plates. T he n u mber of conidia (revertants) wa s estimated using a hematocytometer after 5 days of incubation at a temperature of 28C. Chlorobenzene was tested at one concentration only (200 fig/ml). At this concentration there were no significant differences in survival or number of revertants between the controls and treated. It m a y be worthwhile to note that this particular test system has not been evaluated and validated to the same extent as, for example, the Salmonella/mammalian microsome assay, the L 5 1 7 8 Y mouse lymphom a assay, or the micronucleus test, and it is not clear whether the testing conditions were optimal with regard to, for example, temperature or p H (known to be of importance at least in other types of tests involving fungi).
Sex-linked recessive lethal mutations in Drosophila melanogaster: Apparently there is at least one unpublished report (90) on the effects of chlorobenzene in the so-called Drosophila sex-linked recessive lethal test (the S L R L test). T he ability of chlorobenzene to induce sex-linked recessive lethal mutations in postmeiotic germ cells was evaluated in males (wild-type stock, Canton-S) that had been exposed to at least 9,000 p p m chlorobenzene for 4 hr (36,000 ppm-hr) or 10,700 p p m for 3 x 4 hr (128,400 ppm-hr) before the surviving flies were mated with three sets of virgin "Base" females for 72 hr each. There was no indication of any mutagenic effect in any germ cell stage. The original report was not available at the time of the present evaluation and it has consequently not been possible to evaluate the experimental conditions, etc., in great detail.
Forward mutations in m a m m a l i a n cells in vitro: The L 5 1 7 8 Y mouse cell l y m p h o m a assay is a well-established test system w h e n screening for gene mutations in vitro. The test system identifies agents that can induce forward mutations in the thymidine kinase locus (TK-locus). Cultures of L5178Y, clone 3.7.2C, were exposed to chlorobenzene for 4 hr and then cultured for 2 days, before plating in soft agar with or without trifluorothymidine (62). Four experiments were performed without S9 (postmitochondrial supernatant fractions of liver homogenate from male Fischer 344 rats pretreated with Aroclor 1254), and two experiments in the presence of the metabolic activation system. Well established mutagens were included in the test as positive controls, and the solvent ( D M S O ) as a negative control. The dose range varied from 6.25 to 195 fig/ml (without S9) and from 70 to 190 (with S9). The highest concentrations were toxic to the cells. Without S9, two of the four tests yielded inconclusive results, the two others were positive (lowest effective concentration being 100 pg/ml). The two experiments with S9 gave significant and consistent positive responses, showing a mutagenic effect of chloroben zene.
The final draft of the health effect criteria document from E P A (32) and the G e r m a n B U A report (19), also mentioned the results of an unpublished mouse l y m p h o m a L 5 1 7 8 Y cell culture assay from Monsanto 1976 (testing being performed by Litton Bionetics). In contrast to the study referred to above, chlorobenzene was found to lack mutagenic effects . The Monsanto study was not available for evaluation and it has consequently not been possible to judge the significance of the reported "negative" results. The ability of chlorobenzene to induce chromosomal aberrations has been investigated in cultured Chinese hamster ovary cells, CHO-cells, with and without the addition of a metabolic activation system (60). The S9 rat liver microsomal fraction, which was used as metabolic activation system, was prepared from Aroclor-1254-induced male Sprague-Dawley rats. D M S O was used as vehicle and cyclophosphamide (with activation) and mitomycin C (without metabolic activation) as positive controls. Chlorobenzene was tested at the following concentrations: 0, 30, 100, 300, or 500 (without activation), 0, 50, 150, or 510 (experiment one with activation) and 0, 150, 300, or 500 (experiment two with metabolic activation) pg/ml. Approximately 24 hr before the exposure, cultures were initiated at a cell density of 1.75 x 106 cells/flask. In the experiment without activation, the cells were incubated with chlorobenzene for 8 hr, before they were treated with colchemid for 2-2.5 hr before harvest. In the experiments with activation, the cells were incubated with the substance and S9 for 2 hr, then cultivated with me d i u m only for another 8 hr. Colchemid was then added 2 hr before cell harvest. O n e hundred cells were scored for each of three concentrations (the highest test concentration containing sufficient metaphase cells, and two lower concentrations, covering a 1-log range). A n increase of chromatid breaks was seen at one intermediate dose (150 fig/ml) in the first of two trials with S9. However, this effect was not reproducible in the second experiment. N o aberrations were induced in the absence of a metabolic system up to a dose of 500 Jig/ml, a concentration that was found to be clearly toxic to the cells. L D 50 value. The total amount of substance was divided into two equal doses (i.e., 2 x 1 1 2 . 5 to 2 x 450 mg/kg) and given 24 hr apart. Each group consisted of 5 exposed animals. The number of com-oil-treated controls was 10. The frequency of micronucleated polychromatic erythrocytes ( M N P C E ) was recorded 30 hr after the first injection. T w o smears per femur were prepared and coded, and from each bone marrow smear, 1,000 polychromatic erythrocytes were analyzed for the presence of chromosomal fragments. There was a statistically significant and dose-related increase in the number M N P C E in the mice given chlorobenzene when compared to the vehicle-treated controls. N o information was given with regard to the potential bone marrow toxicity of the comp o u n d (i.e., the ratio between the number of normochromatic and polychromatic erythrocytes).
Apart from the two above-mentioned studies, there is also a Russian study available on the cytogenetic effects of chlorobenzene in bone marrow cells from mice (35). In contrast to benzene, chlorobenzene was reported to be without cytogenetic activity. N o effects were seen in a micronucleus test, in a test for chromosomal aberrations in cells arrested in metaphase or in a dominant-lethal test. In each case the doses varied between 3.2 and 400 mg/kg b.wt. The article was written in Russian (apart from a short su m m a r y in English without any information on the number of animals involved, survival times, etc.) and it has consequently not been possible to judge the significance of the reported results.
Chlorobenzene has also been tested and reported negative for induction of chromosomal aberrations in CHO-cells in an EPA-sponsored, unpublished study by Loveday (cited in reference 60). In the latter study, chlorobenzene was tested at lower concentrations than those used in the more recent study presented above.

# Num erical C h r o m o s o m a l Alterations
As early as 1943, Ostergren and Levan reported that chlorobenzene could induce an abnormal mitotic cell-division in a test system based on the onion Allium cepa (98). In a short abstract without details on experimental design, etc., it was stated that full c-mitosis disturbances were observed at a chlorobenzene concentration of 1 m M (precipitate in water); partial disturbances at 0.3 m M (clear aqueous solution); and normal mitosis at 0.1 m M . The authors suggested that the c-mitotic property of chlorobenzene was due to the physical properties of the comp o und and not to its chemical properties.
With the possible exception of the "positive" finding in Allium cepa (the significance of this remains uncertain) no studies were available on the ability of chlorobenzene to induce aneuploidy, polyploidy or nondisjunction (i.e., numerical chromosomal aberrations). H o w ever, the reported increase in the incidence of micronuclei in bone marrow cells from chlorobenzene-exposed mice (65,66), could apart from being interpreted as showing an ability to induce microscopically observable additions, deletions or rearrangement of parts of chromo somes, possibly also be interpreted as showing a chlorobenzene-induced aneuploidization (gain or loss of one or more intact chromosomes).

# Prim ary DNA-Dam age and Binding to DNA
Chemical damage to D N A can be studied by a variety of methods. S o m e techniques are nonspecific, others are limited to specific types of injuries. With regard to the D N A -d a m a g i n g effects of chlorobenzene, only one published study was available in the literature (93). In this study it was reported that chlorobenzene lacked effect on the unscheduled D N A synthesis in a rat hepatocyte DNA-repair test. The so-called hepatocyte/DNA-repair test is a well-established, nonspecific test for D N A damages. A n increased DNA-repair synthesis, measured as an increased incorporation of tritiated thymidine in nondividing cells, seems a general response to various types of D N A damages. Another approach that has been used was to measure the DNA-binding capacity of chlorobenzene, both in vivo and in vitro (20,40,73). Using this procedure, chlorobenzene was reported to interact directly with D N A .
Induction of DNA-repair in m a m m a l i a n cells in vitro: After hepatocytes from adult male F344 rats had been isolated, freshly prepared monolayer cultures were simultaneously exposed to chlorobenzene and 3H-thymidine (93). The exposure time was not clearly stated, but was somewhere in the interval of 5-20 hr. After exposure, the cultures were fixed and the thymidine incorporation was measured autoradiographically. The criteria used for positive response were the following: at least two concentrations must have yielded net nuclear grain counts sig nificantly greater than the concurrently run solvent controls; a positive dose-response relation ship up to toxic concentrations and at least one of the increased grain counts must have been a positive value. Following these criteria, chlorobenzene did not induce DNA-repair synthesis in the primary cultures of rat hepatocytes w h e n given in a concentration up to 9.3 x 10'4 M (highest nontoxic concentration tested; the dose interval was not given).
The final draft of the health effect criteria document from E P A (32) also mentioned an unpublished in vitro study  on the DNA-damaging effects of chloroben zene in a prokaryotic test system. Chlorobenzene was reported to lack D N A -d a m a g i n g effects in the so-called pol A-test, since it was equally toxic to repair-proficient and repair-deficient strains of E. coli w h e n tested at concentrations of 10 or 20 pl/plate.

# Interaction with D N A in vivo and in vitro:
The binding of chlorobenzene and other halogenated hydrocarbons to nucleic acids and proteins was studied in various organs of mice and rats, both in vivo and in vitro (20, 40, 73). In the in vivo experiments, chlorobenzene (20 mCi/mmol) was given in an amount of 127 pCi/kg b.wt. (corresponding to 8.7 pmol/kg b.wt.) to groups of 4 male Wistar rats and 12 adult male B A L B / c mice. Th e animals were sacrificed 22 hr after the injection. D N A , R N A , and proteins were isolated from the livers, kidneys, and lungs. In the in vitro experiments, microsomal and cytosolic fractions were extracted from liver, lungs, and kidneys from male B A L B / c mice and male Wistar rats, pretreated for 2 days with phenobarbital. 14C-labelled chlorobenzene was incubated with necessary co-factors and microsomal proteins + N A D P H , or cytosolic proteins + G S H , for 60-120 min at 37C. Similar experimental designs were employed for the other agents tested (e.g., bromobenzene, 1,2-dichlorobenzene and benzene).
Radioactivity from all compounds tested, including chlorobenzene, was found to bind covalently to the macromolecules in all organs investigated, both in rats and mice, in vivo as well as in vitro. The binding appeared to be mediated by the liver microsomes. Although there were no profound differences in DNA-binding capacity of chlorobenzene between the various organs in vivo, the highest value was observed in the livers from the exposed rats (0.26 pmol/mol DNA-P), giving a covalent binding index of 38. This value has been suggested to be typical for agents with a weak oncogenic potency (78). The relative reactivity, expressed as covalent binding index to rat liver D N A in vivo, decreased in the following order: 1,2-dibromoethane > bromobenzene > 1,2-dichloroethane > chlorobenzene > epichlorohydrine > benzene.
Indirect evidence for the ability of chlorobenzene to interact with D N A has also been presented in a study on the elimination of urinary metabolites in rats given a single i.p. injection of 500 m g chlorobenzene/kg b.wt. (55). L o w levels of a guanine adduct, probably identical with N7-phenylguanine, were found in the urine on days 1 and 2 and between days 4 and 6 after the injection.

# Other Effects on the G enetic Material
In this report, data on sister chromatid exchanges has been treated separately under the heading "Other Effects on the Genetic Material." Representing rearrangements between chromatides within a chromosome (only observable with a special staining technique), S C E s do not constitute true mutations. However, there is a general agreement that there is a close correlation between an increased incidence of S C E s and various types of genotoxic effects. Consequently, sister chromatid exchanges m a y be looked upon as nonspecific indicators of genotoxicity. A s shown in Table 2, chlorobenzene was found to induce S C E s in Chinese hamster ovary cells.
The ability of chlorobenzene to induce S C E s was investigated using cultured Chinese hamster ovary cells, both with and without the addition of a metabolic activation system (60). The activation system used was the S9 rat liver microsomal fraction prepared from Aroclor-1254induced male Sprague-Dawley rats. Chlorobenzene was dissolved in D M S O , which also was used as the negative control. Mitomycin C was used as the positive control in the absence of, and cyclophosphamide in the presence of, the metabolic activation system. Chlorobenzene was tested at the following concentrations: 0, 100, 300, or 999 (experiment one without activation); 0, 100, 300,500, or 1,000 (experiment two without activation) and 0, 30, 100, and 300 (with metabolic activation) |ig/ml. Approximately 24 hr after the cultures had been initiated (1.25 x 106 cells/flask), the m e d i u m was replaced and the cells were exposed to chlorobenzene or the control substances. In the experiments without metabolic activation, the cells were exposed for 2 hr before bromodeoxyuridine was added. The incubation then continued for an additional 24-hr period. The m e d i u m was removed and the cells were rinsed before n e w m e d i u m with bromodeoxyuridine and colchemid was added for an additional 2-hr culture period. A similar design was used in the experiment with S9, but the m e d i u m containing the test substance and the metabolic system was replaced after 2 hr of exposure. After cell harvest and fixation on slides, the cells were stained with Hoechst 33258. Selection of cells for scoring was based on well-spread chromosomes with good morphology. The total number of chromosomes analyzed for S C E s was over 1,000 for each concentration of chlorobenzene.
Chlorobenzene was found to induce a dose-related increase of S C E s in both experiments without S9. Chlorobenzene was reported to be slightly insoluble and toxic at the concentrations that gave the increased incidence of S C E s (in experiment one: 300 and 500 |ug/ml; in experiment two: 500 and 1,000 pg/ml), but there were no significant decreases in the number of M 2 cells. Chlorobenzene did not increase the number of S C E s in the presence of S9 up to a dose of 300 jag/ml (a concentration that was clearly toxic to the cells).
In a review of the genotoxicity of hexachlorobenzene and other chlorinated benzenes (18), it was stated that monochlorobenzene failed to actively induce S C E s in a cultivated h u m a n cell line. Since no reference was given to the original paper, it has been impossible to evaluate and judge the validity of this information. Chlorobenzene has also been tested and reported negative for induction of S C E s in an earlier study on CHO-cells than that reported above. In this E P A sponsored, unpublished study by Loveday (cited in reference 60), chlorobenzene was tested at lower concentrations than those employed in the more recent study presented above.
Chlorobenzene was reported to induce reciprocal recombination in the yeast S. cerevisiae, strain D3. The number of recombinants/105 survivors was increased w h e n chlorobenzene was tested at concentrations of 0.05 or 0.06% in the presence of a metabolic activation system (32). However, since these findings originate from an unpublished report from 1979 by Simmon, Riccio, and Peirce, it has not been possible to evaluate the significance of the reported "positive" finding. N o studies were available on the ability of chlorobenzene to induce other types of genetic effects such as reciprocal exchanges between homologous chromosomes, gene conversion, gene amplification, insertional mutations, etc.

# Cell Transform ation and Tum or Prom otion
Cell transformation tests m a y provide s o m e information of the ability of chemicals to induce neoplastic transformation of cultured somatic cells. These tests do not generally provide any direct information on the molecular mechanisms of action that could be either genotoxic or epigenetic. Chlorobenzene has apparently been tested in such an assay (29). Cultured adult rat liver cells were exposed to various concentrations of chlorobenzene: 0, 0.001, 0.005, 0.05, or 0.01%. The cells were exposed 12 times. Each exposure lasted 16 hr with sufficient time to recover from toxicity between each exposure. It was reported that chlorobenzene induced a low, but definitive, anchorage independency in the cells, indicating an ability of the substance to induce cell transformation in vitro. This study, originating from the American Health Foundation, has apparently not been published. The information was obtained from a con densed abstract in the T S C A T S database and it has consequently not been possible to evaluate the data in great detail.
The ability of chlorobenzene and other halogenated benzenes to promote hepatocarcinogenesis has also been evaluated in a rat liver foci bioassay (45). The end point of the assay (i.e., the occurrence of altered foci of hepatocytes in vivo) is considered to s h o w putative preneoplastic lesions. Male and female Sprague-Dawley rats were subjected to a partial hepatectomy before being given an oral dose of the liver tumor initiator diethylnitrosamine (0.5 mmole/kg b.wt.). One and five weeks after the injection of the carcinogen, groups of rats (5-7 animals) were given an i.p. injection of 0.5 m m o l e chlorobenzene/kg b.wt. (the total amount corresponding to 112 mg/kg). T w o weeks after the final injection, the rats were sacrificed. Pieces of the liver were removed and stained for the presence of T-glutamyltranspeptidase activity (GGT-foci). In contrast to 1,2,4,5-tetrachlorobenzene and hexachlorobenzene, monochlorobenzene was reported to be without tumor promoting activity in male and female rats. However, since the data were presented in a summarized form only, without any indication of having been subjected to statistical analysis, it is difficult to judge the significance of the reported findings. The G G T foci/cm2 was, for example, 0.67  0.31 (mean  S E M ) for male rats given chlorobenzene; 0.17  0.15 for male controls given tricaprylin and 1.20  0.34 for male rats given 1,2,4,5-tetrachlorobenzene (judged "positive").

# CARCINOGENICITY
The potential carcinogenicity of chlorobenzene has been tested in rats and mice but no epidemiological data were available with regard to its carcinogenic effects in humans.

# Anim al Studies
In a 2-year cancer bioassay on male and female F344/N rats and B 6 C 3 F 1 hyurid mice, groups of 50 males and 50 females were given chlorobenzene by gavage, 5 days/week for 103 weeks (51, 68). Rats and female mice were given 0 (com oil; vehicle), 60 or 120 mg/kg b.wt./day; and male mice were given 0, 30, or 60 mg/kg/day. The highest doses used differed by factors of 2-4 from those required to produce severe tissue injury in the previously mentioned subchronic toxicity studies (see p. 24). Also included in the study were 50 untreated animals of each sex and species (untreated controls). The animals were observed daily for mortality, and those animals appearing moribund were sacrificed. Complete necropsies were performed on all animals and a number of tissues were taken for histopathological examination.
The administration of chlorobenzene for 2 years did not significantly affect the body weights of the animals, and there were no overt clinical signs of toxicity. Although the survival rates were slightly reduced in s o m e chlorobenzene-treated groups, a closer analysis showed that this was not due to the compound. The only tumor type found to occur at a statistically significant increased frequency in the chlorobenzene-exposed animals was neoplastic nodules in the livers of male rats in the highest dose group. T he increased incidence was significant by dose-related trend tests, and by pair-wise comparisons between the vehicle controls and the highest dose group. Neoplastic nodules are of a benign nature, and the only hepatocellular carcinomas diagnosed a m o n g the male rats affected two control animals.
The tumor incidences in the male and female mice and in the female rats given chlorobenzene for 2 years did not exceed those in the corresponding vehicle or untreated controls. However, although not being a significant effect, two rare tumor types were also observed in rats given chlorobenzene: transitional-cell papillomas of the urinary bladder (one male in the low dose group, and one male in the high dose group) and a tubular-cell adenoma of the kidney (one female rat in the high dose group). The historical incidences of these tumors in Fischer F344/N rats were, at the time of the study, 0/788 for transitional cell-papilloma of the urinary bladder in com-oil-treated males, and 0/789 for renal tubular-cell adenocarcinoma in female controls given c o m oil.
The conclusion that chlorobenzene caused a slight increase in the frequencies of male rats with neoplastic nodules of the liver has been challenged, mainly for statistical reasons (77). The authors of the cancer study disagreed with most of the criticisms, but said that the increased incidence of benign liver tumors in male rats should be considered only as equivocal evidence of carcinogenicity, not sufficient to conclude that chlorobenzene is a chemical carcinogen (52).
Using their weight of evidence classification scheme (30), the U.S. E P A rated chlorobenzene in Group D: inadequate evidence of carcinogenicity (6, 33).
In a sum m a r y of the results from 86 different 2-year carcinogenicity studies conducted by NTP, Ha seman et al. (41) divided the various studies into four different categories: studies showing carcinogenic effects (43/86), studies with equivocal evidence of carcinogenicity (5/86), studies showing no carcinogenic effects (36/86), and inadequate studies (2/86). The increased incidence of neoplastic nodules in the livers of male rats was regarded as evidence showing carcinogenic effects of chlorobenzene. However, no attempt was m a d e to distinguish between "clear" and "s o m e " evidence of carcinogenicity (these agents were pooled into one group).

# Epidem iological Studies
T w o different surveys of cancer mortality rates for U.S. counties revealed an increased mortality rate from bladder cancer in some northwestern counties in Illinois during the periods 1950- 69 and 1970-79; these surveys resulted in a bladder cancer incidence study in eight of the counties incorporated in that region (61). Eligible cases were those diagnosed with bladder cancer during the period of 1978 and 1985. Age-adjusted standardized incidence ratios (SIRs) were calculated for each county and for the 97 zip codes within these counties. W h e n the data were analyzed, only two zip codes were found to have an elevated risk level, and one of these, with a total population of 13,000 inhabitants in 1980, had a significantly increased risk for bladder cancer in both males (number of cases: 21; SIR: 2.6; 9 5 % confidence interval: 1.1-2.6) and females (number of cases: 10; SIR: 2.6; Cl: 1.2-4.7).
Since it was revealed that there could have been a potential environmental exposure to trichloroethylene, tetrachloroethylene, benzene, and other organic solvents from the drinking water wells used by that community, a follow-up cross-sectional etiologic study was initiated (74). N o risk factors unique to the reported cluster, such as smoking and occupation, could be identified, and the only factor that stood out was the fact that most of the cases had lived in the community for twenty years or more . With regard to the potential environmental exposure to chlorobenzene, this was probably insignificant, since no trace of the comp o u n d was ever found in the community wells themselves, even though it was found in the landfill close to the wells.
There are no case reports or epidemiological studies available concerning the potential carcinogenicity of chlorobenzene in humans.
Data on the potential teratogenicity and reproductive toxicity of chlorobenzene are limited to findings obtained in experimental animals. F r o m these experiments there were no indications of any teratogenic effects. However, there was some evidence of embryotoxicity, but only at doses of chlorobenzene that also affected the adult animal. T he biological consequences of these effects are difficult to interpret. N o adverse effects on reproductive performance or fertility have been observed in animals exposed to chlorobenzene.

# Teratogenicity
Until now, there have been no reports on chlorobenzene-induced adverse effects on h u m a n fetal development available in the literature. T he experimental data on the potential embryotoxicity and teratogenicity of chlorobenzene derives from an inhalation teratology study in rats and rabbits (48). The study, which was performed by D o w Chemicals, also exists in an unpublished version (44). The results have been reported elsewhere in, for example, a review of teratological data on several industrial chemicals (49).
Adult virgin female Fischer F344 rats were mated with adult males of the same strain (48).
Groups of 32 to 33 bred females were then exposed to 0, 75, 210, or  T w o separate experiments were performed with pregnant rabbits. Groups of adult female N e w Zealand White rabbits were artificially inseminated and exposed to 0, 75, 210, or 590 p p m (experiment 1) and to 0, 10, 30, 75, or 590 p p m (experiment 2) chlorobenzene, 6 hr/day from day 6 to day 18 of gestation. Each group consisted of 30 to 32 rabbits. The animals were sacrificed on day 29 of gestation. The same types of fetal observations were m a d e as those described above for the rats. The number of pregnant animals examined varied between 28 and 31. The only evidence of maternal toxicity observed a m o n g the rabbits was a significantly increased incidence of animals with enlarged livers in the two highest dose groups.
In the first experiment, there was a slightly increased incidence of a variety of malformations in all groups examined. A m o n g those were several cases of external and visceral malformations scattered a m o n g the chlorobenzene exposed groups. There was no apparent trend for a dose-related increase in any of the single malformations that occurred, with the possible exception of a low incidence of heart anomalies in the highest dose groups (controls: 0/117; 75 ppm: 0/110; 210ppm: 1/193; and 590ppm: 2/122). With regard to skeletal anomalies, there was a significantly increased incidence of fetuses with an extra thoracic rib in the highest dose group.
In the second experiment, there was a significantly increased incidence of implantations undergoing resorption (showing early embryonic death) in the highest dose group. The percentage of litters containing resorptions was 4 1 % in the control group, 4 8 % in the group exposed to 10 ppm, 5 0 % in the 30 and 75 p p m groups and 6 1 % in the 590 p p m group. The second experiment in rabbits did not show any compound-related increases of any type of malformation. Taken together, the experiments performed on the pregnant rats and rabbits showed some evidence of embryotoxic effects of chlorobenzene at the highest exposure concentration. L O E L with regard to embryotoxicity (delayed skeletal development in rats, an extra rib, and possibly also an increased incidence of early embryonic deaths in rabbits) was 590 p p m (2,714 m g / m 3), an exposure concentration that was found to induce toxic effects in the adult animal. John et al. (49) considered that the absence of significant adverse fetal effects in the pregnant experimental animals was evidence enough to suggest that the T L V (at that time, 75 p p m in the United States) afforded an adequate margin of safety for the unborn h u m a n child. In 1986, a similar attempt to evaluate the prenatal risks following from occupational exposure to various industrial chemicals was m a d e in Ge r m a n y (85). Chlorobenzene was one of eighteen agents considered safe at the occupational exposure limit (at that time, 50 p p m in Germany).

# Reproduction Toxicity
In a final test rule for chlorobenzene, released in July 1986 (31), the U.S. E P A required manufacturers and processors of chlorobenzene to conduct reproductive effects testing of chlorobenzene to elucidate the potential reproductive hazard of the compound. At that time, E P A believed that the information available from general toxicity tests (probably those deriving from IBT, see p. 25) on testicular effects in dogs exposed to chlorobenzene, suggested a potential reproductive hazard in humans.
T o satisfy the need for reproductive effects testing for chlorobenzene, Monsanto C o m p a n y conducted a two-generation reproductive study on rats (67). Groups of 30 male and 30 female Sprague-Dawley C D rats (FO-generation) were exposed to 0, 50,150, or 450 p p m (i.e., 0,230, 690, or 2,070 m g / m 3) chlorobenzene vapor for 10 weeks prior to mating and through mating, gestation, and lactation. The exposure took place 6 hr/day, 7 days/week. A selected number increased incidence of animals with a degeneration of the testicular germinal epithelium a m o n g the FO-males in the highest dose group (bilateral changes), and FI-males in the two highest dose groups (unilateral changes only).
Despite the testicular lesions observed in the male rats of the highest dose groups, there were no chlorobenzene-induced adverse effects on the reproductive performance or fertility of the adult animals. The maternal body weights during the gestation and lactation were comparable with those of the controls, mating and fertility indices were unaffected in both the F O and FI-generation, and the pup and litter survival indices for all exposed groups were comparable with those of the corresponding controls.

# DOSE-EFFECT AND DOSE-RESPONSE RELATIONSHIPS
Tables 5-8 on the following pages summarize the various toxic effects of chlorobenzene. It is suggested that the following effects and dose levels should be taken into consideration w h e n establishing permissible levels of occupational exposure:
(1) The prenarcotic and irritating effects of chlorobenzene (observed in humans exposed to 60 p p m for 3-7 hr).
(2) The clear hepatotoxic effects of chlorobenzene ("lowest" L O E L value reported was 50 p p m in rats exposed for 11 weeks).
(3) The possible hematopoietic toxicity of chlorobenzene (leukopenia was reported in mice exposed to 22 p p m for 3 months).

# A cute Exposure
Table 5 summarizes some data obtained in various acute toxicity studies on experimental animals. The table also includes some information on the acute toxicity of chlorobenzene in humans. However, our knowledge of the acute toxicity of chlorobenzene in humans derives almost exclusively from isolated case reports of poisonings or accidental occupational ex posures, showing that chlorobenzene m a y induce significant CNS-depression (i.e., narcotic effects such as drowsiness, incoordination, and unconsciousness) at high acute dose exposures.
Unfortunately, these reports cannot be used for the establishment of dose-effect relationships, mainly because they do not include any information on the actual levels of exposures.
The critical effect of acute exposure to chlorobenzene vapors appears to be the prenarcotic effects of the substance. A n exposure chamber study on five male volunteers exposed to 60 p p m (275 m g / m 3) for 7 hr (71) showed that these concentrations induced acute subjective symptoms such as drowsiness, headache, irritation to the eyes, and sore throat. A significant decrease in flicker-fusion values, indicating a lowered perception, was observed after 3 hr of exposure to the same concentration of chlorobenzene vapor (71). T he information on the h u m a n recognition odor threshold for chlorobenzene varies, but is probably about 0.68 p p m (i.e., 3.1 m g / m 3) (3).

# Repeated Exposure
The various effects following repeated exposures of chlorobenzene have been summarized in Tables 6-8. S o m e previously cited studies in this report, are not included in these tables. The main reason for this is that they were considered insufficient with regard to information on dose-effect and dose-response relationships, thereby preventing a meaningful evaluation of N O E L and L O E L values. T o get a complete picture of the toxicity of chlorobenzene after repeated exposure, the reader is referred to earlier sections of this document.

# RESEARCH NEEDS
(1) The data on the genotoxic and tumor-promoting effects of chlorobenzene are not consistent. This is an area requiring further research, especially with regard to the reported ability of chlorobenzene (or more likely of some of its metabolites) to bind covalently to D N A .
(2) The structural resemblance between chlorobenzene and benzene, and the reported hematopoietic toxicity of chlorobenzene in experimental animals, call for further studies addressing the potential problem with the chlorobenzene-induced bone marrow (i.e., hematopoietic) toxicity, especially with regard to potential dose-effect and dose-response relationships.
(3) Chlorobenzene has been used in large quantities in industry for several years. However, there is still a paucity of data on actual exposure levels of chlorobenzene in occupational settings today. A survey of the potential exposure to chlorobenzene in relevant industries is therefore recommended.
(4) There are only limited epidemiological data available on the health status of workers chronically exposed to chlorobenzene. Recent data on a limited number of volunteers showed, for example, that exposure to chlorobenzene vapors at previous threshold limit values (e.g., 75 p p m in the United States and 50 p p m in Germany) can induce prenarcotic effects, and animal data s h o w that repeated exposure to chlorobenzene at these levels m a y affect the liver. Information from epidemiological studies examining dose-effect and dose-response relationships, especially with regard to the prenarcotic, hepatotoxic and possibly also hematopoietic effects of chlorobenzene, would be useful.
(5) Further studies should be m a d e to explore and assess the potential risks from the extrahepatic bioactivation of chlorobenzene (e.g., in the nasal mucosa).
According to E X I C H E M (34), an O E C D database on projects on existing chemicals, there are ongoing or planned activities in several countries with regard to the evaluation and assessment of the potential adverse health and environmental effects of chlorobenzene. Most of these activities seem to involve gathering of scientific data on toxicological and ecotoxicological effects, monitoring of environmental levels, and health and/or environmental hazard evalua tions. It m a y be worthwhile to note that chlorobenzene has been designated "future high priority" by L A R C (34).

# DISCUSSION AND EVALUATION OF HEALTH EFFECTS
Chlorobenzene (also k n o w n as monochlorobenzene or benzene chloride) is 1 of 12 possible chemical species in the group of chlorinated benzenes. At room temperature, chlorobenzene is a colorless, volatile liquid with an odor that has been described as almondlike, or like that of mothballs and benzene. Chlorobenzene is hardly soluble in water, but is freely soluble in lipids and various organic solvents.
Chlorobenzene has been used extensively in industry for m a n y years, and its main use is as a solvent and intermediate in the production of other chemicals. In occupational settings, the main exposure is that following inhalation of chlorobenzene vapors.
Once absorbed, chlorobenzene is rapidly distributed to various organs in the body. Highest levels are found in fat, liver, lungs, and kidneys. Chlorobenzene is metabolically activated to two different intermediate electrophilic epoxides by cytochrome P450/P448-dependent microsomal enzymes. Chlorobenzene is not only bioactivated in the liver, but also in other organs and tissues such as the lungs and nasal mucosa. The reactive metabolites of chloroben zene are converted either nonenzymatically to various chlorophenols, or enzymatically to the corresponding glutathione conjugates and dihydrodiol derivatives. The glutathione conjugates are then either eliminated as such, or transferred to even more water-soluble products and excreted in the urine as mercapturic acids. The dihydrodiol derivatives are converted to catechols and excreted as such in the urine. The absolute quantities and ratios between the various metabolites formed differs a m o n g various species.
The major h u m a n urinary metabolites of chlorobenzene are the free and conjugated forms of 4-chlorocatechol and p-chlorophenol. It has been recommended that measurements of these should be used as biological exposure indicators for monitoring occupational exposure. The toxic effects of chlorobenzene in experimental animals are relatively well documented, although m a n y toxicity studies were unpublished reports or written in a language not familiar to the evaluator. N o major data gaps could be identified, and the majority of the identified studies appeared to be of acceptable quality, permitting a meaningful risk identification. The amount of h u m a n data on the toxicity of chlorobenzene is, however, limited.
The acute toxicity of chlorobenzene in experimental animals is relatively low. The lowest acute inhalation L C 50 value identified was 8,800 m g / m 3 (female mice exposed for 6 hr). The acute exposure to high concentrations of chlorobenzene is mainly associated with various CNS-effects. These are generally manifested as initial excitation followed by drowsiness, adynamia, ataxia, paraparesis, paraplegia and dyspnea. Death is generally a result of respiratory paralysis. CNS-depressant effects (drowsiness, incoordination and unconscious ness) have also been observed in humans after acute poisoning or occupational exposure to high concentrations of chlorobenzene. The h u m a n probable oral acute lethal dose of chlorobenzene has been estimated at 0.5-5 g/kg b.wt.
A n exposure chamber study of five male volunteers exposed to 60 p p m (275 m g / m 3) for 7 hr showed that this concentration of chlorobenzene induced acute subjective symptoms such as drowsiness, headache, irritation to the eyes, and sore throat. A significant decrease in flicker-fusion values, indicating lowered perception, was observed after 3 hr of exposure to the same concentration of chlorobenzene vapor. Inhalation of chlorobenzene vapor is irritating to the eyes and the mucous membranes of the upper respiratory tract. Prolonged skin contact m a y lead to mild chemical bums.
Repeated administration of chlorobenzene to experimental animals for several weeks or months, is mainly associated with various effects in the liver and kidneys. These organs are, with the C N S , the primary targets for chlorobenzene-induced toxicity. Th e hepatotoxicity of chlorobenzene is manifested as increased activities of serum liver enzymes, increased liver weight, hepatic porphyria and hepatocellular necrosis. Similar effects have also been observed in a m a n w h o ingested 140 ml of a 9 0 % chlorobenzene solution in a suicide attempt. It m a y be useful to note that there is some evidence from in vitro studies, showing that humans, due to metabolic differences, m a y be more susceptible to the hepatotoxic effects of chlorobenzene than rodents. The nephrotoxic action is mainly manifested as increased kidney weight, focal coagulative degeneration, and necrosis of the proximal tubules.
Chlorobenzene differs from m a n y polychlorinated aromatic hydrocarbons in not being a general inducer of the cytochrome P450/P448 enzyme system. Instead, chlorobenzene appears to lower the cytochrome P450 levels. Since administration of chlorobenzene induces an initial, but transient, depletion of the glutathione levels in the liver, exposure to this c o m p o u n d seems associated with a lowered capacity of both bioactivating and detoxifying enzyme systems.
Repeated administration of chlorobenzene to experimental animals is also associated with lesions of the thymus (lymphoid depletion and necrosis), spleen (lymphoid or myeloid depletion), bone marrow (leukopenia, myeloid depletion, general bone marrow depression), lungs (increased lung weights, necrotic lesions in the bronchial epithelium), and testes (bilateral or unilateral degeneration of the germinal epithelium). O f these effects, the hematopoietic toxicity is of special interest. W h e n male and female mice were exposed to 100 m g / m 3 (22 ppm) of chlorobenzene, 7 hr/day for 3 months, they were reported to develop leukopenia and a general bone marrow depression.
It is generally assumed that the toxic effects of chlorobenzene are mediated by covalent binding of reactive metabolites to critical cell structures in the target organs. However, the exact molecular mechanisms of action behind the various toxic effects of chlorobenzene are still unknown. Several possible toxicological mechanisms m a y be involved. Whereas, for example, the hepatotoxic and nephrotoxic action of chlorobenzene m a y be a direct result of covalent binding to critical structures and/or an indirect effect of oxidative stress, the CNS-depressant effect is most likely mediated by other toxicological mechanisms, probably induced by the unmetabolized substance itself.
It appears as if halogenated aromatic monocyclics form a complex group w h e n it comes to the interpretation of their genotoxicity. In the case of chlorobenzene there is no problem with lack of information. At least 12 different published investigations representing various types of genetic endpoints and/or test systems were identified. Apart from the published information, there are also several unpublished studies mentioned in the present document. Even if some results only are presented as a figure or symbol in a summarizing table, the conceivable problem with condensed presentations of study designs, protocols and results do not seem of major importance in the case of chlorobenzene. There were no obvious differences in study qualities between those reporting absence of genotoxic effects and those showing effects.
The major problem in interpreting the existing genotoxicity data for monochlorobenzene relates to the fact that the compo u n d was reported "negative" in s o m e test systems, and "positive" in others. The interpretation becomes even more complex w h e n one also has to consider that whereas some authors reported that chlorobenzene was genotoxic/mutagenic in a given test system, other investigators reported a "negative" result. In the case of chloroben zene, this seems to be the situation in the L 5 1 7 8 Y mouse cell l y m p h o m a assay, the micro nucleus test, and when measuring S C E s in vitro (at least w h e n one includes unpublished information).
The combination of being positive in an L 5 1 7 8 Y gene mutation assay and in a S C E assay and simultaneously being negative in the A m e s test and in an assay for chromosomal aberrations in C H O cells, is not unique for chlorobenzene (1, 89). However, the mutagenic effect of chlorobenzene observed in the L 5 1 7 8 Y cells, with and without exogenous metabolic activa tion, and its ability to induce sister chromatid exchanges in cultivated Chinese hamster cells in the absence of metabolic activation, are not isolated positive responses. Consequently, chlorobenzene has also been shown to increase the incidence of micronuclei in bone marrow cells of exposed mice in a dose-dependent manner. Although chlorobenzene apparently lacked D N A -d a m a g i n g effects in a rat hepatocyte DNA-repair test, radioactivity from 14C-chlorobenzene was reported to bind covalently directly to D N A in various organs, including the liver. This was shown in both mice and rats, in vivo as well as in vitro. The latter findings suggest that chlorobenzene, or more likely, s o m e of its metabolites, can interfere directly with the DNA-molecule.
The data on DNA-binding should be interpreted with some care because it cannot be excluded that the relatively low levels of DNA-binding is an artifact resulting from protein contamina tion. The reported binding of chlorobenzene-associated radioactivity to nucleic acids deserves particular attention and should be further examined. At present, it is suggested that chlorobenzene should be regarded as an agent capable of inducing a certain degree of DNA-binding after administration of large doses.
Beside the above-mentioned "positive" results from various short-term tests, chlorobenzene has also been reported to induce point mutations in Actinomyces antibioticus 400, abnormal mitotic cell-division in Allium cepa, and reciprocal recombination in Saccharomyces cerevisiae. However, the significance of these results remains unclear for various reasons.
Previously, w h e n the number of available genotoxicity studies was limited, it was suggested that chlorinated benzenes, including chlorobenzene, appeared to lack significant genotoxic properties (18). However, paying attention to more recent findings it m a y be wise to reconsider such a conclusion, or at least to initiate more careful and exhaustive reevaluation of the potential genotoxicity of chlorobenzene. Although not always consistent and clear, the overall data are judged to show "limited evidence of genotoxicity" of chlorobenzene. This judgment is based on the fact that chlorobenzene has been reported "positive" in at least three different test systems measuring mutagenicity, chromosomal anomalies, and D N A damage/DNA-binding, at the same time as the majority of test results were reported as "negative." With regard to the question of h o w potent genotoxic agent chlorobenzene might be, the available "positive" studies showed that its genotoxic potential is low. The effects were generally observed only after administration of relatively high concentrations of chlorobenzene.
The ability of chlorobenzene to induce neoplastic transformation has also been tested with conflicting results. Whereas the comp o u n d was found to induce a low, but definite, anchorageindependency in cultured rat liver cells, it was without activity in a rat liver foci bioassay. The significance of these results remains unclear.
Chlorobenzene induced benign liver tumors in male rats, but was without tumorigenic effects in female rats and male and female mice given the c o m p o u n d by gavage, 5 days/week for 103 weeks (60 or 120 mg/kg b.wt./day). The inadequate/equivocal evidence of carcinogenicity in experimental animals, in combination with the limited evidence of genotoxicity from short-term tests and the absence of epidemiological data, implies that chlorobenzene, at present, should be regarded as an agent not classifiable as to h u m a n carcinogenicity.
Animal experiments on the potential teratogenicity and reproductive toxicity of chlorobenzene did not show any significant teratogenic potential of the compound. However, there was some evidence of embryotoxic effects in both rabbits (skeletal anomalies and an increased incidence of early embryonic deaths) and rats (delayed skeletal development), but these effects were only seen at doses found toxic to the adult animal ( L O E L with regard to embryotoxicity was established to 590 p p m (i.e., 2,714 m g / m 3). A two-generation reproductive toxicity study in rats did not show any chlorobenzene-induced adverse effects on the reproductive performance or fertility.
Apparently, chlorobenzene is without immunotoxic effects in mice after multiple exposures at 75 p p m (345 m g / m 3), and the comp o u n d was reported not to induce skin sensitization in a maximization test on male guinea pigs.
C N S effects (i.e., prenarcotic effects) are judged to be the most critical effects following acute exposure to chlorobenzene vapors. A n exposure chamber study involving five male volunteers exposed to 60 p p m (276 m g / m 3) for up to 7 hr, showed that this relatively low concentration of chlorobenzene vapor resulted in acute subjective symptoms such as drowsiness, headache, irritation of the eyes, and sore throat.
Based on what is presently k n o w n about the various toxic effects of chlorobenzene, the hepatotoxic and nephrotoxic ( L O E L in the most sensitive species after 11 weeks of inhalation was 50 ppm), and possibly also the hematopoietic effects (leukopenia was observed in mice after 3 months of exposure to 22 ppm), are judged to be the most critical effects observed after exposure to chlorobenzene. Consequently, it is on these effects that various threshold limit values should be based.
So far, there is no reliable scientific data showing that oral doses and/or inhalation of air concentrations below the indicated L O E L values would induce other types of significant adverse effects in experimental animals. In the present document, relevant data are summarized and evaluated for the purpose of establishing permissible levels of occupational exposure to chlorobenzene. O f the various effects described, the effects on the central nervous system (prenarcotic effects) of chlorobenzene, together with its hepatotoxic effects, should be considered in setting occupational exposure limits. At present, there is "limited evidence" indicating that chlorobenzene is genotoxic and that it m a y induce hematopoietic toxicity at relatively moderate doses. It is presently not classifiable as to h u m a n carcinogenicity. 105 references.

# SUMMARY
Key-words: Chlorobenzene; occupational exposure limits; C N S effects; hepatotoxicity; genotoxicity, hematopoietic toxicity.
-f the offspring from the FO-generation (30 males and 30 females/group) formed the FI-generation. These animals were exposed to the same concentrations of chlorobenzene as the FO-generation, starting 1 w e e k post-weaning; lasting 11 weeks prior to mating; and through mating, gestation, and lactation. The progeny of the FI-generation, the F2-pups, were observed during weaning, and then they were sacrificed. A number of parameters were investigated, including body weights, food consumption, mating and fertility indices, pup and litter survival, and histopathological examinations of selected organs (liver, kidneys, pituitary gland, and male and female reproductive organs). Chlorobenzene did not significantly affect the body weights or food consumption in any of the generations studied. However, the histopathological examination showed dose-related changes in the livers, kidneys, and testes of F 0 and FI males. T he hepatotoxicity was manifested both as hepatocellular hypertrophy and significantly increased m e a n and absolute liver weights. Th e lowest L O E L for the latter effect was 50 p p m (i.e., 230 m g / m 3), the FO-males being most sensitive.
The renal changes appeared as an increased incidence of animals with tubular dilation with eosinophilic material, interstitial nephritis, and foci of regenerative epithelium. There was an  97) showed that male and female Swiss mice developed leukopenia after having been exposed to 22 ppm (100 mg/m3) chlorobenzene 7 hr/day for 3 months and i t has been reported in secondary sources of information (32, 92) that Varhavskaya observed various types of pathological changes in the bone marrow of male rats given oral doses of 0.01 mg chlorobenzene/kg b.wt./day for 9 months (the significance of the latter study is questionable; such low doses have not induced hematopoietic toxicity in any other study).