B11 Vinyl Chloride

King Lit Wong, Ph.D.

Johnson Space Center Toxicology Group

Biomedical Operations and Research Branch

Houston, Texas

PHYSICAL AND CHEMICAL PROPERTIES

Vinyl chloride is a colorless gas with a high flammability and an odor like that of ether (ACGIH, 1986).

Synonyms:

Chloroethene, chloroethylene

Formula:

CH2CHCl

CAS number:

75-01-4

Molecular weight:

62.5

Boiling point:

−13.9°C

Melting point:

Not applicable

Vapor pressure:

2530 mm Hg at 20°C

Conversion factors at 25°C, 1 atm:

1 ppm = 2.55 mg/m3

1 mg/m3 = 0.39 ppm

OCCURRENCE AND USE

Vinyl chloride is used primarily in the manufacture of polyvinyl chloride resins (ACGIH, 1986). It is also used in organic syntheses. There is no known use of vinyl chloride in spacecraft, and it has never been found in air samples taken during space-shuttle missions. However, vinyl chloride has been predicted to be off-gassed in the space station (Leban and Wagner, 1989).



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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants B11 Vinyl Chloride King Lit Wong, Ph.D. Johnson Space Center Toxicology Group Biomedical Operations and Research Branch Houston, Texas PHYSICAL AND CHEMICAL PROPERTIES Vinyl chloride is a colorless gas with a high flammability and an odor like that of ether (ACGIH, 1986). Synonyms: Chloroethene, chloroethylene Formula: CH2CHCl CAS number: 75-01-4 Molecular weight: 62.5 Boiling point: −13.9°C Melting point: Not applicable Vapor pressure: 2530 mm Hg at 20°C Conversion factors at 25°C, 1 atm: 1 ppm = 2.55 mg/m3 1 mg/m3 = 0.39 ppm OCCURRENCE AND USE Vinyl chloride is used primarily in the manufacture of polyvinyl chloride resins (ACGIH, 1986). It is also used in organic syntheses. There is no known use of vinyl chloride in spacecraft, and it has never been found in air samples taken during space-shuttle missions. However, vinyl chloride has been predicted to be off-gassed in the space station (Leban and Wagner, 1989).

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants PHARMACOKINETICS AND METABOLISM When inhaled, vinyl chloride is absorbed quite well by human subjects. In male human volunteers exposed to 7.5-60 mg/m3 (3-24 ppm) for 6 h, about 42% of the inhaled vinyl chloride was retained by the respiratory system; i.e., the exhaled concentration was 42% less than the inhaled concentration (Krajewski et al., 1980). The degree of respiratory retention achieved a relatively stable level 30 min into the 6-h exposure and appeared not to vary significantly with the exposure concentration, which ranged from 3 to 24 ppm. Rats tend to readily absorb vinyl chloride in inhalation exposure. In rats exposed to 14C-labeled vinyl chloride at 20,000 ppm for 5 min, radioactivity was detected in the liver, bile duct, kidney, and gastrointestinal tract within 10 min (Duprat et al., 1977). Similarly, in rats exposed to 14C-labeled vinyl chloride at 50 or 100 ppm for 5 or 6 h, liver and kidney had the highest concentration of radioactivity among all the tissues after the exposure (Bolt et al., 1976; Watanabe et al., 1976). In rats exposed to vinyl chloride at 1000 ppm for 1-6 h, it has been postulated that vinyl chloride was oxidized to 2-chloroacetaldehyde via three pathways, listed as follows (Hefner et al., 1975). (1) At low concentrations, vinyl chloride is oxidized first to 2-chloroethanol, which is further oxidized by alcohol dehydrogenase to 2-chloroacetaldehyde. (2) When the first pathway is saturated, vinyl chloride is oxidized by mixed function oxidase to an epoxide, 2-chloroethylene oxide, which arranges to 2-chloroacetaldehyde. (3) Alternatively, when the first pathway is saturated, vinyl chloride is oxidized by catalase to 2-chloroethyl hydroperoxide, which in turn is converted to 2-chloroacetaldehyde. Because pretreatment of rats with 6-nitro-1,2,3-benzothiadiazole, which inhibits some microsomal cytochrome P-450 pathways, completely blocked vinyl chloride metabolism in rats exposed to vinyl chloride, Bolt et al. (1977) postulated that microsomal mixed function oxidase is the major enzyme for vinyl chloride metabolism. Most of the 2-chloroacetaldehyde formed from vinyl chloride reacts with sulfhydryl groups in the cells, but some of it is oxidized by aldehyde dehydrogenase to 2-chloroacetic acid (Hefner et al., 1975). The reaction of 2-chloroacetaldehyde with sulfhydryl groups explains the formation of n-acetyl-S-(2-hydroxyethyl) cysteine, S-(2-hydroxyethyl) cysteine, and mercaptoacetic acid in the urine of rats exposed to vinyl chloride (Wata-

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants nabe et al., 1976, 1978a). Urinary mercaptoacetic acid has also been found in workers exposed to vinyl chloride (Shu, 1986). Bolt et al. (1977) exposed rats to vinyl chloride at 50-1200 ppm in a closed inhalation system and they found that vinyl chloride metabolism was saturated at about 250 ppm. Similarly, Buchter et al. (1980) discovered that vinyl-chloride metabolism in rhesus monkeys was saturated at 200 ppm. In rats exposed to vinyl chloride at 10, 1000, or 5000 ppm for 6 h, Watanabe's group presented evidence that vinyl chloride metabolism appeared to be saturated at 1000 ppm (Watanabe et al., 1976, 1978a; Watanabe and Gehring, 1976). At 10 ppm, 68% of the body burden was excreted in the urine, 12% was expired as CO2, 4% was excreted in the feces, and about 2% was expired unchanged (Watanabe et al., 1976; Watanabe and Gehring, 1976). When the exposure level was increased to 1000 ppm, however, the fraction of the body burden eliminated in the urine was reduced to 56% and that expired as vinyl chloride increased to 12%. There is evidence that vinyl chloride's epoxide metabolite, 2-chloroethylene oxide, and its rearrangement product, 2-chloroacetaldehyde, could bind to macromolecules in rats. Watanabe and his colleagues showed that, in rats exposed to vinyl chloride, the amount of macromolecular binding directly increased with the exposure concentration or phenobarbital pretreatment (Watanabe et al., 1978a,b; Guengerich and Watanabe, 1979). It has been postulated that the epoxide metabolite is the active metabolite of vinyl chloride and the macromolecular binding is responsible for vinyl chloride 's carcinogenicity. There are few data on vinyl chloride metabolism in humans. However, it has been shown that incubation of Salmonella typhimurium with S-9 fraction isolated from human liver increased the mutational frequency in a similar magnitude as incubation with rat S-9 fraction (Sabadie et al., 1980). This indicates that electrophilic metabolites could be formed by the action of human mixed function oxidase on vinyl chloride. TOXICITY SUMMARY Acute and Short-Term Toxicity Mucosal Irritation Vinyl chloride is known to cause mucosal irritation at over 500 ppm in

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants humans (Lefaux, 1968). Baretta et al. (1969) reported that two of seven human subjects complained of dryness of nose and eyes in an exposure to 500-ppm vinyl chloride lasting for 3.5 h. The dryness of nose and eyes was probably an indication of very slight mucosal irritation. It appears that 500 ppm is probably the threshold for vinyl chloride's mucosal irritation. Since none of the four subjects exposed to 250 ppm for 7.5 h complained of dryness of nose or eyes (Baretta et al., 1969), the no-observed-adverse-effect level (NOAEL) for mucosal irritation is 250 ppm. Miscellaneous Symptoms In a study by Lester et al. (1963) nausea was reported in five of six human subjects exposed to vinyl chloride at 16,000 ppm for 5 min. In that study, one of six human subjects exposed to vinyl chloride at 20,000 ppm for 5 min experienced headache, which lasted for 30 min. Baretta et al. (1969) reported that two of seven men exposed to 500 ppm for 3.5 h complained of mild headache, but they did not complain of nausea. Therefore, the concentration of vinyl chloride causing headache is lower than that causing nausea. Because nausea was detected only at such a high exposure concentration (it was absent in a 5-min exposure at 12,000 ppm or less), nausea will not be relied on in setting the SMACs. None of the four men in the study of Baretta et al. (1969) complained of headache in a 7.5-h exposure to 250-ppm vinyl chloride. The NOAEL for headache is 250 ppm in acute vinyl chloride exposures. Liver Toxicity As will be discussed later, liver is the major target organ of vinyl chloride in subchronic and long-term exposures. Whether vinyl chloride could cause non-neoplastic liver toxicity in humans in acute exposures is debatable. A group of Hungarian scientists exposed mice, rats, and rabbits to vinyl chloride at 1500 ppm for 24 h (Tatrai and Ungvary, 1981). No liver pathology was found in rats and rabbits. However, in mice, vasomotor paralysis and shock developed during the exposure, followed by hepatic histopathology, which included coagulation necrosis and confluent hemorrhages in the centrilobular zone, and ultrastructural changes, such as dilation of the cisterns of rough endoplasmic reticulum and Golgi apparatus

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants and atrophy of some mitochondria. All the mice died as a result of the 24-h exposure at 1500 ppm, whereas a 12-h exposure killed 16 of 20 mice. In contrast, no rats or rabbits were killed by the 24-h exposure (Tatrai and Ungvary, 1981). In a human study, Baretta et al. (1969) failed to detect any changes in the serum levels of glutamyl pyruvate transaminase, alkaline phosphatase, lactic dehydrogenase, bilirubin, blood urea nitrogen, and creatinine in four men 24 h after a 7.5-h exposure to vinyl chloride at 500 or 250 ppm. Therefore, if vinyl chloride is indeed acutely toxic to the liver in humans, 500 ppm appears to be the NOAEL. Central Nervous System Toxicity Vinyl chloride could impair the central nervous system (CNS). Lester et al. (1963) reported that light-headedness, dizziness, and dulling of vision and hearing were detected in five of six human subjects exposed at 16,000 ppm for 5 min. When these six subjects were exposed at 12,000 ppm for 5 min, two subjects felt slight dizziness. At 8000 ppm, only one of six subjects felt light-headed. No CNS symptoms were detected in a 5-min exposure at 4000 ppm (Lester et al., 1963). According to Lefaux (1968), vinyl chloride at 1000 ppm produces drowsiness, slight visual disturbances, tingling sensation on the limbs, numbness, and faltering gait. Vinyl chloride has no perceptible action on the CNS below 1000 ppm (Lefaux, 1968). Baretta et al. (1969) reported no CNS impairment in seven subjects exposed to vinyl chloride at 500 ppm for 3.5 h or in four subjects exposed for 7.5 h. The NOAEL for acute CNS impairment is, therefore, 500 ppm. Mortality A person was reported killed by a massive exposure to vinyl chloride at an unknown concentration (Damziger, 1966). Although the lethal concentration of vinyl chloride in humans is not known for certain, it is probably over 10,000 ppm based on the animal data of Mastromatteo et al. (1960). This group of scientists exposed five mice, five rats, and five guinea pigs to various concentrations of vinyl chloride for 30 min. No deaths occurred at 10,000 ppm. One of five mice died but no rats or guinea pigs died at 20,000 ppm. At 30,000 ppm, all five mice and all five rats died, and one of the five guinea pigs died. Because the lethal concentration of vinyl

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants chloride is estimated to be much higher than the concentrations required to cause other toxic effects, mortality is not used as a toxic end point in deriving the SMACs. Subchronic and Chronic Toxicity Liver Toxicity Liver is the major target organ of vinyl chloride. Liver function impairment and hepatic histological changes have been reported in workers employed in places where vinyl chloride was manufactured or used (Lillis et al., 1975; Popper and Thomas, 1975; Tamburro et al., 1984). Clinically, occupational exposure to vinyl chloride might cause abdominal pain in the upper right-hand quadrant, hepatomegaly, portal hypertension, esophageal varices, and liver cirrhosis (Lillis et al., 1975; Popper and Thomas, 1975; Lee et al., 1977). No exposure-concentration data were given in these reports. In a study conducted with 168 workers in two Romanian factories where they were exposed to vinyl chloride from 1962 to 1972, the investigators compared, among other things, the rates of nervous symptoms and gastroenterological symptoms in the workers in 1962 with those in 1966 (Suciu et al., 1975). Without specifying the analytical method, the investigators reported that the average vinyl chloride concentrations in 1962 and 1966 were 2298 and 98 mg/m3 (896 and 38 ppm), respectively (Suciu et al., 1975). They detected a higher rate of euphoria, dizziness, somnolence, nervousness, headache, complete narcosis, weight loss, anorexia, epigastric pains, and hepatomegaly in the year the workers were exposed to 896 ppm than in the year they were exposed to 38 ppm (Suciu et al., 1975). The rate of pains in the right hypochondrium was lower, however, at 896 ppm than at 38 ppm. Because no control group was used and because of a lack of information on how the exposure concentrations were determined, the Romanian data are not used in setting SMACs. Nevertheless, the data illustrate the potential toxicity of vinyl chloride in the CNS and liver in human workers. Kramer and Mutchler (1972) did a medical study with 98 workers employed over two decades in two vinyl chloride polymerization facilities. These workers were exposed mainly to vinyl chloride, but there were co-exposures to vinylidene chloride, which was at lower concentrations.

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants The exposures to vinylidene chloride in the second decade of the occupational exposure were probably negligible because vinylidene chloride was frequently detected only at trace levels. During the two decades of employment, there were environmental monitoring and medical surveillance. The investigators did not report all the results of the environmental monitoring during the two decades, but they stated that “in more recent measurements, infrared and gas chromatographic techniques have established that the vinyl chloride concentrations average 10 ppm.” When compared with a control group, the vinyl-chloride workers had no significant disease. There were no differences in chest x-rays and electrocardiograms between the two groups. No acroosteolysis was detected. Based on the medical history taken periodically in the medical surveillance program, the exposed workers reported a higher history of asthma and kidney stone and blood urine but a lower history of gastrointestinal and hepatic trouble and nervous symptoms. By physical examinations, the investigators found a higher rate of anal and rectal abnormalities in the exposed workers than in the controls. From a regression analysis of the data collected, the investigators estimated that, in 60-y-old workers who had been on the job for 20 y, vinyl chloride at 300 ppm time-weighted average (TWA) would increase the bromsulphalein clearance time by five-fold, 150 ppm would raise it by two-fold, and 50 ppm would increase it by 80%. They concluded that repeated exposure to vinyl chloride at 300 ppm or higher for a working lifetime could cause some impairment in liver function. In a study by Ho et al. (1991) 12 of over 100 workers in a polyvinyl chloride plant in Singapore were found to have elevated serum glutamic pyruvic transaminase and gamma glutamyl transpeptidase levels, when they worked in an environment with the vinyl chloride concentrations ranging between 1 and 21 ppm, with a geometric mean of 6 ppm for 1-13 y. Nine of the 12 workers had mild-to-moderate nonspecific fatty changes on liver biopsies. None of them had a history of jaundice, Raynaud's disease, or blood transfusion. No liver function impairment attributable to vinyl chloride was detected in the workers after the vinyl chloride was lowered to 0.6-2.9 ppm, with a geometric mean of 1.5 ppm, in 1983. In workers afflicted by vinyl-chloride-induced liver disease, their liver function improved within 0.5-2 y after they were removed from further exposure (Ho et al., 1991). Based on the human data of Ho et al., the NOAEL for non-neoplastic liver toxicity is about 1.5 ppm. Lee et al. (1977) showed that increased cell turnover and DNA synthesis were detected in the livers of rats exposed to vinyl chloride at 50 or 250

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants ppm, 6 h/d, 5 d/w for 12 mo. Because increased cell turnover and DNA synthesis, by themselves, are not considered adverse clinically, the SMACs are not set to prevent them. Similarly, increased liver weight by itself is not considered a significant toxic end point. So the SMACs are not derived from the discovery made by Bi et al. (1985) that the liver weight increased in rats exposed to vinyl chloride at 10, 100, or 3000 ppm, 6 h/d, 6 d/w for 6 mo. Torkelson et al. (1961) found that there were species differences in the sensitivity to vinyl chloride's liver toxicity. In a 6-mo exposure, at 7 h/d, 5 d/w, of guinea pigs, rats, and rabbits to 200-ppm vinyl chloride, no changes were seen in guinea pigs, but increased liver weight was detected in rats and the liver in rabbits developed centrilobular degeneration and necrosis (Torkelson et al., 1961). Therefore, the rat is more sensitive than the guinea pig. However, it is not clear whether the rabbit is more sensitive than the rat because only three male and three female rabbits were used in the experiment, making it difficult to draw a conclusion. In the study of Torkelson et al. (1961) a 4.5-mo exposure of rats at 500 ppm resulted in centrilobular degeneration in liver. A similar exposure of rats at 100 or 200 ppm led to increased liver weight, but an exposure at 50 ppm failed to cause any significant changes. There are reports of liver toxicity in animals subchronically exposed to very high concentrations of vinyl chloride. The group of Feron showed that an exposure of rats to 5000 ppm, 7 h/d, 5 d/w for 52 w produced degeneration, hyperplasia, hepatocellular carcinoma, and angiosarcoma in the liver (Feron and Kroes, 1979). Viola (1970) discovered hepatomegaly, hepatitis, and liver necrosis in male rats exposed to vinyl chloride at 30,000 ppm for 4 h/d, 5 d/w for 1 y. Kidney Toxicity Kidney is the second major organ affected by vinyl chloride. At a very high concentration of 30,000 ppm, Viola (1970) found that vinyl chloride produced tubular nephrosis and chronic interstitial nephritis in rats exposed 4 h/d, 5 d/w for 1 y. Tubular nephrosis was also produced in rats exposed to vinyl chloride at 5000 ppm for 7 h/d, 5 d/w for 1 y (Feron and Kroes, 1979). Torkelson et al. (1961) discovered that 500-ppm vinyl chloride could cause histopathology in the interstitial and tubular areas of the kidney

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants in rats exposed 7 h/d, 5 d/w for 4.5 mo. Because a similar exposure of rats at 200 ppm failed to produce any histological changes (Torkelson et al., 1961), the NOAEL for kidney toxicity is 200 ppm. Neurological Toxicity Psychiatric disease and mild distal axonal neuropathy have been reported in workers exposed to vinyl chloride repetitively at unknown concentrations (Halama et al., 1985; Perticonti et al., 1986). Since the exposure concentrations were not measured, these human data cannot be used to set the SMACs. In the experiment conducted by Viola (1970) an exposure to vinyl chloride at 30,000 ppm for 4 h/d, 5 d/w for 1 y led to diffuse degeneration of the white and gray matter in the brain and atrophy of granular cells in the cerebellum in rats. Because Feron and Kroes (1979) showed that 5000 ppm failed to cause any nonneoplastic injuries in the brain of rats exposed 7 h/d, 5 d/w for 1 y, the NOAEL for brain toxicity is 5000 ppm. Effects on the Extremities Occupational exposures to vinyl chloride are known to cause circulatory disturbance in the extremities, Raynaud's disease (Lillis et al., 1975; Preston et al., 1976), acroosteolysis, and scleroderma (Dinman et al., 1971; Wilson et al., 1967; Sakabe, 1975). Unfortunately, the exposure concentrations at which these effects were seen in workers are not known. Because Raynaud's disease was usually detected before acroosteolysis in vinyl-chloride workers, vascular lesion is believed to precede the bone changes (Dodson and Dinman, 1971). Viola (1970) reported that an exposure of rats at 30,000 ppm for 4 h/d, 5 d/w for 1 y resulted in pathology in the paws, such as metaplasia of metatarsal bones, chondroid metaplasia, epidermal edema, epidermal hyperkeratosis, and degeneration of basal cells. These pathological changes in rats somewhat resemble the acroosteolysis and scleroderma seen in vinyl-chloride workers. Because there are no concentration-response data and 30,000 ppm is a very high concentration, Viola's data on the paws of rats are not suitable for setting the SMACs.

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants Effects on the Respiratory System In an epidemiology study performed by Wong et al. (1991), a significant mortality excess from emphysema and chronic obstructive pulmonary disease was found in vinyl-chloride workers. Since emphysema and chronic obstructive pulmonary disease have not been found to be associated with occupational vinyl chloride exposures in other epidemiology studies, it is uncertain whether vinyl chloride causes emphysema or chronic obstructive pulmonary disease in humans. However, there is some evidence of the pulmonary toxicity of vinyl chloride in animals. Suzuki (1978, 1980) exposed male mice to vinyl chloride at 2500 or 6000 ppm for 5 h/d, 5 d/w for 5 or 6 mo. In the exposed mice, he found hyperplasia of the alveolar epithelium, degeneration of the alveolar septal cells, hypertrophy and hyperplasia of Clara cells and ciliated epithelial cells in the terminal bronchioles, and bronchiolitis. It appears that the lowest-observed-effect level (LOEL) for lung toxicity is 2500 ppm. Taken together, the epidemiology data of Wong et al. and Suzuki's data in mice show that vinyl chloride might produce lung injuries in humans, so the SMACs are prudently set to prevent this toxic end point. Effects on the Reproductive System There were two Soviet reports on the reproductive effects of vinyl chloride in workers (Makarov, 1984; Makarov et al., 1984). A decline in sexual function, which was evaluated by questionnaire, was found in men and women exposed to vinyl chloride occupationally. In the exposed women workers, gynecological examinations revealed increased incidences of ovarian dysfunction, benign uterine growths, and prolapsed genital organs. The Agency for Toxic Substances and Disease Registry characterized the two reports as “not adequately reported for proper evaluation; therefore, such data cannot be used to identify thresholds ” (ATSDR, 1989). So the SMACs are not set relying on the Soviet data. Instead the animal data of Bi et al. (1985) are used. In the study of Bi et al., 74 or 75 male rats were exposed to vinyl chloride at 0, 10, 100, or 3000 ppm, 6 h/d, 6 d/w for 1 y. Sacrifices were made of 8, 30, 6, and 10 rats at the 3rd, 6th, 9th, and 12th mo, respectively, and the surviving rats were killed 6 mo after the 12-mo exposure. Testicular histology was evaluated in the rats sacrificed and also in rats that

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants died before the interim and final sacrifices. A reduction in the testicular-to-body-weight ratio was found in the 100- and 3000-ppm groups after 6 mo of exposure. With the rats killed at different time points taken together, Bi et al. (1985) reported a statistically higher rate of testicular injuries in the 100- and 3000-ppm groups, but not in the 10-ppm group. There was fusion of spermatids or spermatocytes into giant cells. Spermatids disappeared first, followed by sloughing of secondary and primary spermatocytes into the lumen of seminiferous tubules, leaving behind spermatogonia and Sertoli cells. The degeneration and necrosis distributed randomly in the testis without any relationship to the vascular system. Carcinogenicity Vinyl chloride was found to cause liver cancers, especially angiosarcoma, in vinyl-chloride workers in the 1970s (Health et al., 1975; Tabershaw and Gaffey, 1974; Nicholson et al., 1975; Fox and Collier, 1977). An epidemiology study showed that the mortality excess from liver cancers increased with duration of employment (Health et al., 1975). Other than liver cancers, there have been epidemiological reports that vinyl-chloride exposures may produce cancer in other tissues. For instance, Heldaas et al. (1987) observed 6 cases of malignant melanoma in 454 vinyl-chloride and polyvinyl-chloride workers in Norway, where only 1.1 cases were expected. Nevertheless, Heldaas et al. admitted that it was difficult to make a solid conclusion on the causality between vinyl chloride and malignant melanoma. More recently, there have been reports of a multiplant cohort study in the United States by Wong et al. (1991) and one in Europe by Simonato et al. (1991). Both the U.S. and European studies confirmed the findings of earlier epidemiology studies on the excesses of liver cancers in general and angiosarcoma in particular caused by occupational exposures to vinyl chloride (Wong et al., 1991; Simonato et al., 1991). Both studies also found an increase in brain tumors in vinyl-chloride workers. In addition, an increase in biliary-tract cancers was discovered in the U.S. study and an increase in lymphoma was found in the European study. In the European study, the mortality excess from liver cancers was found to be related to time since initial exposure to vinyl chloride, duration of employment, and estimated exposure levels (Simonato et al., 1991).

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants = 500 ppm × (square root of 4)/10 × 7.5 h/24 h = 30 ppm. Based on the occupational data of Ho et al. (1991), liver dysfunction is possible when the vinyl chloride concentration in the workplace averages 6 ppm and no liver dysfunction would be found at about 1.5 ppm. It appears that the NOAEL for non-neoplastic liver toxicity is about 1.5 ppm in occupational exposure. The NOAEL of 1.5 ppm is based on data from over 100 workers exposed to vinyl chloride for at least 1 y since 1983 (Ho et al., 1991). For simplicity sake, the NOAEL is assumed to be based on a 1-y occupational exposure. Because this type of liver dysfunction is believed to be reparable, a NOAEL for a 1-y occupational exposure ought to be devoid of liver toxicity for 7, 30, or 180 d. 7-d, 30-d, and 180-d ACs based on non-neoplastic liver toxicity = 1-y NOAEL = 1.5 ppm. Kidney Toxicity As discussed in “Toxicity Summary,” the NOAEL for non-neoplastic kidney toxicity is 200 ppm, based on data from a 6-mo exposure of rats (Torkelson et al., 1961). 7-d and 30-d ACs based on kidney toxicity = 6-mo NOAEL × 1/species factor = 200 ppm × 1/10 = 20 ppm. Because vinyl chloride's kidney injuries are believed to be reparable, the 180-d AC is set to equal the 30-d AC of 20 ppm. Since kidney injuries have never been reported in acute vinyl chloride studies, no 1-h and 24-h ACs are needed for this end point. Lung Toxicity Vinyl chloride has been shown to produce non-neoplastic lung injuries in

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants mice (Suzuki, 1978, 1980). The LOAEL based on a 6-mo exposure of mice is 2500 ppm (Suzuki, 1978, 1980), so the NOAEL is estimated to be 250 ppm. 7-d and 30-d ACs based on lung toxicity = 6-mo NOAEL × 1/species factor = 250ppm × 1/10 = 25 ppm. 180-d AC based on lung toxicity = 6-mo NOAEL × time adjustment × 1/species factor = 250 ppm × (5 h/d × 5 d/w × 26 w)/(24 h/d × 180d) × 1/10 = 250 ppm × 650 h/4320 h × 1/10 = 250 ppm × 0.15 × 1/10 = 4 ppm. No 1-h and 24-h ACs are needed because vinyl chloride is not known to cause lung toxicity acutely. Testicular Toxicity According to the data of Bi et al. (1985), vinyl chloride is known to cause testicular injuries in rats in long-term exposures. A reduction in testicular weight was noted in rats exposed to vinyl chloride at 100 or 3000 ppm for 6 h/d, 6 d/w for 6 mo. Bi et al. expressed the pathology data by combining the histopathological data of rats sacrificed after a 3-, 6-, 9-, or 12-mo exposure to vinyl chloride at 0, 10, 100, or 3000 ppm. They found that exposures to 100 or 3000 ppm produced a higher percent of rats with fusion of cells and degeneration of seminiferous tubules in the testis than the control. The NOAEL was 10 ppm. Since the number of rats sacrificed after 6 mo of exposure approximately equaled the combined number of rats sacrificed immediately after a 3-, 9-, or 12-mo exposure, the NOAEL of 10 ppm is assumed to represent a NOAEL based on a 6-mo exposure. 7-d and 30-d ACs based on testicular toxicity = 6-mo NOAEL × 1/species factor = 10 ppm × 1/10 = 1 ppm.

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants Because the cell types that could be injured by vinyl chloride are spermatids and spermatocytes, vinyl chloride's testicular injuries are believed to be reversible. As a result, the 180-d AC is set to equal the 30-d AC. 180-d AC based on testicular toxicity = 30-d AC = 1 ppm. Because there is no evidence that acute vinyl chloride exposures are toxic to the testis, the 1-h and 24-h ACs are not derived. Carcinogenicity Vinyl chloride exposures could lead to the production of tumors in several organs, especially in the liver. Based on the rat data from Maltoni and his colleagues (Maltoni, 1977), the U.S. Environmental Protection Agency, using the linearized multistage model, estimated that a life-time exposure of humans at 1 ppm has a tumor risk of 6.80 × 10−3 (EPA, 1984). The life-time exposure concentration that would yield a 10−4 tumor risk, which is the tumor risk accepted by NASA, is calculated as follows: Life-time exposure concentration that would generate a 10−4 tumor risk = (1 ppm/6.80 × 10−3) × 10−4 = 0.0147 ppm. This life-time exposure concentration is converted to the ACs using the Crump and Howe approach as suggested by the NRC's Committee on Toxicology (NRC, 1992; Crump and Howe, 1984). Setting k = 3, t = 25,550 d, and s1 = 10,950 d, the adjustment factor is calculated to be 26,082 for estimating a near-instantaneous exposure level that would yield the same excess tumor risk as a continuous life-time exposure. 24-h AC based on carcinogenicity = 0.0147 ppm × 26,082 = 380 ppm. For the 7-d, 30-d, and 180-d ACs based on carcinogenicity, the adjustment

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants factors are 3728, 871, and 146.7, respectively, assuming k = 3, t = 25,550 d, and the earliest age of exposure to be 30 y. 7-d AC based on carcinogenicity = 0.0147 ppm × 3728 = 55 ppm. 30-d AC based on carcinogenicity = 0.0147 ppm × 871 = 13 ppm. 180-d AC based on carcinogenicity = 0.0147 ppm × 146.7 = 2 ppm. Establishment of SMACs By selecting the lowest ACs among the various toxic end points for an exposure duration, the 1-h, 24-h, 7-d, 30-d, and 180-d SMACs are set at 130, 30, 1, 1, and 1 ppm, respectively. Because these toxic end points are not expected to be affected by any microgravity-induced physiological changes, the SMACs are not adjusted any further. TABLE 11-5 Acceptable Concentrations   Acceptable Concentration, ppm     Toxic End Point 1 h 24 h 7 d 30 d 180 d Mucosal irritation 500 500 50 50 50 Headache 130 50 — — — CNS impairment 130 30 — — — Liver toxicity 130 30 1.5 1.5 1.5 Kidney toxicity — — 20 20 20 Lung toxicity — — 25 25 4 Testicular toxicity — — 1 1 1 Carcinogenicity — 380 55 130 2 SMAC 130 30 1 1 1

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