Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 2 (1996)

King Lit Wong, Ph.D.

Johnson Space Center Toxicology Group

Biomedical Operations and Research Branch

Houston, Texas

Methylene chloride is a volatile and colorless liquid (ACGIH, 1986). Its vapor is not flammable or explosive (Merck, 1989).

Methylene chloride is a widely used solvent (NTP, 1986). Examples of its use are as a paint remover and a degreasing agent. There is no known use of methylene chloride in spacecraft, but methylene chloride has been shown to off-gas in space shuttles reaching typically 0.1 ppm in a few days (NASA, 1989).

The blood equilibrates with inhaled methylene chloride sooner at rest than at exercise. DiVincenzo et al. (1972) showed that the methylene chloride concentration in blood in 11 human volunteers, who exercised during one-third of the exposure duration, did not plateau 2 h into an exposure to methylene chloride at a concentration of 100 or 200 ppm. Similarly, in a study conducted by Astrand et al. (1975) with five human subjects who were exposed to methylene chloride at 500 ppm for 2 h, with the first 30 min at rest, followed by 30 min of exercise at a 50-watt workload, 30 min of exercise at a 100-watt workload, and 30 min of exercise at a 150-watt workload, both the arterial and venous concentrations of methylene chloride did not reach a plateau in 2 h (Astrand et. al., 1975). In contrast, DiVincenzo and Kaplan (1981) showed that the methylene chloride concentration in venous blood reached a plateau in 2 h during a 7.5-h exposure of four to six sedentary human volunteers to methylene chloride at 50-150 ppm. However, when exposed to methylene chloride at 200 ppm, the blood concentration failed to plateau in 7.5 h (DiVincenzo and Kaplan, 1981).

Experiments demonstrated that methylene chloride is quite well absorbed during inhalation exposures. DiVincenzo et al. (1972) reported that methylene chloride vapor was rapidly absorbed by the lung during the first few minutes of exposure of 11 human volunteers to methylene chloride at 100 or 200 ppm. Astrand et al. (1975) showed that human subjects at rest absorbed 55 % of the amount of methylene chloride inhaled in a 30-min exposure at 250 or 500 ppm. The absorption decreased to 40% when the subjects were working at a load of 50 watts, which is equivalent to light exercise (Astrand et al., 1975). In a study conducted by DiVincenzo and Kaplan (1981), up to 70% of the methylene chloride inhaled in a 7.5-h exposure at 50-200 ppm was absorbed by resting human subjects.

In rats, methylene chloride is distributed, after a 1-h exposure at 560

ppm, mainly to white adipose tissue (Carlsson and Hultengren, 1975). The tissues, ranked according to methylene chloride concentrations in decreasing order, are white adipose tissue, liver, kidneys, and brain.

Methylene chloride is metabolized by two enzyme pathways in rodents (Kubic et al., 1974; Ahmed and Anders, 1978). The glutathione transferase pathway metabolizes methylene chloride into hydrogen chloride, formaldehyde, and carbon dioxide. Methylene chloride is also metabolized by the cytochrome P-450 system into hydrogen chloride, carbon monoxide, and carbon dioxide. McKenna et al. (1982) showed that metabolism of methylene chloride was saturable in rats; the percentage that was metabolized in 48 h after a 6-h methylene chloride exposure decreased from 95 % to 69% to 45% as the exposure concentration increased from 50 ppm to 500 ppm to 1500 ppm, respectively. McKenna et al. reported that the major metabolites of methylene chloride in rats were carbon monoxide and carbon dioxide, which were exhaled.

DiVincenzo and Kaplan (1981) showed that, in a 7.5-h exposure of four to six sedentary human subjects to methylene chloride at 50-200 ppm, about 30% of the absorbed methylene chloride was converted into carbon monoxide, leading to a carboxyhemoglobin (COHb) concentration of 1.9-6.8% in blood. Even though the methylene chloride concentration in blood was approaching a plateau 2 h into the exposure at 50-150 ppm, the increase in COHb concentration did not slow down in the same period. According to Stewart et al. (1972), formation of 2.6-8% COHb in blood occurred in 11 men after a 1-2-h inhalation exposure to methylene chloride at 515-986 ppm.

DiVincenzo and Kaplan (1981) reported that, after a 7.5-h methylene chloride exposure at 50-200 ppm in four to six human subjects, less than 5 % of the absorbed methylene chloride was excreted unchanged in the expired air, and 25-34% was excreted as carbon monoxide during

and after the exposure. DiVincenzo et al. (1972) showed that methylene chloride's blood concentration follows a bi-exponential decay in humans. The first phase of the decay is very rapid, followed by a slower phase with a half-life of about 40 min. A physiologically based pharmacokinetic model has been developed by Andersen et al. (1987). The model's predictions of the blood concentration of methylene chloride in mice, rats, hamsters, and humans agreed quite well with experimental data. Peterson (1978) also modeled the uptake, metabolism, excretion of methylene chloride in man. The model was used to predict the exhaled concentration of methylene chloride and the blood COHb concentration after an acute methylene chloride exposure.

After a methylene chloride exposure ends, the COHb concentration in blood might continue to rise, depending on the length of the exposure. In two studies in which humans were exposed to methylene chloride at 250-986 ppm for 1 or 2 h, the COHb concentration rose an average of 33% within 1 or 2 h after the exposure ended and then decreased with time (Stewart et al., 1972; Astrand et al., 1975) This suggests that, after the 1-2 h exposure, methylene chloride is released from some of the tissues and metabolized into carbon monoxide, leading to a temporary accumulation of COHb in blood. It is interesting that such a phenomenon does not occur in longer methylene chloride exposures. Serial samplings failed to demonstrate any further increase in COHb concentrations after a 7.5-8-h exposure of methylene chloride in two human studies (DiVincenzo et al., 1981; Andersen et al., 1987).

Acute exposures to methylene chloride are known to adversely affect the central nervous system (CNS) and the liver. These adverse effects are summarized below.

Because carbon monoxide is one of methylene chloride's metabolites,

the acute toxicity of methylene chloride resembles that of carbon monoxide. Putz et al. (1979) showed that a 4-h exposure to methylene chloride at 200 ppm, resulting in 5% COHb in the fourth hour, impaired the hand-eye coordination and increased the reaction time in 12 human volunteers. In the same study, these CNS effects were reproduced by a carbon monoxide exposure that yielded 5 % COHb.

Acute methylene chloride exposures could impair vigilance performance in humans. In the 4-h exposure of 12 human volunteers to methylene chloride at 200 ppm conducted by Putz et al. (1979), impaired auditory vigilance was found. Winneke and Fodor (1976) also studied visual vigilance in eight women by measuring their abilities to correctly detect random drops in the intensity of a train of pulses of white noise. The vigilance performance started to deteriorate 1 h into the exposure to methylene chloride at 500 ppm. The eight women also subjectively felt a more rapid decline in their soberness, and they felt tired more rapidly during the 2-h and 20-min exposure to methylene chloride at 500 ppm than during the sham exposure to air. Winneke (1981) reported that visual vigilance was impaired by acute methylene chloride exposures as low as 300 ppm, so he concluded that ''prolonged monotonous observation-tasks are easily disturbed by'' methylene chloride.

Methylene chloride also could impair visual or CNS alertness in human subjects. In the study of Winneke and Fodor (1976), there was decreased visual or CNS alertness as early as 50 min into an exposure of 12 women to methylene chloride at 500 ppm for 2 h and 20 min, as measured by a drop in the monocular critical flicker frequency. Similar drops in the critical flicker frequency were detected by Winneke (1981) in a 95-min exposure to methylene chloride at 300 ppm. Stewart et al. (1972) also reported that a 2-h exposure to methylene chloride at 986 ppm, resulting in 10.1% COHb in the blood, changed the amplitude of visual-evoked potentials triggered by 100 strobe flashes in three out of three human volunteers.

Unlike other aspects of CNS function, Winneke's group showed that cognitive performances of human subjects were quite resistant to methylene chloride's depressive effect on the CNS (Winneke and Fodor, 1976; Winneke, 1981). DiVincenzo et al. (1972) exposed 11 men to methylene chloride at 100 and 200 ppm for 2 to 4 h, with the men exercising approximately one-third of the exposure duration. The expo-

sure did not change the time for the men to complete tests of adding single-digit numerals. Winneke and Fodor (1976) showed that, in an exposure of 12 women to methylene chloride at 500 ppm for 2 h and 20 min, there were no differences in their performances in an addition test and a letter-canceling test. Even a 2-h exposure at 1000 ppm failed to reduce cognitive performances in human subjects, as determined by an addition test, the learning and retention of nonsense syllables, and the reproduction of visual patterns (a test of short-term memory) (Winneke, 1981).

As the exposure concentration increases, methylene chloride produces more overt CNS depression. Winneke (1981) reported that a 4-h methylene chloride exposure at 800 ppm results in depressive mood and motor impairment. As the concentration approached 1000 ppm, Stewart et al. (1972) reported that two of three human subjects complained of mild light-headedness after 1 h of exposure; one of the two developed a sensation of "thick tongue." Moskowitz and Shapiro (1952) reported four cases of accidental exposures to unknown but presumably very high concentrations of methylene chloride for 1-3 h, which produced unconsciousness in all the victims; three men finally recovered after 3-6 h and one man never regained consciousness and died.

Other than acting on the CNS, methylene chloride might also affect the liver. A 6-h exposure at 5000 ppm or higher increases the hepatic triglyceride concentration in guinea pigs (Balmer et al., 1976).

Subchronic exposures to methylene chloride have been reported to produce COHb and toxic effects in the liver, kidney, and the respiratory system.

Kim and Carlson (1986) compared COHb formation in rats exposed

to the same concentrations of methylene chloride at 200, 550, or 960 ppm for either 8 h/d for 5 d or 12 h/d for 4 d. They found no significant difference in the COHb concentrations in rats exposed to the same concentration for 8 h/d or 12 h/d after the first and last exposures of the week. Similar results were found in mice. The half-lives of the disappearance of COHb in the 8-h or 12-h groups of rats also did not differ. However, the half-lives depended on the exposure concentration: the half-lives were 50 and 130 min in rats exposed to 550 ppm for 8 h or 960 ppm for 8 h, respectively. They concluded that unusual work shift would probably not change methylene chloride's toxicity mediated via COHb formation.

Subchronic exposure to methylene chloride could produce liver and kidney toxicity. MacEwen et al. (1972b) reported cellular vacuolization, nuclear enlargement, and iron pigmentation in portal areas of the liver and cortical tubular-cell degeneration in the kidney of rats exposed to methylene chloride at 1000 ppm, 24 h/d, for 100 d. In similarly exposed mice, ductal proliferation and large masses of brown pigment were found in or around the portal areas. In addition, a mild ballooning degeneration of cytoplasm and chromatin clumping were noted in the livers of these mice. In the kidneys, a very faint granular staining with hemosiderin was observed in some tubules of half of the mice examined. MacEwen et al. (1972b) also reported marked fatty liver in four dogs and mild fat accumulation in the liver of four monkeys exposed to methylene chloride at 1000 ppm for 100 d, but the kidney was not affected in the dogs and monkeys.

In 20 rats continuously exposed to methylene chloride at 25 ppm for 100 d, Haun et al. (1972) detected fatty changes and cytoplasmic vacuolization in the liver, as well as nonspecific tubular degeneration and regeneration in the kidney. In 20 mice exposed at 100 ppm, the only pathology discovered was fatty liver. No histopathology was found in any tissues of four dogs and four monkeys exposed at 100 ppm. At a lower concentration of 25 ppm, the only species affected was rats, which had fatty liver and nonspecific tubular degeneration in their kidneys.

Evaluation of these data indicates that the liver is more sensitive than

the kidney to methylene chloride. These data also revealed species differences in the sensitivity toward methylene chloride's hepatic effects. The sensitivity of four test species ranked in decreasing order as rats, mice, dogs, and monkeys is shown in Table 8-1.

The National Toxicology Program (NTP, 1986) sponsored subchronic and chronic toxicity studies conducted at exposure concentrations much higher than those in studies performed by MacEwen et al. (1972b) and Haun et al. (1972). The NTP's studies failed to show that rats were clearly more sensitive toward the non-neoplastic effects of methylene chloride than mice. In the NTP's subchronic toxicity study, rats and mice were exposed to methylene chloride at 1000, 2100, or 4200 ppm, 6 h/d, 5 d/w, for 90 d (NTP, 1986). The exposure at 1000 or 2100 ppm did not cause any histopathology, and the exposure at 4200 ppm produced mild centrilobular hydropic degeneration in mice but not in rats.

In the NTP's chronic toxicity study, rats were exposed at 1000, 2000, or 4000 ppm, and mice were exposed at 2000 or 4000 ppm, 6 h/d, 5 d/w, for 2 y (NTP, 1986). Rats and mice suffered different types of histopathology in the liver; rats were afflicted with more types of histopathology than mice. Mice in both the 2000- and 4000-ppm groups developed only cytological degeneration in the liver. In comparison, several types of hepatic pathology were found in the 1000-, 2000-, and 4000-ppm groups: focal granulomatous inflammation, focal necrosis, hemosiderosis, and cytoplasmic vacuolization.

In a chronic toxicity study sponsored by several chemical companies, Burek et al. (1984) exposed rats and hamsters to methylene chloride at 500, 1500, or 3500 ppm, 6 h/d, 5 d/w, for 2 y, and showed differences

in the sensitivities between the two species. The methylene chloride exposures failed to increase the incidence of liver or kidney histopathology in hamsters, and some of the exposures affected the liver and kidney in rats. Similar to the findings of the NTP (1986), Burek et al. found that chronic methylene chloride exposures were more damaging to the liver than the kidney in rats. Methylene chloride did not cause any concentration-dependent increases in the incidence of glomerulonephropathy in female rats. In male rats, however, chronic methylene chloride exposures led to glomerulonephropathy at 1500 and 3500 ppm. In terms of liver injuries, chronic methylene chloride exposures at 500, 1500, or 3500 ppm produced vacuolization consistent with fatty liver in both male and female rats and they also caused multinucleared hepatocytes in female rats. Exposures at 1500 or 3500 ppm resulted in necrosis of individual hepatocytes in male rats, and the exposure at 3500 ppm produced coagulation necrosis and foci of altered hepatocytes in female rats.

Repetitive exposures of mice to methylene chloride at 4000 ppm, 6 h/d, 5 d/w, for up to 13 w, have been shown by Foster et al. (1992) to produce cytoplasmic vacuoles in bronchiolar Clara cells. The lesion appeared only on the second day of each week of exposure and resolved after the second day. The disappearance of the lesion correlated with a decrease in cytochrome P-450 monooxygenase activity in Clara cells, suggesting that Clara cells developed tolerance to methylene chloride with time by the inactivation of one of the pathways of methylene chloride metabolism. In contrast to mice, rats are not susceptible to this toxicity of methylene chloride (Foster et al., 1986). Since the Clara cell lesion did not appear to be too serious and disappeared with time, SMACs are not set according to the Clara cell lesion.

In the chronic toxicity study by the NTP (1986), exposures at 4000 ppm, 6 h/d, 5 d/w, for 2 y have been shown to cause squamous metaplasia in the nasal cavities of female rats but not those of male rats or female and male mice. Similar exposures at 2000 ppm failed to produce such a change. Squamous metaplasia in the nose is not relied on in setting methylene chloride's SMACs because it is a toxic effect seen only at very high exposure concentrations.

Two 2-y bioassays showed that methylene chloride was carcinogenic in rats and mice, but not in hamsters (Burek et al., 1984; NTP, 1986). In a chronic exposure of rats and hamsters to methylene chloride at 3500 ppm conducted by Burek et al. (1984), salivary gland sarcomas were the only kind of tumor found, and these sarcomas were observed only in the male rats. In the NTP's chronic toxicity study (1986), methylene chloride produced leukemia and benign mammary tumors in female rats and alveolar and bronchiolar adenomas and carcinomas in mice, as well as hepatocellular adenomas and carcinomas in mice. The incidences of lung tumors in female mice were 3 of 50, 16 of 48, and 40 of 48 in the 0-, 2000-, and 4000-ppm groups, respectively. The corresponding incidences of liver tumors were 3 of 50, 30 of 48, and 41 of 48. According to the NTP, methylene chloride shows clear evidence of carcinogenicity in female F344/N rats and male and female B6C3F₁ mice. An epidemiological study did not find any significant increase in cancer-related mortality in workers exposed to methylene chloride at 30-1200 ppm for up to 30 y (Friedlander et al., 1978). The ACGIH (1986) has classified it as a suspected human carcinogen.

The methylene chloride metabolites via the glutathione transferase pathway have been postulated to be the active metabolites in causing its carcinogenicity (Andersen et al., 1987). One of the metabolites formed is formaldehyde. Casanova et al. (1992) studied DNA-protein cross-links in rodents exposed to methylene chloride. They exposed mice and hamsters to methylene chloride at 4000 ppm, 6 h/d, for 2 d and then to ¹⁴C-methylene chloride on the third day for 6 h at a concentration decaying from 4500 to 2500 ppm. They found DNA-protein cross-links in mouse liver, but not in mouse lung, while the cross-links failed to show up in either organs of hamsters. Casanova et al. stated that the failure to detect DNA-protein cross-links in mouse lung did not rule out the possibility that the cross-links existed in subpopulations of lung cells. They attributed the DNA-protein cross-links to formaldehyde formed from methylene chloride's metabolism via the glutathione transferase pathway.

Methylene chloride is mutagenic in Salmonella typhimurium (Jongen

et al., 1978). It has been shown to cause chromosomal aberrations, but not sister chromatid exchange, in Chinese hamster ovary cells in vitro (Thilager and Kumaroo, 1983). It also failed to produce micronuclei in mice (Gocke et al., 1981).

It should be noted that methylene chloride has not been found to be teratogenic. Schwetz et al. (1975) exposed rats and mice to methylene chloride at 1225 ppm, 6 h/d, on gestation days 6-15 and failed to find any malformations in the fetuses. Because the exposure duration used by Schwetz et al. might not be long enough for a chemical that acts via its metabolites, Hardin and Manson (1980) exposed five female rats to methylene chloride at 4500 ppm, 6 h/d, 7 d/w, for 12-14 d before breeding and on days 1-17 of gestation. Hardin and Manson did not detect any increases in the incidence of skeletal or soft-tissue malformations or external anomalies.

No evidence of interaction involving methylene chloride and other chemicals has been found in the literature.

The acceptable concentrations (ACs) for the three major toxic end points of CNS depression, liver toxicity, and carcinogenicity are estimated for continuous exposures lasting 1 h, 24 h, 7 d, 30 d, or 180 d. The lowest AC among the three end points will be chosen as the SMAC for each exposure duration.

As discussed in the Toxicity Summary, one of the major acute effects of methylene chloride is CNS depression, which appears to be due to carbon monoxide formed from methylene chloride's metabolism. A 4-h exposure to methylene chloride at 200 ppm, which yields 5 % COHb in

blood, impairs the hand-eye coordination and auditory vigilance (Peterson et al., 1978), but there are no data on the no-observed-adverse-effect level (NOAEL) of methylene chloride. It makes sense to adopt the NOAEL of COHb used in setting the 1-h and 24-h SMACs of carbon monoxide as a potential basis for setting the 1-h and 24-h SMACs of methylene chloride. Three percent COHb is the target COHb concentration used to set both the 1-h and 24-h SMACs for carbon monoxide (Wong, 1990). The task here is to determine the methylene chloride concentrations that produce about 3% COHb in 1 and 24 h. Assuming a baseline COHb concentration of 0.6% due to endogenous CO production, the task is to determine the methylene chloride concentrations that would increase the COHb concentration by 2.4%. The increases in COHb concentrations produced by various methylene chloride exposure scenarios are shown in Table 8-5.

To derive the 1-h AC based on CO formation, a linear regression line was fitted through the data of percent COHb increase versus C x T by forcing the fitted line through the origin. All the data in the above table were used except the data points at 3750 and 1972 ppm-h because their corresponding responses of 10% and 9.3% increases in COHb were too far away from the region of interest, 2.4%. The linear regression yielded a line with a slope of 0.0038, r² of 0.74, and a 95 % confidence limit of 100 ppm-h at a 2.4% increase in COHb. Accordingly, 100 ppm is selected as the 1-h AC based on CO formation.

To derive the ACs based on CO formation for a 24-h, 7-d, 30-d, or 180-d methylene chloride exposure, the physiologically based pharmacokinetic (PB-PK) model of Andersen et al. (1991) was used. For a 70-kg man with a starting COHb of 0.6%, this model predicted that an exposure to methylene chloride at 35 ppm would produce a final COHb of 3 % in 24 h, so 35 ppm is chosen to be the 24-h AC based on CO formation.

To calculate the acceptable 7-d, 30-d, and 180-d methylene chloride concentrations based on the carbon monoxide metabolite, the target COHb concentrations of 1.6% were adopted from the 7-d, 30-d, and 180-d carbon monoxide SMACs. According to the PB-PK model of Andersen et al. (1991), a continuous exposure to methylene chloride at 14 ppm would raise the COHb concentration from 0.6% to 1.6% in a 70-kg man in 7, 30, or 180 d. The 7-d, 30-d, and 180-d ACs based on CO formation are all set at 14 ppm.

It should be noted that the National Research Council's Subcommittee on SMACs recognized the potential that methylene chloride's toxicity due to COHb formation could be aggravated by a reduction in the mass of red blood cells (RBCs) in the bodies of astronauts in microgravity. However, the ACs based on CO formation derived from data gathered on earth should be valid because microgravity reduces astronauts' RBC mass by only about 10%. Thus, any aggravation on the

COHb concentration formed in astronauts during methylene chloride exposures will not be significant.

Subchronic or chronic exposures of rats to methylene chloride have been shown to produce these hepatic changes: fatty liver in the studies of MacEwen et al. (1972a,b) and Haun et al. (1972); cytoplasmic vacuolization (which might reflect fatty changes) and necrosis in the studies of Burek et al. (1984) and NTP (1986); and hemosiderosis and focal granulomatous inflammation in the NTP (1986) study.

Methylene chloride's hepatic effects depend on exposure duration to a certain extent. MacEwen et al. (1972b) reported that livers of rats exposed to methylene chloride at 5000 ppm, 24 h/d, 7 d/w, for 4 w exhibited the same degree of cellular vacuolization, nuclear enlargement, and iron pigmentation as that in rats exposed at 5000 ppm, 24 h/d, 7 d/w, for 14 w. However, the NTP (1986) study showed that a subchronic exposure of rats at 2100 ppm, 6 h/d, 5 d/w, for 13 w was not hepatotoxic, but a chronic exposure at 2000 ppm, 6 h/d, 5 d/w, for 2 y produced liver injuries. Similarly, in a comparison of methylene chloride's hepatic effects in mice exposed at 100 ppm, 24 h/d, for 3 d, for 1, 2, 3, 4, or 10 w, Weinstein and Diamond (1972) showed that liver histopathology became somewhat more severe as the exposure was lengthened, but the hepatic triglyceride concentration did not increase linearly with the exposure duration. Their results are summarized in Table 8-6.

Therefore, the bulk of the data indicate that methylene chloride's liver toxicity is somewhat time-dependent in rats. As a result, the prudent approach in deriving an AC based on hepatic toxicity would be to lower the AC as the exposure time is lengthened.

Instead of using the data gathered by MacEwen et al. (1972b) and Haun et al. (1972) to derive the ACs, data from the NTP (1986) study and the Burek et al. (1984) study were used. These studies are more recent and they were peer-reviewed, but the studies of MacEwen et al. and Haun et al. were not. The NTP study showed that repetitive exposures to methylene chloride at 2100 ppm, 6 h/d, 5 d/w, for 13 w caused no histopathology in rats.

The NTP (1986) study also showed that exposures to rats at 1000, 2000, or 4000 ppm, 6 h/d, 5 d/w, for 2 y could lead to cytoplasmic vacuolization, hemosiderosis, and focal granulomatous inflammation in liver. Burek et al. (1984) found that a similar 2-y exposure of rats to methylene chloride produced cytoplasmic vacuolization, indicative of fatty liver, at as low as 500 ppm, so the LOAEL for non-neoplastic hepatotoxicity is 500 ppm.

No evidence of liver toxicity has been found in the literature for acute methylene chloride exposures; therefore, 1-h and 24-h ACs based on liver toxicity are not derived.

A 2-y exposure to methylene chloride at 0, 2000, and 4000 ppm produced 3 of 50, 30 of 48, and 41 of 48 cases of lung tumors and 3 of 50, 16 of 48, and 40 of 48 cases of liver tumors, respectively, in female B6C3F₁ mice in the NTP study (1986). Instead of using the airborne methylene chloride concentrations to calculate the 10^-4 tumor dose, it is better to use the doses of active metabolite produced by the glutathione transferase pathway in the lung and liver, as estimated by a physiologically based pharmacokinetic model (Andersen et al., 1987). According to this pharmacokinetic model, 2000 and 4000 ppm of airborne methylene chloride are equivalent to 123 and 256 mg of methylene chloride metabolized per liter of lung per exposure day. Similarly, 2000 and 4000 ppm are equivalent to 851 and 1811 mg of methylene chloride metabolized per liter of liver per exposure day. By substituting these values in the linearized multistage model using GLOBAL86 (Howe and Crump, 1986), 0.011 mg of methylene chloride metabolized per liter of lung per day and 0.24 mg of methylene chloride metabolized per liter of liver per day are the lower 95% confidence limit of the dose that will yield a 10-4 lung and liver tumor risk, respectively. Based on the pharmacokinetic model (Andersen et al., 1987), 0.011 mg/L lung and 0.24 mg/L liver are equivalent to about 6 and 12 ppm of methylene chloride for humans, respectively. The lower concentration of 6 ppm is used in the risk assessment.

Instead of the physiologically based pharmacokinetics model, EPA (1990) used the body-surface-area ratio to extrapolate the tumor data in mice to humans. With the linearized multistage model, EPA estimated that a continuous lifetime exposure to methylene chloride at 0.02 mg/m³ or 5.8 x 10^-3 ppm would produce an excess tumor risk of 1 in 10,000

in humans. In comparison, EPA's estimate is 190 times more conservative than the risk assessment estimate based on the physiologically based pharmacokinetics model.

According to the Committee on Toxicology, setting k = 3 (the number of stages in the carcinogenic process affected by methylene chloride), t = 25,550 d (lifetime of 70 y), and t_o 10,950 d (an initial exposure age of 30 y), the adjustment factor for a near instantaneous exposure is calculated to be 26,082 (NRC, 1992).

Similarly, by setting k = 3, t = 25,550 d, and t_o = 10,950 d, the adjustment factor for estimating the 7-d exposure concentration that would yield the same excess tumor risk as that for a lifetime exposure is 3728 (NRC, 1992).

With a similar approach, 871 and 146.7 are calculated to be the adjustment factors for converting a lifetime exposure concentration to 30-d and 180-d exposure concentrations for the same excess tumor risk (NRC, 1992).

The ACs for the three toxic end points are listed in Table 8-7. The

lowest AC for each exposure duration is selected to be the SMAC. As a result, the 1-h, 24-h, 7-d, 30-d, and 180-d SMACs are set at 100, 35, 15, 5, and 3 ppm, respectively.

No adjustments of the SMACs are needed for any microgravity-induced physiological changes. The reason is that the inflight hemoglobin concentrations obtained in Skylabs were higher than the preflight values by only 10%, so the carbon monoxide produced from methylene chloride metabolism is not going to be significantly more toxic inflight than on earth.

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