Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 3 (1996)

John T. James, Ph.D., Harold L. Kaplan, Ph.D.,

and Martin E. Coleman, Ph.D.

Johnson Space Center Toxicology Group

Biomedical Operations and Research Branch

National Aeronautics and Space Administration

Houston, Texas

Dichloroacetylene (DCA) is a liquid with the following properties (Torkelson and Rowe, 1981):

DCA is the major product formed from trichloroethylene (TCE) in carbon dioxide scrubbers containing alkaline materials (Saunders, 1967). Although TCE is not used in the spacecraft, it has been found in numerous atmospheric samples collected from the cabin of the space

shuttle (Coleman, 1985) and is a potential source of DCA in spacecraft. In a National Aeronautics and Space Administration (NASA) Toxicology Laboratory experiment, all the TCE disappeared when passed over heated alkaline adsorbent, with at least 75 % conversion to DCA (Rippstein, 1980). Alkaline decomposition of 1, 1, 2, 2-tetrachloroethane also results in formation of DCA.

DCA appears to be efficiently absorbed by the respiratory system of rodents; however, toxic effects are not expressed there (Reichert et al., 1984; Kanhai et al., 1989).

DCA has marked organotropism for the kidney where it reacts with neutrophils to yield compounds that cause tissue injury (Kanhai et al., 1989).

In rats exposed to 36 ppm of a DCA-diethyl ether complex, a large fraction of inhaled DCA is metabolized to N-acetyl-S-(1, 2-dichlorovinyl)-L-cysteine (N-ADCVC) and excreted in the urine. Kanhai et al. (1989) estimated that 10% of the complex introduced into a nose-only inhalation system during 1-h exposures was eliminated within 24 h as N-ADCVC in the urine of male rats. The metabolite was postulated to be a detoxification product leading away from pathways that produce compounds capable of acylation of macromolecules in the kidney (Kanhai et al., 1989).

In vitro studies have shown that DCA can be readily metabolized to S-(1,2-dichlorovinyl)-glutathione (DCVG) by microsomes from rat liver

and kidney; however, the metabolism is slower in cytosol fractions from the same tissue (Kanhai et al., 1989). Metabolic pathways have been proposed that result in conversion of DCVG to S-(1, 2-dichlorovinyl)-L-cysteine (DCVC). DCVC could then be cleaved to form reactive metabolites (e.g., enethiols), which lead to acylation of macromolecules. Alternatively, DCVC could be acetylated to form N-ADCVC, which is eliminated in the urine of rats (Kanhai et al., 1989).

Human exposures to DCA have occurred as a result of its formation from TCE in closed-circuit anesthesia machines (Defalgue, 1961). These exposures have shown that DCA is neurotoxic to humans, characteristically producing functional impairment of certain cranial nerves, particularly the trigeminal (Carden, 1944; Humphrey and McClelland, 1944; Reichert et al., 1976). Predominant symptoms reported include headache, nausea, and vomiting, which are followed, some hours later, by numbness in the perioral region, anesthesia throughout the facial skin and mucosa, and analgesia or reduced sensitivity over the entire distribution of the trigeminal nerve, with loss of the corneal reflex (Carden, 1944; Humphrey and McClelland, 1944; Reichert et al., 1976). In addition, labial sores, visual disturbances, and impaired taste and smell sensations frequently occur (Carden, 1944; Humphrey and McClelland, 1944; Reichert et al., 1976). Two fatalities occurred among several cases of cranial-nerve palsies attributed to the anesthetic use of TCE in a hospital (Humphrey and McClelland, 1944). The concentration of DCA was not measured in any of the poisonings involving the use of TCE in anesthesia machines (Siegel et al., 1971).

In animals, the principal target organs of acute exposure to DCA are the kidney and, to a lesser extent, the liver and brain. A 1-h exposure of NMRI mice to DCA at a concentration of 101 ppm caused the most marked pathological changes in the kidneys, consisting of extensive necrosis of the distal proximal tubules (Reichert et al., 1975). In the liver, there were fat accumulations, vacuolation, and basophilic cytoplasm. The brain showed generalized tissue edema and axonal swelling, atrophied cells in the brain stem, and shrinkage of ganglion cells.

A 1-h exposure of New Zealand albino rabbits to DCA at 126, 202, or 307 ppm caused extensive necrosis of the collecting tubules of the kidneys, with subsequent tubule calcification and scar formation (Reichert and Henschler, 1978). A slight and transitory increase in blood urea nitrogen (BUN) was evident at 126 ppm, and, at 202 ppm, the increase was large and prolonged, indicating a severe uremia. At 202 and 307 ppm, there were morphological changes in the liver and serum enzyme changes indicative of acute but reversible liver cell damage. At 126 ppm, liver pathological changes were less severe, and marker enzymes for liver damage were normal. The effects on the kidneys and livers of rabbits exposed at 17-23 ppm for 6 h were similar to those at 126 ppm for 1 h (Reichert and Henschler, 1978).

Histological changes in the brains of New Zealand albino rabbits after a 1-h exposure to DCA at 126, 202, or 307 ppm were evident in the sensory cortical regions and were concentration-related (Reichert et al., 1976). The most severe injury was to the sensory trigeminal nucleus, followed in decreasing intensity by the facial and oculomotor nerves and the motor trigeminal nucleus. A 6-h exposure at 17 ppm caused somewhat less severe damage than a 1-h exposure at 126 ppm (Reichert et al., 1976).

Human exposures to DCA also have resulted from its formation from TCE in environmental control systems containing alkaline, carbon dioxide scrubbers (Saunders, 1967). One serious incident occurred in a test of a sealed environmental chamber (Saunders, 1967). Within 48 h of the start of the test, a distinct, sweet-sour odor developed, and became increasingly irritating and nauseating to the five-man crew. After the third and fourth day, the crew experienced headaches, vomiting, itching around the eyes, sore gums, and painful jaws, and the test was terminated. Shortly thereafter, severe cold sores developed. Analyses of desorbed charcoal samples identified several volatile compounds, including TCE as the most prevalent contaminant, DCA, and monochloroacetylene (MCA). Saunders (1967) attributed the symptoms to DCA, but did not provide any information on its concentration in the chamber. The American Conference of Governmental Industrial Hygienists (ACGIH) cited this report as the reference for its statement that dis-

abling nausea occurred in at least 85% of individuals exposed to DCA at about 0.5 to 1 ppm for prolonged periods (ACGIH, 1991). Apparently, the ACGIH was referring to another incident and provided the wrong reference.

In a simulated flight test, the performance and health of a pig-tailed monkey deteriorated after 2 or 3 d (Saunders, 1969). DCA and TCE were detected at concentrations of 0.1 and 0.3 ppm, respectively. Although Saunders (1969) was of the opinion that DCA was the causative agent, the evidence was weak.

The principal target organ of repeated and continuous exposures of the NMRI:O(SD) rat to DCA is the kidney. Exposures to DCA at 15.5 ppm (with TCE at 150 ppm as a stabilizer) for 6 h/d, 5 d/w, for 6 w caused large increases in cytoplasmic and nuclear mass of epithelial cells of proximal convoluted tubules (Siegel et al., 1971). Similar repeated exposures to DCA at 9.8 ppm (with TCE at 50 ppm) produced some nonspecific toxic effects without any morphological changes to the kidneys, and DCA at 2.8 ppm (with TCE at 3.2 ppm) resulted in neither toxic signs nor pathological changes. Continuous exposure of rats at 2.8 ppm (with TCE at 5.3 ppm) for 24 h/d for 90 d caused kidney pathological changes similar to those from repeated exposures at 15.5 ppm (Siegel et al., 1971). In addition, some animals exposed continuously developed weakness in the hind legs and had difficulty walking, indicating neurological effects. The investigators attributed the renal changes to DCA, and not to TCE or acetaldehyde, which was present as an impurity at 4 ppm in the repeated and continuous 2.8-ppm DCA studies (Siegel et al., 1971). As evidence for their conclusions, they cited the work of Prendergast et al. (1967) in which repeated exposures to TCE at 713 ppm and continuous exposure at 35 ppm did not produce any morphological changes in the kidneys or other organs of rats. A continuous 90-d exposure of rats to acetaldehyde at 5 ppm also did not produce renal changes (Siegel et al., 1971).

Similar histopathological changes were observed in the kidneys of NMRI:O(SD) rats exposed to DCA at 4.8 ppm (with ether at 33 ppm as

a stabilizer) for 24 h/d for up to 28 d (Jackson et al., 1971). The renal lesion was observed initially after 4 d of exposure and increased in prevalence and severity up to 28 d of exposure. Of six rats observed for up to 9 d, four showed signs of hind-leg weakness, two had self-inflicted bite wounds, and three died. Renal pathological changes were not present in control animals exposed continuously to ether at 49 ppm for up to 30 d or to 50 ppm for 90 d (Jackson et al., 1971).

A small number of studies suggest that DCA might be carcinogenic to both NMRI mice and Wistar rats. In male and female NMRI mice, exposures to DCA (with acetylene as a stabilizer) at 9 ppm for 6 h/d, 1 d/w, for 12 mo (group 1), 2 ppm for 6 h/d, 1 d/w, for 18 mo (group 2), or 2 ppm for 6 h/d, 2 d/w, for 18 mo (group 3) resulted in apparent increases in the incidence of cystic kidney tumors (Reichert et al., 1984). Unfortunately, this finding was confounded by the presence of numerous oncocytomas in the kidneys of some control groups. Cystadenocarcinomas in male mice were found at an incidence of 12/30 in group 2 (low dose for 1 d/w), but the incidence in group 3 (low dose for 2 d/w) was only 3/30. No adenocarcinomas were reported in female mice. In some cases, lung tumors were more common in controls than in exposed mice. For example, male controls in group 2 had an incidence of lung adenomas of 8/30, whereas the exposed group had an incidence of only 3/30. Malignant lymphomas were much more common in male and female controls for group 1, with incidences of 14/30 and 17/30, respectively, versus incidences of 1/30 and 2/30 in exposed male and female mice, respectively. Harderian gland tumors, commonly seen in this strain of mice, were increased in DCA-exposed mice, but this gland is not present in human beings.

The carcinogenic findings were no clearer in male and female Wistar rats exposed to DCA (6 h/d, 2 d/w, 18 mo). The incidence of renal cystadenomas was higher in exposed males (7/30) than in controls (0/30); however, far more oncocytomas occurred in male controls (14/30) than in exposed males (0/30). In exposed rats, liver cholangiomas increased in both sexes. However, because only one group was exposed, a quantitative risk estimate was not attempted on these tumors. Overall, the data are considered to provide limited evidence of carcino-

genicity, a finding that agrees with the assessment of DCA by the International Agency for Research on Cancer (IARC, 1986).

DCA is mutagenic for strain TA100 Salmonella typhimurium if the bacteria are suspended in Oxoid medium to promote active bacterial growth (Reichert et al., 1983).

Lethality data suggest possible biological interactions between DCA and TCE or ether, but the data are sparse (Siegel et al., 1971). In NMRI:O(SD) rats, the 4-h LC₅₀ value for TCE was 12,500 ppm, and the values for DCA in DCA-TCE (1:7) and DCA-ether (1:9) mixtures were 55 and 219 ppm, respectively. Also, in NMRI:ASH and FTD: Hartley guinea pigs, the 4-h LC₅₀ value for DCA in a DCA-ether (1:9) mixture was approximately 4 times higher than that for DCA in a DCA-TCE (1:10) mixture.

Based on accidental exposures of human beings to DCA during trichloroethylene anesthesia and during closed environmental testing, it is apparent that the principal target organ of DCA is the nervous system for exposures of a few days or less. The human data cannot be used to set safe exposure concentrations because the DCA concentrations were never reported in descriptions of the accidents. DCA has been tested in a number of animal species and has elicited neurotoxicity, nephrotoxicity, hepatotoxicity, respiratory distress, and lethality. The only animal exposures that result in neurotoxicity similar to that found in human beings (trigeminal nerve injury) are those involving rabbits (Reichert et al., 1976). The results of the only cancer study conducted on DCA provided limited evidence of its carcinogenicity; however, the study

protocol was unconventional, and the results are difficult to interpret (Reichert et al., 1984). For these reasons, a cancer risk assessment was not attempted on DCA. Reported respiratory-system injury was probably due to DCA decomposition into phosgene when appropriate stabilizer concentrations were not used during high-concentration exposures (Reichert et al., 1975). This decomposition does not occur at DCA concentrations below a few parts per million even without a stabilizer; hence, respiratory-system injury is not considered in detail below.

Renal injury appears to be the primary finding in rats (Jackson et al., 1971) and mice (Reichert et al., 1975) exposed to DCA. The lethality seen in mice has been attributed to the severe renal damage caused by DCA; however, this type of injury has not been observed in humans exposed to DCA unless severe neurological symptoms were already apparent (Reichert et al., 1975). Renal injury has been demonstrated in rabbits, and neurotoxicity similar to that in humans has also been shown (see below). The acceptable concentrations (ACs) to prevent nephrotoxicity in humans were estimated from acute (1- and 6-h) exposures in rabbits and longer (2- to 90-d) exposures in rats. Exposure concentrations causing mild-to-moderate effects, as evident by serum urea nitrogen increases and histopathological changes, were reduced by a factor of 10 to reach a no-observed-adverse-effect level (NOAEL) and by a factor of 10 for species differences. Using Haber's rule, which appears to represent the acute data well, the 6-h NOAEL concentration of 0.2 ppm (20 ppm ÷ 100) was reduced by a factor of 4 to estimate a human 24-h NOAEL of 0.05 ppm. The 1-h AC (renal) was set at 1.3 ppm. Rat data were used in a similar manner to derive ACs (renal) for 7-d, 30-d, and 180-d exposures. A factor of 20 was used to estimate a NOAEL from the 28-d, 5-ppm exposures because 11 of 11 rats showed moderate-to-severe changes (see footnote a to Table 5-4).

The liver injury associated with DCA exposures has been seen in several species, although the extent of injury is typically less severe

than renal injury. Again, rabbit and rat data were used along with factors of 10 for reaching a NOAEL (if appropriate) and for species extrapolation. Haber's rule was used to estimate 24-h and 180-d levels from 6-h and 90-d data, respectively. The 30-d AC was set at 0.1 ppm because 0.1 ppm was determined to be a safe concentration for 7 and 180 d.

Neurotoxic effects in rabbits and rats were used to set ACs that would protect human beings from DCA-induced neurotoxicity. The cranial nerve effects in rabbits are a good model of human neurotoxicity, whereas hind-limb weakness was the major manifestation of the neurotoxicity in rats. The moderate histological effects noted in the rabbits exposed at 126 ppm for 1 h were not considered a lowest-observed-adverse-effect level (LOAEL). Based on the severe effects seen at 202 ppm, a factor of 2 applied to the moderate effects at 126 ppm was estimated to give a LOAEL. The effects reported after 17 ppm exposures for 6 h were mostly mild; hence, the factor of 2 was not applied. From the calculated or observed LOAELs, factors of 10 were used to reach NOAELs and to extrapolate animal data to human. Using the approach recommended by the National Research Council (NRC, 1992) for extrapolating to shorter exposure times, the 7-d and 30-d ACs were determined from the 90-d NOAEL of 0.03 ppm without increasing the concentration.

Lethality data were available for mice and rats. Using the NRC-recommended ''benchmark'' approach, the 95% limit of the LC₁₀ was calculated from the dose-response curves in mice exposed for 1 h or 6 h, and these values were reduced by factors of 10 for species extrapolation and setting a NOAEL; a factor of 4 was applied to the 6-h value to determine a 24-h estimate. The resulting values were not used because the estimated concentrations to protect against lethality were far below concentrations estimated to protect against neurotoxicity, the major effect seen in human beings. Estimates for the 7-d, 30-d, and 180-d ACs

(lethality) were based on concentrations that caused no deaths in rats after prolonged exposures. Only a species factor was applied to these "NOAEL" concentrations. This approach led to reasonable ACs (lethality), which were approximately 10-fold above concentrations that protect against sublethal injury to the kidney, liver, or nervous system.

The SMACs were set to protect against kidney injury and neurotoxicity at each potential exposure time. For 1- and 24-h exposures, hepatotoxicity was also a factor in setting SMACs. Human exposure data were useful only to the extent that they demonstrated that neurotoxicity is the primary effect in humans. Analysis of acute lethality data in mice led to values that were inconsistent with sublethal effects, so the lethality results were not used. Data on carcinogenic potential were not used because there was not an appropriate way to apply the linearized multistage model to the data.

ACGIH. 1991. Dichloroacetylene. In Documentation of the Threshold Limit Values and Biological Exposure Indices, 6th Ed. American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio.

ACGIH. 1995. 1995-1996 Threshold Limit Values and Biological Exposure Indices. American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio.

Carden, S. 1944. Hazards in the use of closed-circuit technique for trilene anesthesia. Br. Med. J. 1:319-320.

Coleman, M. 1985. Summary Report of Postflight Atmospheric Analysis for STS-1 to STS-41-C. JSC Memo. SD4-84-351. National Aeronautics and Space Administration, Johnson Space Center, Houston, Tex.

Defalgue, R. J. 1961. Pharmacology and toxicology of trichloroethylene. A critical review of the world literature. Clin. Pharmacol. Ther. 2:665-688.

Humphrey, J. H., and M. McClelland. 1944. Cranial nerve palsies with herpes following general anesthesia. Br. Med. J. 1:315-318.

IARC. 1986. Some chemicals used in plastics and elastomers. Pp. 369-378 in IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans, Vol. 39. Lyon, France: International Agency for Research on Cancer.

Jackson, M. A., J. P. Lyon, and J. Siegel. 1971. Morphologic changes in kidneys of rats exposed to dichloroacetylene-ether. Toxicol. Appl. Pharmacol. 18:175-184.

Kanhai, W., W. Dekant, and D. Henschler. 1989. Metabolism of the nephrotoxin dichloroacetylene by glutathione conjugation. Chem. Res. Toxicol. 2:51-56.

NRC. 1992. Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants. Washington, D.C.: National Academy Press.

Prendergast, J. A., R. A. Jones, L. J. Jenkins, Jr., and J. Siegel. 1967. Effects on experimental animals of long-term inhalation of trichloroethylene, carbon tetrachloride, 1, 1, 1-trichloroethane, dichlorodifluoromethane, and 1, 1-dichloroethylene. Toxicol. Appl. Pharmacol. 10:270-289.

Reichert, D., and D. Henschler. 1978. Nephrotoxic and hepatotoxic

effects of dichloroacetylene. Food Cosmet. Toxicol. 16:227-235.

Reichert, D., D. Ewald, and D. Henschler. 1975. Generation and inhalation toxicity of dichloroacetylene. Food Cosmet. Toxicol. 13:511-515.

Reichert, D., G. Liebaldt, and D. Henschler. 1976. Neurotoxic effects of dichloroacetylene. Arch. Toxicol. 37:23-38.

Reichert, D., T. Neudecker, U. Spengler, and D. Henschler. 1983. Mutagenicity of dichloroacetylene and its degradation products trichloroacetyl chloride, trichloroacryloyl chloride and hexachlorobutadiene. Mutat. Res. 117:21-29.

Reichert, D., U. Spengler, W. Romen, and D. Henschler. 1984. Carcinogenicity of dichloroacetylene: An inhalation study. Carcinogenesis 5:1411-1420.

Rippstein, W. J. 1980. Halogenated Hydrocarbon Conversions in Lithium Hydroxide Beds. NASA Memo SD4-80-61. National Aeronautics and Space Administration, Johnson Space Center, Houston, Tex.

Saunders, R. A. 1967. A new hazard in closed environmental atmospheres. Arch. Environ. Health 14:380-384.

Saunders, R. A. 1969. Another Incident of Dichloroacetylene Intoxication. AMRL-TR69-130. Aerospace Medical Research Laboratory Wright-Patterson Air Force Base, Dayton, Ohio.

Siegel, J., R. A. Jones, R. A. Coon, and J. P. Lyon. 1971. Effects on experimental animals of acute, repeated and continuous inhalation exposures to dichloroacetylene mixtures. Toxicol. Appl. Pharmacol. 18:168-174.

Torkelson, T. R., and V. K. Rowe. 1981. Halogenated aliphatic hydrocarbons containing chlorine, bromine and iodine. P. 3584 in Patty's Industrial Hygiene and Toxicology, 3rd Rev. Ed., Vol. 2B. New York: John Wiley & Sons.