Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 4 (2000)

Hector D. Garcia, Ph.D.

Johnson Space Center Toxicology Group

Medical Operations Branch

Houston, Texas

Trichlorofluoromethane is a colorless, nonflammable liquid or gas at room temperature and has a faint ether-like odor detectable at a threshold concentration of about 5 ppm (ACGIH 1991).

Trichlorofluoromethane (CFC-11) does not occur naturally. It is an ozonedepleting chlorofluorocarbon and is used principally as a plastic foam blowing agent, an aerosol propellant for pharmaceuticals for asthmatic patients and toiletries, a refrigerant, a heat-transfer medium, and a solvent-degreasing agent in the aerospace and electronics industries. Low concentrations of CFC-11 have been seen in the spacecraft atmosphere six times of 28 space-shuttle missions and five Spacelabs (once at ≤ 1.75 ppm, four times at ≤ 0.175 ppm, and once at ≤ 0.0175 ppm) (James et al. 1994).

Uptake and excretion of CFC-11 have been studied in animals. During a 10-min inhalation exposure of dogs and rabbits to concentrations varying from 0 to 5%, CFC-11 rapidly diffused into the blood, cerebrospinal fluid, urine, and bile and reached steady-state concentrations in the blood within about 10 min. (Paulet et al. 1975a). After cessation of exposure, CFC-11 was eliminated, primarily (98%) through the breath, within 20-50 min. Small quantities of CFC-11 were eliminated in the urine and bile, the bile containing higher concentrations than the urine. Upon cessation of an inhalation exposure to CFC-11, its concentration in blood dropped sharply (Dollery et al. 1970). Although CFC-11 was rapidly cleared from the blood, it was retained for longer periods in some tissues. Niazi and Chiou (1975) showed that the kinetics of elimination of CFC-11 after intravenous infusion into dogs reflects three tissue compartments with half-lives of 3 min, 16 min, and 93 min for the initial, intermediate, and final phases of pulmonary clearance. Thus, multiple doses might result in accumulation of much higher concentrations of CFC-11 in some tissues than is evident by measuring blood concentrations.

Lack of appreciable metabolism has been demonstrated in humans. Radiolabeled CFC-11 inhaled for 7-17 min at 1000 ppm by a man and a woman was recovered quantitatively in the exhaled breath with only trace amounts of radioactivity found in exhaled carbon dioxide (0.13% and 0.10%) and recovered as nonvolatile materials in the urine (0.07% and 0.09%) (Mergner et al. 1975). It is likely that the trace amounts of metabolites were products of radiolabeled impurities. Cox et al. (1972) showed that, although CFC-11 is not significantly metabolized and does not produce free radicals, it exhibits characteristic binding spectra with hepatic microsomal preparations, giving a type I spectrum with a binding constant, K_s, value similar to carbon tetrachloride,

indicating that CFC-11 binds to cytochrome P-450. Although CFC-11 does not appear to be metabolized, Paulet and co-workers reported that CFC-11 had measurable effects on other metabolic processes—at 50,000 ppm, it produced corticosteronemia in the rat (Paulet and Rochcongar 1974)—and slight hyperglycemia with hyperlactacidemia and decreased oxygen consumption in rats, rabbits, and dogs (Paulet et al. 1975b).

CFC-11 is considered one of the most cardiotoxic of the CFCs. Demonstrated toxic effects include sensitization to epinephrine, which results in induction of cardiac arrhythmia, and changes in respiration and narcosis at high doses. CFC-11 does not appear to be mutagenic or carcinogenic.

In five groups of humans (9, 8, 10, 8, and 11 subjects in each group; a total of 46 subjects) exposed in a test chamber to CFC-11 at 1000 ppm for 1, 2, 8, or 10 h, including one group of 8 exposed 8 h/d for 18 d, no adverse effects were seen in EKGs or in a variety of other tests for toxic effects (Stewart et al. 1978).

In dogs inhaling CFC-11 at 3500 ppm, Reinhardt et al. (1971) reported ventricular fibrillation and cardiac arrest following injection of epinephrine. No cardiac sensitization was seen in dogs inhaling up to 1300 ppm. In a later report by Trochimowicz and Reinhardt (1975) from the same DuPont laboratory, cardiac sensitization to induction of arrhythmia by epinephrine was observed during experimental exposure of dogs to CFC-11 at 5000 ppm but not 1000 ppm. The average blood concentration associated with cardiac sensitization in these experiments was 25 µg/mL (arterial) or 20 µg/mL (venous). Arrhythmia was induced in dogs inhaling 800,000 ppm plus 20% oxygen when the dogs were frightened by a loud noise to induce release of endogenous epinephrine. Exercising on a treadmill, likewise known to induce release of endogenous epinephrine, did not induce arrhythmia in dogs inhaling CFC-11 at up to 10,000 ppm (Mullin et al. 1972).

In rats given CFC-11 with simultaneous injections of epinephrine, arrhythmia was seen at 25,000 ppm (Watanabe and Aviado 1975); pentobarbital anesthesia reduced the incidence of arrhythmia and increased the required

concentration of CFC-11 to 100,000 ppm (Doherty and Aviado 1975). Increased sensitivity was seen in rats with cardiac necrosis or pulmonary arterial thrombosis but not pulmonary emphysema (Watanabe and Aviado 1975).

Mice under pentobarbital anesthesia exhibited second-degree atrioventricular block upon inhalation of CFC-11 at 100,000 ppm (Aviado and Belej 1974). Injection of epinephrine reduced the required CFC-11 concentration to 50,000 ppm.

In anesthetized monkeys, CFC-11 at 50,000 ppm elicited cardiac arrhythmia. Epinephrine infusion lowered the required concentration to 25,000 ppm, and coronary arterial occlusion reduced it to 12,500 ppm (Belej et al. 1974). The combination of the two procedures further reduced the threshold concentration of CFC-11 to 5000 ppm

The respiratory effects of exposure of humans to CFC-11 were reported by two laboratories. Stewart et al. (1978) reported finding no adverse effects in pulmonary-function tests, including computerized spirometry in two groups of humans (eight were exposed to CFC-11 at 1000 ppm for 6 h/d, 1 d/w for 4 w, and seven were exposed at 1000 ppm for a single 6-h exposure). Valic et al. (1977) studied the effects of much shorter exposures to higher concentrations. In 10 young male volunteers, a 15-s inhalation of 53,000 ppm led to highly significant but "not clinically alarming" reductions of up to about 10% in ventilatory capacity (maximum expiratory flow) lasting about 45 min (Valic et al. 1977). The magnitude of the effect did not change appreciably for a 45-s exposure. Exposure to 50:50 or 10:90 mixtures of Freon 11 and Freon 12 at individual concentrations of 3,000-18,000 ppm produced a greater effect (up to 14.1% and 11.0% reductions, respectively) than obtained with either Freon alone.

Aviado's laboratory studied the respiratory effects of CFC-11 in several animal species. In monkeys, 50,000 ppm caused a significant reduction in respiratory minute volume that was not preceded by stimulation of breathing (Belej et al. 1974). In dogs, rats, and mice, similar effects were seen at 25,000, 100,000, and 25,000 ppm, respectively. A concentration of 10,000 ppm was a no-observed-adverse-effect level (NOAEL) and 25,000 ppm was a lowest-observed-adverse-effect level (LOAEL) for decreased pulmonary resistance in anesthetized dogs exposed for 5 min (Belej and Aviado 1975). Bronchodilation was observed at 50,000 ppm in the anesthetized monkey (Aviado and Smith 1975) and at 25,000 ppm in the anesthetized dog; bronchoconstriction was seen

in rats and mice at concentrations of 25,000 and 10,000 ppm, respectively (Watanabe and Aviado 1975). Decreased pulmonary compliance was found in rats exposed to CFC-11 at 25,000 ppm (Watanabe and Aviado 1975) and mice exposed at 10,000 ppm (Brody et al. 1974) but not in monkeys (Aviado and Smith 1975) or dogs exposed at up to 50,000 ppm.

In humans exposed to CFC-11 at 1000 ppm for up to 10 h, no adverse effects were seen in EEGs, neurological tests, visual-evoked responses, and 7 of 11 cognitive tests. The eight male subjects repetitively exposed at 1000 ppm for 8 h/d, 5 d/w for 18 d, however, showed statistically significant but not clinically significant performance decrements in a sound stimulus test (4%), light stimulus test (35%), 10-s estimation test (but not the 30-s estimation test), and the Flanagan arithmetic test (5%) (Stewart et al. 1978). No such decrements were seen in eight male subjects exposed for a single 8-h exposure. The authors interpret these results as showing no effect at the concentrations studied because of the absence of a consistent decrement in test performance or a dose-related response.

In guinea pigs exposed to CFC-11 at concentrations ranging from 8000 ppm to 106,000 ppm for durations of 5, 30, 60, or 120 min, the following signs were noted:

• A NOAEL for all signs at 8000 to 12,000 ppm for all durations up to 2 h.

• A NOAEL for all signs at 21,000 to 25,000 ppm for a 5-min exposure.

• Slight tremors progressing to increasingly severe CNS effects up to unconsciousness with increasing concentration or exposure duration beginning at 21,000 ppm for 30 min up to 106,000 ppm for 2 h.

• Unconsciousness with occasional weak convulsive movements and audible, irregular breathing at 100,000 to 106,000 ppm for 2 h, and apparent recovery within 2 d, but autopsy 8 d after 2-h exposures showed mottled area of resolved congestion or light hemorrhage in lungs (Nuckolls 1933).

Other than carcinogenicity studies, no reports on the effects of long-term exposures to CFC-11 were found.

No reports were found indicating that CFC-11 might be carcinogenic in either humans or animals. An analysis by Gold and Zeiger (1997) of the available literature on CFC-11 found no evidence of carcinogenicity in rats and mice. Epidemiological studies (e.g., of refrigeration workers) have been inconclusive because of the difficulty in finding a large enough study cohort with well-defined exposures to CFC-11 and without confounding exposures to other toxicants (Axelson 1985).

Maltoni et al. (1988) found that exposures to CFC-11 at 1000 or 5000 ppm for 4 h/d, 5 d/w for 104 and 78 w were not carcinogenic to Sprague-Dawley rats or Swiss mice, respectively. The TLV Committee of the American Conference of Governmental Industrial Hygienists (ACGIH 1991) and the World Health Organization (WHO Working Group 1990) reviewed the results of National Cancer Institute (NCI) studies in rats and mice fed CFC-11 at gavage doses of approximately 500 or 1000 mg/kg and 2000 or 3900 mg/kg for 78 w, respectively. The studies yielded no significant increase in tumor incidence (NCI 1978). The NCI considered the study in rats to be inadequate because of poor survival rates and the study in mice to be negative. There was no evidence of carcinogenicity in either male or female Swiss ICR/Ha mice given subcutaneous injections of CFC-11 in tricaprylin shortly after parturition and observed for the following year.

Negative results for genotoxicity have been obtained for CFC-11 in vitro using bacteria and mammalian cells with or without metabolic activation and in the dominant lethal test (WHO 1990). CFC-11 was shown to be nongenotoxic at doses up to 10,000 µg per plate in Salmonella typhimurium strains TA100, TA1535, TA1537, and TA98 when tested in the Ames preincubation assay with and without rat liver and hamster liver homogenate (Zeiger et al. 1987; Gold and Zeiger 1997). CFC-11 was found to be negative for mutagenicity when tested in S. typhimurium G 46 strains TA1535 and 1538 at 3.6 mM for 60 min with active and inactive microsomes (Uehleke et al. 1977). It was negative for mutagenicity in the Ames Salmonella bacterial mutation assay when tested at a concentration of 1% for 72 h in strains TA1535 and TA100 with and without S9 (Longstaff et al. 1984); negative in Escherichia coli K12 and S. typhimurium strains TA1535 and TA1538 in the presence of microsomes at 3.6 mM for 1-2 h (Greim et al. 1977); and negative in S. typhimurium strains TA98, TA100, TA1535, TA1537, and E. coli WP2 uvra with and without

metabolic activation (Araki et al. 1994). CFC-11 gas was not mutagenic to cultured mammalian cells in the CHO/HGPRT assay (20-200 µL/3 mL culture medium, 5 h with and 18-19 h without metabolic activation) (Uehleke et al. 1977; Krahn et al. 1980) and did not transform mammalian cells in the Styles transformation assay in the presence of S9 (Longstaff 1988).

No studies were found on CFC-11's potential effects on reproduction or development.

Spaceflight, on rare occasions, has been accompanied by non-life-threatening cardiac dysrhythmias (but no life-threatening arrhythmias) at a higher frequency than observed in tests of the affected individuals on earth. Such a putative spaceflight-induced predisposition to cardiac dysrhythmias might enhance the arrhythmogenic effects of CFC-11 in a manner similar to the sensitization seen in animals upon injection of epinephrine.

The interaction of CFC-11 with epinephrine in producing cardiac arrhythmia has been described in preceding sections. This effect appears to have a threshold concentration of CFC-11 (near 1000 ppm) below which arrhythmias are not produced, even with injection of epinephrine.

Table 10-1 presents a summary of the toxicity data on CFC-11.

Table 10-2 presents exposure limits for CFC-11 set by other organizations and Table 10-3 presents the SMACs established by NASA.

To set SMAC values for CFC-11, acceptable concentrations (ACs) were calculated for the induction of each adverse effect (cardiac arrhythmia, respiratory effects, and CNS effects) using the guidelines established by the NRC (1992). For each exposure time (1 h, 24 h, 7 d, 30 d, and 180 d), the lowest AC was selected as the SMAC value (Table 10-4).

ACs for cardiac effects are based on the Stewart et al. (1978) report of a NOAEL for 46 humans exposed to CFC-11 at 1000 ppm for ≤ 1 h. Five groups comprised 9, 8, 10, 8, and 11 subjects. Safety factors of 10/√46 = 1.47 for the low number of human subjects and 5 for potential spaceflight effects on the cardiovascular system were applied. Paulet et al. (1975a) showed that steady-state concentrations in body fluids are achieved quickly; therefore, the resulting AC value of 140 ppm is used for all exposure durations ≥ 1 h.

1-h, 24-h, 7-d, 30-d, and 180-d ACs = 1000 ppm ÷ 1.47 ÷ 5 = 140 ppm.

ACs for respiratory effects are based on the Stewart et al. (1978) report of NOAELs for respiratory effects in 15 humans (two groups of eight and seven subjects each) exposed to CFC-11 at 1000 ppm for 6 h/d, 1 d/w for up to 4 w. An adjustment for the low number of human subjects of 10/√15 = 2.56 was applied for all exposure durations. Because no effects were seen in the Stewart experiments even at the highest dose, although a reduced ventilatory capacity was reported by Valic at much higher concentrations of CFC-11, the ACs calculated below are probably quite conservative.

1-h and 24-h ACs = 1000 ppm ÷ 2.56 = 390 ppm.

A 1-h AC for CNS effects is based on the Stewart et al. (1978) report of a NOAEL for 46 humans exposed to CFC-11 at 1000 ppm for ≥ 1 h. An adjustment of 10/√46 = 1.47 was applied for the low number of human subjects.

1-h AC = 1000 ppm ÷ 1.47 = 680 ppm.

The ACs for 24-h, 7-d, 30-d, and 180-d exposures were based on the Stewart et al. (1978) report of a NOAEL of 1000 ppm for 27 humans for ≤ 8 h exposures. (The groups comprised 3, 8, 4, 4, and 8 subjects each). An adjustment

of 10/√27 = 1.92 for the low number of human subjects was applied. No adjustment was made for exposure duration because the concentration of CFC-11 in the brain should parallel the concentration in the blood, which reaches steady state quickly (within 2 h).

24-h, 7-d, 30-d, 180-d ACs = 1000 ppm ÷ 1.92 = 520 ppm.

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