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

Dichlorofluoromethane is a colorless, nonflammable gas at room temperature with a slight etherlike odor (ACGIH 1991).

HCFC-21 does not occur naturally. It has been manufactured for use principally as a specialty refrigerant, a solvent, an aerosol propellant, and a heat-

exchange fluid in geothermal-energy applications. The shuttle orbiter carries about 650 pounds of HCFC-21, which is used in a heat exchanger. Toxicological reports for the 28 missions from STS-26 to STS-55 show that HCFC-21 has been detected in shuttle cabin air at concentrations of 0.1 to 1 mg/m³ in one mission and 0.01 to 0.1 mg/m³ in three missions (James et al. 1994).

Limited data are available on the toxicokinetics of HCFC-21 in animals, and no data were found for human exposures. Trochimowicz et al. (1977) showed that HCFC-21 is partially metabolized in rats and dogs, probably by a cytochrome-P-450-dependent pathway. Peter et al. (1986) injected HCFC-21 into rats intraperitoneally (ip) and showed that it was only partially exhaled, indicating some metabolism, in contrast to CFC-22, which is completely exhaled and not metabolized (Peter et al. 1986). Rats exposed to HCFC-21 at 150 ppm for 6 h/d, 5 d/w for 46 d showed increased fluoride concentrations in urine (Industrial Bio-Test Labs 1979), as did rats exposed at 1000 and 5000 ppm for 6 h/d, 5 d/w for 13 w (E.I. du Pont de Nemours 1977). Although no reports were found that examined the metabolic products of HCFC-21, its hepatotoxicity suggests that its metabolism might be similar to that of chloroform, presumably yielding highly reactive species, such as phosgene.

The only studies found of the effects of HCFC-21 exposure on humans were case reports of bronchoconstriction in some patients inhaling bronchodilator aerosols in which the propellant contained a mixture of HCFC-21 and CFC-11 and one German study of arterial hypoxic patients inhaling propellants from pressurized aerosols. In animals, the toxicity of HCFC-21 appears to be more similar to chloroform than to other chlorofluorocarbons, particularly in its ability to induce liver toxicity from subchronic or chronic exposures. HCFC-21 exhibits the cardiotoxicity (sensitization to arrhythmia) common to many chlorofluorocarbons.

In general, toxicity due to acute exposures to HCFC-21 is low, but HCFC-21 is appreciably more toxic than related difluorinated methanes. Short-term exposures to high concentrations of HCFC-21 can produce adverse effects on

weight gain, the central nervous system (CNS), liver, heart, and respiratory system. Very high concentrations can cause unconsciousness or death.

A 1935 report states that a 1-h exposure of guinea pigs to HCFC-21 at 12,000 ppm induced slight stupor (Dufour 1935). In a more recent study, groups of 10 Charles River-CD male rats exposed to HCFC-21 at 10,000 ppm for 6 h/d, 5 d/w for 2 w showed no appreciable change in behavior as measured by motor coordination and response to noise (Kelly and Trochimowicz 1976). Unconsciousness was produced in guinea pigs after 30-min exposures at 50,000 ppm (Nuckolls 1959).

The LC₅₀ (lethal concentration for 50% of the animals) value in rats for a 4-h exposure is 49,900 ppm (C.H. Tappan and R.S. Waritz, E.I. du Pont de Nemours, unpublished data, 1964). Exposure to HCFC-21 at 100,000 ppm killed rats and guinea pigs in 1 h (Weigand 1971).

In a series of papers from the laboratory of Domingo Aviado, HCFC-21 was shown in rats to be the fastest-acting apnea inducer of a series of aerosol propellants, including FC-11, FC-114, FC-12, FC-115, and FC C-318, and was shown to be a potent depressant for respiratory minute volume in monkeys (Belej and Aviado 1973; Friedman et al. 1973; Belej et al. 1974; Aviado 1975). HCFC-21 did not, however, increase bronchoconstriction as indicated by pulmonary resistance in anesthetized rats (Friedman et al. 1973), and in monkeys, HCFC-21 acted as a bronchodilator (Belej and Aviado 1973).

Bronchoconstriction was reported in a small percentage (4.4%) of asthmatic patients after inhaling a metered dose (90 µg) of the bronchodilator, salmetrol, which contained a mixture of HCFC-21 and trichlorofluoromethane as propellant (Wilkinson et al. 1992).

The cardiac effects of inhaling pressurized aerosols were examined in 10 patients with marked arterial hypoxia resulting from bronchial pulmonary

disease and 10 subjects without lung disease. Diseased subjects were administered 10 12.6-mL puffs (1 puff per breath for 10 consecutive breaths) of propellant, consisting of 60% HCFC-21 and 40% CFC-11. EKGs were recorded immediately after administration and at 10 min following administration. Normal subjects were administered 16 puffs, according to the following schedule: 1 puff + 1 puff + 2 puffs + 4 puffs + 4 puffs + 4 puffs, with 25 min between each administration. EKGs were recorded at 2, 5, 10, and 20 min following each administration. No statistically significant changes in the EKG (conduction disturbances or contour changes) and no bradycardia or tachycardia were seen in either set of subjects (Fabel et al. 1972).

Life-threatening arrhythmias were seen in 2 of 12 beagle dogs exposed for 10 min to HCFC-21 at 10,000 ppm with intravenous epinephrine and in one of one beagle dog exposed to HCFC-21 at 25,000 ppm with intravenous epinephrine (E.I. du Pont de Nemours 1989). Although HCFC-21 at a concentration of 2.5% in air was shown to induce arrhythmia in monkeys and to sensitize the mouse heart to epinephrine, it did not induce bradycardia in rats, in contrast to several other halocarbons (Friedman et al. 1973; Belej et al. 1974). HCFC-21 induced tachycardia and hypotension (Aviado 1975), reduced the pulmonary arterial blood pressure, and depressed the myocardial contractility in monkeys at 2.5% (Belej and Aviado 1973); in dogs, 2.5% induced tachycardia, but hypotension was not seen at concentrations of <10% HCFC-21 (Belej and Aviado 1975).

Longer term exposures to HCFC-21, primarily in rats, result in decreased weight gain, hair loss, respiratory, hepatic and other systemic effects. Reports of carcinogenicity and testicular toxicity appear to be unfounded.

A case report from Italy implicates chronic occupational exposure to HCFC-21 in the development of chronic nasopharyngeal irritation and edema (Tanturri et al. 1988). Unfortunately, however, no measurement or estimate was made of the concentration of Freon in the work atmosphere. That was because Freon was determined to be the cause of the patient's complaints only when his symptoms cleared up after he quit working in the Freon-containing environment. The report did not give any indication of whether any other similarly exposed workers suffered similar complaints.

Rats exposed to HCFC-21 at 500 ppm for 6 h/d, 5 d/w for 51 d developed portal cirrhosis of the liver at 100% incidence, whereas rats similarly exposed to 150 ppm for up to 131 d had no incidence of cirrhosis (Table 8-1) (Industrial Bio-Test Laboratories 1979). The data reported by Industrial Bio-Test had not been reviewed internally by their Quality Assurance department because the department had been closed down. Results of other tests run by that testing laboratory were found questionable by the scientific community during this time period. Nevertheless, the findings of hepatotoxicity for HCFC-21 were confirmed by tests done at E.I. du Pont de Nemours.

Testing by DuPont's Haskell Laboratory found liver damage in rats exposed to HCFC-21 at 10,000 ppm, 6 h/d, 5 d/w for 2 w, and in rats exposed at 1,000 ppm, 6 h/d, 5 d/w for 13 w (E.I. du Pont de Nemours 1977). A no-effect level was not determined in rats; dogs exposed for 6 h/d, 5 d/w for 13 w showed liver damage at 5000 ppm but not at 1000 ppm.

Two of 10 rats exposed to HCFC-21 at 50 ppm, 6 h/d, 5 d/w for 51 d (the lowest tested dose) developed interstitial edema of the pancreas (Industrial Bio-Test Laboratories 1979). That is not considered an adverse effect, but rather a normal, reversible, physiological response.

Testing by DuPont's Haskell Laboratory found heavy spleens, enlarged

lymph nodes, decreased plasma proteins, increased plasma enzyme activities, increased urine volume, and increased urine fluoride in rats exposed to HCFC-21 at 1000 and 5000 ppm for 6 h/d, 5 d/w for 13 w (E.I. du Pont de Nemours 1977).

Of 40 rats (20 male and 20 female) examined after exposure to HCFC-21 at 500 ppm, 6 h/d, 5 d/w for 99 d, 3 males developed ''leukemia," which was described as being localized in the lung in two animals and the liver in a third (Industrial Bio-Test Laboratories 1979). None of 40 rats examined after exposure to HCFC-21 at 0, 50, or 150 ppm, 6 h/d, 5 d/w for 99 d had developed "leukemia" (Industrial Bio-Test Laboratories 1979). The strain of rat tested was not given, other than "albino rat." From the description given and the failure to report large increases in the absolute number of peripheral lymphocytes, it is unlikely that these were leukemias, but rather, according to B. Wagner (Wagner Associates, Inc., Millburn, NJ, personal commun., 1995), were most likely circumscribed lymphomas, which are common in some strains of rats and are not indications of exposure to carcinogens.

Testing by DuPont's Haskell Laboratory found increased mortality in both male and female rats exposed to HCFC-21 at 1000 and 5000 ppm for 6 h/d, 5 d/w for 13 w (E.I. du Pont de Nemours 1977). In another subchronic toxicity study (Industrial Bio-Test Laboratories 1979), rats (35 males and 35 females per group) were exposed 6 h/d, 5 d/w for 99 days to HCFC-21 at 0, 50, 150, or 500 ppm and followed for 30 d after exposure; 15 exposure-related deaths were reported. One male rat in the 150-ppm group was sacrificed on study day 107 in extremis with back-leg paralysis. Thirteen males and one female in the 500 ppm group were either found dead or sacrificed in extremis on study days 71 through 128.

HCFC-21 did not induce mutations in bacterial tester strains TA1535, TA1538, TA98, and TA100 exposed for 24 h in the Ames Salmonella bacterial reversion assay, nor did it induce cell transformation in BHK21 cells (+S9) in vitro (Longstaff et al. 1984).

Exposure of pregnant rats to HCFC-21 at 8800 ppm for 6 h/d on d 6 through 15 of gestation adversely affected maternal weight gain and appeared to cause a total preimplantation loss of embryos in 15 of 25 dams, but was not teratogenic (Culik and Kelly 1976; Kelly et al. 1978).

Testicular toxicity was reported by Industrial Bio-Test Laboratories in rats exposed to HCFC-21 at 500 ppm for 6 h/d, 5 d/w for 130 d, but the report was internally inconsistent. Different sections of the report gave contradictory statements as to how many animals were affected, at which doses, for which exposure durations, and whether the results were considered exposure-related and significant. A cover letter to the report stated that it had not been reviewed by IBT's quality assurance unit, because the unit had been discontinued due to loss of personnel. No other laboratories have reported testicular toxicity even for exposures to much higher concentrations; therefore, it must be assumed that this effect was not related to exposure to HCFC-21.

Spaceflight, on rare occasions, has been accompanied by non-life-threatening cardiac dysrhythmias at a higher frequency than observed for the affected individuals in tests on earth. Such a putative spaceflight-induced predisposition to cardiac dysrhythmias might enhance the arrhythmogenic effects of HCFC-21 in a manner similar to the sensitization seen in animals upon injection of epinephrine.

Moderate-to-heavy exercise or the use of epinephrine or other heart stimulants should be avoided during exposure to HCFC-21 because of HCFC-21's ability to sensitize the heart to arrhythmias. Concomitant exposure to HCFC-21 and other chlorocarbons, such as chloroform, might produce liver toxicity, which would not be seen during exposure to either compound alone at low concentrations.

Table 8-2 presents a summary of the toxicity data on HCFC-21.

Table 8-3 presents exposure limits for HCFC-21 set by other organizations and Table 8-4 presents the SMACs established by NASA.

To set SMAC values for HCFC-21, acceptable concentrations (ACs) are calculated for the induction of each adverse effect (CNS depression, cardiac effects, and hepatotoxicity) using the guidelines established by the National

Research Council's Committee on Toxicology (NRC, 1992). ACs are not set for the following end points: leukemia and testicular toxicity (because the data are not convincing), heavy spleens, enlarged lymph nodes, decreased plasma proteins, increased plasma enzyme activities, increased urine volume, increased urine fluoride, and interstitial edema of the pancreas. None of those effects is considered an adverse effect per se. For each exposure time (1 h, 24 h, 7 d, 30 d, and 180 d), the lowest AC is selected as the SMAC value (Table 8-5).

An AC for CNS depression is set based on a report that a 1-h exposure of guinea pigs to HCFC-21 at 12,000 ppm induced slight stupor (Dufour 1935). The 12,000-ppm value is divided by 10 to estimate a no-observed-adverse-effect level (NOAEL). The resultant value is further divided by 10 for species extrapolation.

AC = 12,000 ppm ÷ 10 (to NOAEL) ÷ 10 (species) = 120 ppm.

Because CNS effects are generally independent of exposure duration, once blood concentrations have achieved steady state, as would be expected for a 1-h exposure to a chlorofluorocarbon, the resultant value of 120 ppm is set as the AC for all exposure durations—1 h, 24 h, 7 d, 30 d, and 180 d.

An AC for cardiovascular effects (tachycardia and hypotension) is based on the reports by Belej and co-workers of effects in dogs (Belej and Aviado 1975) and similar findings in monkeys (Belej et al. 1974) exposed to HCFC-21 at 25,000 ppm for 5 min. The 25,000-ppm value is divided by 10 to estimate a NOAEL, again by 10 for potential species differences in response, and by 5 for potential spaceflight effects.

No adjustment is made for exposure durations beyond 1 h, because the cardiovascular effects are generally dependent only on blood concentrations.

The ACs for all exposure durations from 1 h to 180 d are therefore set at 50 ppm.

An AC for toxicity to the liver is based on the 1979 report by Industrial Bio-Test Laboratories that subchronic (6 h/d, 5 d/w, 131 d; comparable to 23 d of continuous exposure) exposure of rats to HCFC-21 at 150 ppm was a NOAEL for cirrhosis of the liver. The reliability of the data can be questioned on the basis of allegations that Industrial Bio-Test Laboratories falsified test results in the past, and the fact that the data for HCFC-21 were not reviewed by its quality-assurance department or published in peer-reviewed literature and were only in a report to Allied Chemical Corporation. Nevertheless, the results are consistent with the findings of Trochimowicz et al. (1977), from DuPont's Haskell Laboratories, who, in a 90-d study on ChR-CD rats and beagle dogs exposed to HCFC-21 at 1000 or 5000 ppm for 6 h/d, 5 d/w, found extensive liver damage in rats at either test concentration, minimal morphological change in the liver of dogs exposed at 5000 ppm, and a NOAEL for liver toxicity in dogs exposed at 1000 ppm. Thus, the 150-ppm NOAEL is used as a starting point and divided by 10 for potential species differences in response. The resulting value is adjusted for the duration of potential exposure on the basis of 6 h/d × 131 d, approximating 23 d continuous exposure, except that the value is not increased for exposure durations shorter than 23 d. ACs are not calculated for 1 h and 24 h, because that would require an extrapolation of greater than 10-fold.

7-d AC = 150 ppm ÷ 10 (species) = 15 ppm.

30-d AC = 150 ppm ÷ 10 (species) × 23 d/30 d = 12 ppm.

180-d AC = 150 ppm ÷ 10 (species) × 23 d/180 d = 2 ppm.

ACGIH. 1991. Dichlorofluoromethane. Pp. 434-435 in Documentation of the Threshold Limit Values and Biological Exposure Indices, 6th Ed. Cincinnati, OH: American Conference of Governmental Industrial Hygienists.

ACGIH. 1997. TLVs and BEIs. American Conference of Governmental Industrial Hygienists. Cincinnati, OH .

Aviado, D.M. 1975. Toxicity of aerosol propellants in the respiratory and circulatory system. 10. Proposed classification. Toxicology 3:321-332.

Belej, M.A., and D.M. Aviado. 1973. Acute fluorocarbon toxicity in rhesus monkey [abstract]. Fed. Proc. Fed. Am. Soc. Exp. Biol. 32:814.

Belej, M.A., and D.M. Aviado. 1975. Cardiopulmonary toxicity of propellants for aerosols. J. Clin. Pharmacol. 15:105-115.

Belej, M.A., D.G. Smith, and D.M. Aviado. 1974. Toxicity of aerosol propellants in the respiratory and circulatory systems. IV. Cardiotoxicity in the monkey. Toxicology 2:381-395.

Culik, R., and D.P. Kelly. 1976. Embryotoxic and Teratogenic Studies in Rats with Inhaled dichlorofluoromethane (Freon 21) and 2,2-Dichloro-1,1,1-trifluoroethane (FC-123). Haskell Laboratory, E.I. du Pont de Nemours, Wilmington, DE.

Dufour, R.E. 1935. The Comparative Life, Fire, and Explosion Hazards of Dichloromonofluoromethane (F21). Underwriters' Laboratories, Chicago, IL.

E.I. du Pont de Nemours. 1977. Toxicity Information for Fluorocarbon 21 (FC 21, Dichloromonofluoromethane). E.I. du Pont de Nemours, Wilmington, DE.

E.I. du Pont de Nemours. 1989. Cardiac Sensitization with Cover Sheet and Letter dated 050589. EPA/OTS Doc. No. 86-890000758. E.I. du Pont de Nemours, Wilmington, DE.

Fabel, H., R. Wettengel, and W. Hartmann. 1972. Myokardischamie und arrhythmien durch den gebrauch von dosieraerosolen beim menschen? Dtsch. Med. Wschr. 97:428-431.

Friedman, S.A., M. Cammarato, and D.M. Aviado. 1973. Toxicity of aerosol propellants on the respiratory and circulatory systems. II. Respiratory and bronchopulmonary effects in the rat. Toxicology 1:345-355.

Industrial Bio-Test Laboratories. 1979. Report to Allied Chemical Corporation: Subacute Inhalation Toxicity Study with Genetron 21 in Albino Rats. Industrial Bio-Test Laboratories, Decatur, IL.

James, J.T., T.F. Limero, H.J. Leaño, J.F. Boyd, and P.A. Covington. 1994. Volatile organic contaminants found in the habitable environment of the Space Shuttle: STS-26 to STS-55. Aviat. Space Environ. Med. 65: 851-857.

Kelly, D.P., R. Culik, H.J. Trochimowicz, and W.E. Fayerweather. 1978. Inhalation teratology studies on three fluorocarbons [abstract]. Toxicol. Appl. Pharmacol. 45:293.

Kelly, D.P., and H.J. Trochimowicz. 1976. Two-week inhalation toxicity studies. Haskell Laboratory, E.I. du Pont de Nemours, Wilmington, DE.

Longstaff, E., M. Robinson, C. Bradbrook, J.A. Styles, and I.F. Purchase. 1984. Genotoxicity and carcinogenicity of fluorocarbons: Assessment by short-term in

vitro tests and chronic exposure in rats. Toxicol. Appl. Pharmacol. 72:15-31.

NRC. 1984. Fluorocarbon 21 in Emergency and Continuous Exposure Limits for Selected Airborne Contaminants, Vol. 2. Washington, DC: National Academy Press.

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

Nuckolls, A.H. 1959. The comparative life, fire and explosion hazards of refrigerants. Underwriter's Laboratories, Chicago, IL.

Peter, H., J.G. Filser, L.V. Szentpaly, and H.J. Wiegand. 1986. Different pharmacokinetics of dichlorofluoromethane (CFC 21) and chlorodifluoromethane (CFC 22). Arch. Toxicol. 58:282-283.

Tanturri, G., F. Pia, and M. Benzi. 1988. A case of oedematous pharyngolaryngitis in a subject occupationally exposed to Freon gas. Med. Lav. 79:219-222.

Trochimowicz, H.J., J.P. Lyon, D.P. Kelly, and T. Chiu. 1977. Ninety-day inhalation toxicity studies on two fluorocarbons [abstract 164]. Toxicol. Appl. Pharmacol. 41:200.

Weigand, W. 1971. Studies on inhalation toxicity of fluorine derivatives of methane, ethane and cyclobutane. [in German]. Zentralbl. Arbeitsmed. Arbeitsschutz 21:149.

Wilkinson, J.R., J.A. Roberts, P. Bradding, S.T. Holgate, and P.H. Howarth. 1992. Paradoxical bronchconstriction in asthmatic patients after salmeterol by metered dose inhaler. Br. Med. J. 305:931-932.