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B6 Hydrogen King Lit Wong, Ph.D. Johnson Space Center Toxicology Group Biomedical Operations and Research Branch Houston, Texas PHYSICAL AND CHEMICAL PROPERTIES Hydrogen is a colorless, odorless, and tasteless gas (Sax, 1984). Formula: Hz CAS number: 1333-74-0 Molecular weight: 2.0 Boiling point: -252.8Â°C Melting point: -259.2Â°C Gas density: 0.069 that of air Conversion factors at 25Â°C, 1 atm: 1 ppm= 0.082 mg/m3 1 mg/m3 = 12.2 ppm OCCURRENCE AND USE Hydrogen is present in fuel cells as a by-product of oxygen generation from water. Flatulence is a biological source of hydrogen, which is formed by bacterial degradation of oligosaccharides in the lower intestine (Hopfer, 1982). The production rate in humans is approximately 50 mg/d per person (Olcott, 1972). Preliminary date from space-shuttle flights indicate that hydrogen might be present in the cabin atmosphere at about lOOppm. 139
140 SMACS FOR SELECTED AIRBORNE CONTAMINANTS PHARMACOKINETICS AND METABOLISM No pharmacokinetics and metabolism information on hydrogen was found. TOXICITY SUMMARY Hydrogen is a biological inert gas with no known direct toxic effect (Sax, 1984; NIOSH, 1987). Hydrogen, however, might cause hypoxia via displacement of oxygen in the air. Hypoxic signs and symptoms include tachypnea, cyanosis, sweating, euphoria, impaired judgment, loss of short-term memory, cardiac dysrhythmias, and unconsciousness (Ellenhorn and Barceloux, 1988). EXPOSURE LIMITS No exposure limit was found. The American Conference of Govern- mental Industrial Hygienists classifies hydrogen as a simple asphyxiant with no exposure limit set (TLV Committee, 1990). TABLE 6-1 Spacecraft Maximum Allowable Concentrations Durationa ppm mg/m3 Target Toxicity lh 4100 336 None (explosivity) 24 h 4100 336 None (explosivity) 7 db 4100 336 None (explosivity) 30d 4100 336 None (explosivity) 180 d 4100 336 None (explosivity) :niese SMACs are ceiling values. The current 7-d SMAC = 3000 ppm. RATIONALE Simple asphyxiants should be kept at a level that does not lower the oxygen concentration below 18% (TLV Committee, 1990). It takes 14.3% of hydrogen to lower the oxygen concentration from 21 % to 18%.
HYDROGEN 141 H2 concentration (21 % - 18%) x 100/21 14.3%. However, simple asphyxiants are usually not a problem because oxygen concentration in the spacecraft is continuously maintained at acceptable levels. Because hydrogen's lower explosive limit is less than 14.3% (Sax, 1984), its SMACs are established based on explosivity rather than asphyxi- ating action. With explosion being a very serious matter, a wide margin of safety is called for. Therefore, all the SMACs of hydrogen are set at one-tenth of its lower explosive limit, i.e., at 4100 ppm. A safety factor of 10 agrees with the recommendation of the U.S. Environmental Protection Agency that personnel doing environmental monitoring should evacuate when the concentration of an explosive gas reaches 10% of the lower explosive limit (EPA, 1988). REFERENCES Ellenhorn, M.J. and D.G. Barceloux. 1988. Pp. 11-12 in Introduction and Initial Evaluation in Medical Toxicology, Diagnosis and Treatment of Human Poisoning. Elsevier, New York. Hopfer, U. 1982. Digestion and absorption of basic nutritional con- stituents. P. 1160 in Textbook of Biochemistry with Clinical Correla- tions. T.M. Devlin, ed. John Wiley & Sons, New York. NIOSH. 1987. P. 2726 in Registry of Toxic Effects of Chemical Sub- stances 1985-86. DHHS (NIOSH) Puhl. No. 87-114. National Institute for Occupational Safety and Health, Cincinnati, Ohio. Olcott, T.M. 1972. Development of a Sorbent Trace Contaminant Con- trol System, Including Pre- and Postsorbers for a Catalytic Oxidizer. NASA CR-2027. Johnson Space Center, Houston, Tex. Sax, I., ed. 1984. P. 1549 in Dangerous Properties oflndustrial Materi- als, 6th Edition. Van Nostrand Reinhold, New York. TLV Committee. 1990. Pp. 8 and 26 in Threshold Limit Values and Biological Exposure Indices. American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio. EPA. 1988. Air Surveillance for Hazardous Materials. Environmental Response Team, Office of Emergency and Remedial Response, U.S. Environmental Protection Agency, Washington, D.C.