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7 Hydrogen This chapter summarizes relevant information on hydrogen gas, referred to as hydrogen in this profile. Selected chemical and physical properties are presented. The committee considered all those data in its evaluation of 1-h, 24-h, and 90-day guidance levels for hydrogen. The committeeâs recommendations for hydrogen con- centrations considered the toxicity and explosivity of the gas in the reduced-oxygen environment present onboard submarines. The committeeâs recommendations for maximum hydrogen concentrations are provided at the conclusion of this chapter with a brief summary of the adequacy of the data used for defining them. PHYSICAL AND CHEMICAL PROPERTIES Hydrogen is a colorless, odorless, and tasteless gas (Budavari et al. 1989). It is the lightest gas and is explosive in air at concentrations greater than about 4% (Lewis 1996). In contact with chlorine, oxygen, or other oxidizers, hydrogen is flammable and explosive and burns with a nearly invisible flame (Budavari et al. 1989). Selected chemical and physical properties are listed in Table 7-1. OCCURRENCE, USES, AND SOURCES OF EXPOSURE Hydrogen is the most abundant element and is present in Earthâs atmosphere at about 0.5 ppm (Windholz et al. 1976). It is formed during electrolysis of water as a byproduct of oxygen generation or by passing water vapor over heated iron. It is produced naturally by gut bacterial degradation of oligosaccharides (Hopfer 1982). Humans produce hydrogen at about 50 mg/day (Olcott 1972). Hydrogen is found in aircraft (NRC 2002) and space-shuttle air at about 100 ppm (NRC 1992). Charging shipboard batteries produces hydrogen. Thus, the hydrogen found onboard a sub- marine can reflect the low ambient concentrations found in the air, biologic sources, 151
152 Exposure Guidance Levels for Selected Submarine Contaminants TABLE 7-1 Physical and Chemical Properties of Hydrogen Gas Synonyms Protium CAS registry number 1333-74-0 Molecular formula H2 Molecular weight 2.00 Boiling point â252.77Â°C Melting point â259.2Â°C at 54 mm Hg Flash point NA Explosive limit 4.1% by volume in air (lower limit) Density 0.00008987 g/cm3 at 20Â°C Vapor pressure NA Solubility NA Conversion factors 1 ppm = 0.082 mg/m3; 1 mg/m3 = 12.2 ppm Abbreviations: NA, not applicable or not available. Sources: Explosive limit from Lewis 1996; density from Dean 1979; other data from Buda- vari et al. 1989. and its release from marine batteries as a byproduct. Several measurements of hydrogen on submarines have been reported. Data collected on nine nuclear- powered ballistic missile submarines indicate an average hydrogen concentration of 0.03% (range, 0-0.63%) and data collected on 10 nuclear-powered attack sub- marines indicate an average hydrogen concentration of 0.02% (range, 0-0.75%) (Hagar 2003). Carbon monoxide and hydrogen in submarine air are oxidized to carbon dioxide and water in a specialized burner (U.S. Naval Systems Command 1979). SUMMARY OF TOXICITY At very high concentrations in air, hydrogen is a simple asphyxiant gas be- cause of its ability to displace oxygen and cause hypoxia (ACGIH 1991). Hydrogen has no other known toxic activity. This profile considers only hydrogen gas and excludes health effects associated with other isotopic forms (deuterium or tritium) and hydrogen-containing chemicals (Windholz et al. 1976). Hydrogen-induced as- phyxiation may occur at lower hydrogen concentrations when oxygen concentra- tions are also reduced as onboard a submarine. However, hydrogen concentrations needed to induce hypoxia even in a low-oxygen environment would far exceed the explosive limit of the gas. Thus, occupational exposure standards are set on the ba- sis of the explosivity of hydrogen rather than its toxicity.
Hydrogen 153 Hydrogen As an Asphyxiant Gas Hydrogen can displace oxygen and result in asphyxiation and hypoxia. Air onboard a submarine is maintained at lower oxygen concentrations (about 19.5%) than in the natural environment to reduce the risk of fires. Hagar (2003) reported that the routine average partial pressure of oxygen (PO2) on nuclear-powered attack submarines is 118-180 mm Hg (mean, 149 mm Hg); similar values were reported for nuclear-powered ballistic missile submarines. Minimum values recommended by NRC (2007) for the oxygen 1-h EEGL, 24-h EEGL, and 90-day CEGL are 105, 127, and 140 mm Hg, respectively. Assuming reasonably high humidity, an atmos- phere with 28.2% hydrogen (282,000 ppm) is required to reduce the submarine mean PO2 of 148 mm Hg (19.5%) to the 1-h oxygen EEGL of 105 mm Hg (14%); that is, H2 concentration = [(19.5% - 14%)/19.5%] Ã 100 = 28.2%. Accordingly, hydrogen concentrations of 14.3% and 5.6% are required to re- duce normal mean submarine oxygen concentrations to the 24-h EEGL and 90-day CEGL values, respectively. Health effects associated with hydrogen mimic other forms of hypoxia. As al- veolar partial pressure of oxygen is reduced, visual acuity in dim light declines as a reduction in arterial blood oxygen depresses the function of retinal rod cells. Hy- poxia will lead to stimulation of medullary chemoreceptors and then to a compensa- tory increase in pulmonary ventilation. Important consequences of mild hypoxia include impaired judgment, reduction in ability to perform discrete motor move- ments, short-term memory loss, mental fatigue, headache, occasional nausea, and increase in reaction times. Rapid asphyxiation is characterized by tachypnea, cya- nosis, sweating, cardiac arrhythmia, depression of the central respiratory center followed by loss of consciousness, and coma (reviewed in NRC 2007). Hydrogen as an Explosive Gas Hydrogen is an explosive gas. The U.S. Environmental Protection Agency (EPA 1988) recommends evacuation of personnel when the concentration of an explosive gas reaches 10% of the lower explosive limit. Ten percent of the lower explosive limit, or 4,100 ppm, of hydrogen is less than the hydrogen concentration required to reduce oxygen in submarine air to the 1-h or 24-h EEGL or the 90-day CEGL. Therefore, the hydrogen EEGL and CEGL values are based on hydrogen explosivity rather than adverse health effects arising from asphyxiation. Explosive limits of hydrogen in a lowered-oxygen atmosphere as would be found aboard a submarine are unknown.
154 Exposure Guidance Levels for Selected Submarine Contaminants INHALATION EXPOSURE LEVELS FROM THE NATIONAL RESEARCH COUNCIL AND OTHER ORGANIZATIONS Inhalation exposure levels for hydrogen have been established by the National Aeronautics and Space Administration (NASA) and are shown in Table 7-2. The American Conference of Governmental Industrial Hygienists (ACGIH 2004) classi- fies hydrogen as a simple asphyxiant, and no exposure limit has been assigned. ACGIH (1991) notes that the major hazard posed by hydrogen is due to its flamma- ble and explosive properties. COMMITTEE RECOMMENDATIONS A health-based exposure standard would consider hydrogen-induced as- phyxiation to be the critical effect. As noted earlier, clinical signs associated with hydrogen-induced hypoxia would occur if the oxygen concentration were reduced to below the 1-h EEGL (105 mm Hg), the 24-h EEGL (127 mm Hg), or the 90-day CEGL (140 mm Hg) (NRC 2007). However, the lower explosive limit for hydrogen in air is 41,000 ppm, and 10% of this concentration is 4,100 ppm. That value is ap- preciably lower than hydrogen concentrations required to produce hypoxia. There- fore, the EEGL and CEGL values for hydrogen (see Table 7-3) are based on explo- sivity rather than toxicity arising from its asphyxiant properties. Because of the seriousness of an onboard explosion, a safety factor of 10 was used in deriving the EEGL and CEGL values (to represent 10% of the lower explosive limit). Applica- tion of the safety factor agrees with the approaches used by NASA to derive the spacecraft maximum allowable concentration (Wong 1994) and that used by EPA (1988) to set exposure standards for explosive gases. TABLE 7-2 Selected Inhalation Exposure Levels for Hydrogen Organization Type of Level Exposure Level (ppm) Reference Spacecraft NASA SMAC Wong 1994 1-h 4,100 24-h 4,100 30-day 4,100 180-day 4,100 Abbreviations: NASA, National Aeronautics and Space Administration; SMAC, spacecraft maximum allowable concentration.
Hydrogen 155 TABLE 7-3 Emergency and Continuous Exposure Guidance Levels for Hydro- gen U.S. Navy Values (ppm) Committee Recommended Maximum Exposure Level Current Proposed Values (ppm) EEGL 1-h 10,000 10,000 4,100 24-h 10,000 10,000 4,100 CEGL 90-day 10,000 10,000 4,100 Abbreviations: CEGL, continuous exposure guidance level; EEGL, emergency exposure guidance level. DATA ADEQUACY AND RESEARCH NEEDS Control of submarine air concentration of hydrogen is required to eliminate the explosive threat posed by this gas. Enacting suitable control measures essen- tially eliminates concern about adverse health effects associated with acute or chronic exposure to hydrogen at concentrations associated with an explosive haz- ard. However, the present discussion presumes that hydrogen is biologically inert and acts as a simple asphyxiant. No acute-exposure or repeated-exposure studies of hydrogen are available. Likewise, pharmacokinetic and metabolic information on hydrogen is unavailable (Wong 1994). REFERENCES ACGIH (American Conference of Governmental Industrial Hygienists). 1991. Hydrogen. P. 766 in Documentation of the Threshold Limit Values and Biological Exposure Indi- ces, 6th Ed, Vol. II. American Conference of Governmental Industrial Hygienists, Cincinnati, OH. ACGIH (American Conference of Governmental Industrial Hygienists). 2004. TLVs and BEIs. Threshold Limit Values for Chemical Substances and Physical Agents & Bio- logical Exposure Indices. American Conference of Governmental Industrial Hygien- ists, Cincinnati, OH. Budavari, S., M.J. OâNeil, A. Smith, and P.E. Heckelman, eds. 1989. Hydrogen. Pp. 759 in the Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals, 11th Ed. Rahway, NJ: Merck. Dean, J.A. 1979. Langeâs Handbook of Chemistry, 12th Ed. New York: McGraw-Hill. EPA (U.S. Environmental Protection Agency). 1988. Air Surveillance for Hazardous Mate- rials. Environmental Response Team, Office of Emergency and Remedial Response, U.S. Environmental Protection Agency, Washington, DC. Hagar, R. 2003. Submarine Atmosphere Control and Monitoring Brief for COT Committee. Presentation at the first meeting on Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants, January 23, 2003, Washington, D.C.
156 Exposure Guidance Levels for Selected Submarine Contaminants Hopfer, U. 1982. Digestion and absorption of basic nutritional constituents. P. 1160 in Text- book of Biochemistry with Clinical Correlations, T.M. Devlin, ed. New York: John Wiley and Sons. Lewis, R.J. 1996. Saxâs Dangerous Properties of Industrial Materials, 9th Ed., Vols 1-3. New York: Van Nostrand Reinhold. NRC (National Research Council). 1988. Submarine Air Quality. Monitoring the Air in Submarines. Health Effects in Divers of Breathing Submarine Air Under Hyperbaric Conditions. Washington, DC: National Academy Press. NRC (National Research Council). 1992. Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants. Washington, DC: Na- tional Academy Press. NRC (National Research Council). 2002. P. 95 in The Airliner Cabin Environment and the Health of Passengers and Crew. Washington, DC: National Academy Press. NRC (National Research Council). 2007. Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants, Vol. 1. Washington, DC: The National Academies Press. Olcott, T.M. 1972. Development of a Sorbent Trace Contaminant Control System, Including Pre- and Postsorbers for a Catalytic Oxidizer. NASA CR-2027. Johnson Space Cen- ter, Houston, TX. U.S. Naval Sea Systems Command. 1979. Submarine Atmosphere Control Manual. NVASEA S 9510-AB-ATM-010/(c). Unclassified section provided to the National Research Councilâs Committee on Toxicology (as cited in NRC 1988). Windholz, M., S. Budavari, L.Y. Stroumtsos, and M.N. Fertig, eds. 1976. P. 631 in the Merck Index: An Encyclopedia of Chemicals and Drugs, 9th Ed. Rahway, NJ: Merck. Wong, K.L. 1994. Hydrogen. Pp. 139-141 in Spacecraft Maximum Allowable Concentra- tions for Selected Airborne Contaminants, Vol. 1. Washington, DC: National Acad- emy Press.