Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants: Volume 2 (2008)

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 committee’s recommendations for hydrogen concentrations considered the toxicity and explosivity of the gas in the reduced-oxygen environment present onboard submarines. The committee’s 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.

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.

Hydrogen is the most abundant element and is present in Earth’s 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 submarine can reflect the low ambient concentrations found in the air, biologic sources,

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 submarines 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).

At very high concentrations in air, hydrogen is a simple asphyxiant gas because 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 asphyxiation may occur at lower hydrogen concentrations when oxygen concentrations 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 basis of the explosivity of hydrogen rather than its toxicity.

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 atmosphere 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,

Accordingly, hydrogen concentrations of 14.3% and 5.6% are required to reduce 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 alveolar 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. Hypoxia will lead to stimulation of medullary chemoreceptors and then to a compensatory increase in pulmonary ventilation. Important consequences of mild hypoxia include impaired judgment, reduction in ability to perform discrete motor movements, short-term memory loss, mental fatigue, headache, occasional nausea, and increase in reaction times. Rapid asphyxiation is characterized by tachypnea, cyanosis, sweating, cardiac arrhythmia, depression of the central respiratory center followed by loss of consciousness, and coma (reviewed in NRC 2007).

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.

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) classifies 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 flammable and explosive properties.

A health-based exposure standard would consider hydrogen-induced asphyxiation 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 appreciably lower than hydrogen concentrations required to produce hypoxia. Therefore, the EEGL and CEGL values for hydrogen (see Table 7-3) are based on explosivity 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). Application 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.

Control of submarine air concentration of hydrogen is required to eliminate the explosive threat posed by this gas. Enacting suitable control measures essentially eliminates concern about adverse health effects associated with acute or chronic exposure to hydrogen at concentrations associated with an explosive hazard. 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).

ACGIH (American Conference of Governmental Industrial Hygienists). 1991. Hydrogen. P. 766 in Documentation of the Threshold Limit Values and Biological Exposure Indices, 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 & Biological Exposure Indices. American Conference of Governmental Industrial Hygienists, Cincinnati, OH.

Budavari, S., M.J. O’Neil, 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. Lange’s Handbook of Chemistry, 12th Ed. New York: McGraw-Hill.

EPA (U.S. Environmental Protection Agency). 1988. Air Surveillance for Hazardous Materials. 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.

Hopfer, U. 1982. Digestion and absorption of basic nutritional constituents. P. 1160 in Textbook of Biochemistry with Clinical Correlations, T.M. Devlin, ed. New York: John Wiley and Sons.

Lewis, R.J. 1996. Sax’s 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: National 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 Center, 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 Council’s 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 Concentrations for Selected Airborne Contaminants, Vol. 1. Washington, DC: National Academy Press.