TABLE 7-1 Physical and Chemical Properties of Hydrogen Gas



CAS registry number


Molecular formula


Molecular weight


Boiling point


Melting point

−259.2°C at 54 mm Hg

Flash point


Explosive limit

4.1% by volume in air (lower limit)


0.00008987 g/cm3 at 20°C

Vapor pressure




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 Budavari 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 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.

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