2.1.2 Odor and Toxicity
Hydrogen gas has no color, taste, or smell. Therefore, a gaseous hydrogen leak cannot generally be detected by human senses alone, except perhaps by human hearing.6 Other gaseous fuels, such as methane (natural gas), are also naturally colorless and odorless. However, sulfur-baodorants7 are usually added to pipeline natural gas specifically to aid in detecting leaks. The sulfur in these odorants can poison the catalysts used in automotive fuel cell systems, and satisfactory substitute odorants compatible with fuel cells have not yet been developed.
Hydrogen sensors must be used to detect hydrogen leaks. There are a number of technologies used to sense hydrogen, but many sensors rely on a catalyst that contains palladium, which breaks the chemical bonds between the atoms in a hydrogen molecule. The hydrogen atoms then diffuse into the catalyst, changing its electrical properties (e.g., resistance, capacitance). This change is proportional to the hydrogen concentration and can, therefore, be used to measure hydrogen levels in the air (Speer, 2004).
Hydrogen is not toxic to humans or animals. However, if leaking into an enclosed space, hydrogen gas can displace oxygen in the air and would pose an asphyxiation hazard in high enough concentrations. The risk of asphyxiation from hydrogen leaking into an open area is virtually non-existent because hydrogen is so buoyant that it will rise and diffuse to very low concentrations quickly. Even in an enclosed area the danger from a small leak is slight, but may be greater from a large leak that releases a significant volume of hydrogen relative to the size of the space.
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