Hydrogen selenide, H2Se, is an inorganic chemical gas at room temperature that has a noxious odor reminiscent of decay or rotten eggs like hydrogen sulfide, to which it is closely related. The gas is colorless and flammable, and can cause acute lung damage, eye damage, and death upon exposure, though no human fatalities from it have been reported in the US as of 2011. This is possibly due to the fact that it is heavier than air so it sinks to the floor of a room, and it quickly oxidizes into red selenium when it comes into contact with mucous membranes. Red selenium is a solid allotrope of metallic selenium which has a much lower toxicity level.
Where hydrogen sulfide changes to a solid with the contact of internal skin surfaces in humans, this exposure to red selenium levels requires 6,700 milligrams per 1/10th of a kilogram of body weight to have a 50% death rate in animal testing, which is a much higher tolerance level than for H2Se. Ethanol has a 7,000 mg per 1/10th of a kilogram of body weight exposure level for a 50% death rate, which is an equivalent level of risk. This puts red selenium into a class that is considered non-toxic by current chemical standards and makes it somewhat more practical to work with hydrogen selenide in industrial processes.
The Centers for Disease Control (CDC) in the US have estimated, however, that the toxicity level of hydrogen selenide gas in its original form has an Immediate Danger to Life and Health (IDLH) of just two parts per million. In tests on guinea pigs that were exposed to levels of 1.8 parts per million for two hours, 12.5% of the test animals died and these percentage rose to 25% deaths with four hours of exposure to 2.1 parts per million. The Occupational Safety and Health Administration (OSHA) Permissible Exposure Limit (PEL) for hydrogen selenide in the US is set at 0.05 parts per million.
Since hydrogen selenide is soluble in water, exposure limits are also set for accidental introduction into drinking water supplies. In the US, this safe limit is set at 0.01 milligrams per liter of water by the Environmental Protection Agency (EPA). In European nations like Germany, safe exposure limits are considered to be even lower, at 0.008 milligrams per liter of water.
Despite these drawbacks, hydrogen selenide is an important industrial gas. It serves as an industrial acid or oxidizer in the metal and semiconductor manufacturing industries. Since the gas has the potential to be highly explosive when exposed to air or even a distant spark which can cause a flashback to the source container, a hydrogen selenide cylinder for storing the gas is maintained at less than one atmosphere of pressure to minimize the possibility of out gassing leaks. As an industrial chemical, it is usually delivered in concentrations of 99.99% purity with trace elements of hydrogen sulfide and other gasses present such as nitrogen and methane. Storage containers are made of carbon-reinforced steel and are designed to hold the gas for an estimated shelf-life of 18 months before they pose any risk of degradation.