Hydrogen selenide is a colorless, flammable gas with a noxious odor similar to hydrogen sulfide. It can cause acute lung and eye damage, but no human fatalities have been reported in the US. It rapidly oxidizes to red selenium, which is less toxic. The Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for hydrogen selenide in the United States is set at 0.05 parts per million. Despite its drawbacks, hydrogen selenide is an important industrial gas used in metal and semiconductor manufacturing.
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, although no human fatalities have been reported in the United States as of 2011. This is likely because it is heavier than air so that it sinks to the floor of a room and rapidly oxidizes to red selenium when it comes into contact with mucous membranes. Red selenium is a solid allotrope of metallic selenium which has a much lower level of toxicity.
Where hydrogen sulfide hardens to a solid with contact of internal skin surfaces in humans, this exposure to red selenium levels requires 6,700 milligrams per 1/10 kilogram of body weight to have a 50% mortality rate in animal tests, which is a much higher tolerance level than for H2Se. Ethanol has an exposure level of 7,000 mg per 1/10 kilogram of body weight for a 50% mortality rate, which is an equivalent level of risk. This places red selenium in a class 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 United States 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 of mice that were exposed to levels of 1.8 parts per million for two hours, 12.5% of the test animals died and this rate rose to 25% 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 United States is set at 0.05 parts per million.
Because hydrogen selenide is soluble in water, exposure limits are also set for accidental introduction into drinking water supplies. In the United States, this safe limit is set at 0.01 milligrams per gallon of water by the Environmental Protection Agency (EPA). In European nations like Germany, safe exposure limits are considered 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. Because the gas has the potential to be highly explosive when exposed to air or even a distant spark that can cause the source container to flash back, a hydrogen selenide cylinder to store the gas is kept at less than a atmosphere of pressure to minimize the possibility of gas leaks. As an industrial chemical, it is usually supplied in 99.99% purity concentrations with trace elements of hydrogen sulfide and other gases present such as nitrogen and methane. The storage vessels are made of carbon-reinforced steel and are designed to hold the gas for an estimated 18-month shelf life before posing any risk of degradation.
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