Hydrogen selenide is an inorganic compound with the formula H₂Se. It is the simplest (and virtually the only) hydride of selenium. H₂Se is a colorless, flammable gas under standard conditions. It is the most toxic selenium compound with an exposure limit of 0.05 ppm over an 8-hour period. Even at extremely low concentrations, this compound has a very irritating smell resembling that of decayed horseradish or 'leaking gas', but smells of rotten eggs at higher concentrations.