Hydrogen sulfide

Hydrogen sulfide (formula: $H_{2}S$) is a chemical compound which occurs as a colourless gas with acidic properties. Its characteristic odor, which resembles that of rotten eggs makes the identification of the compound easy.

It is present in the atmosphere (in low concetrations, less than 0.0003ppm) and also works as a signaling molecule for many animals, including humans. Its odor can be identified at concentrations of around 0.1 to 0.15ppm. As the concentration increases, it obtains a sweet odor and becomes deadly.

It's corrosive and flammable.