Methyl mercaptan, or methanethiol, is a colorless, flammable gas with a distinct odor that smells like rotten eggs or cabbage. It is found in coal tar and petroleum distillates and as an emission from paper and pulp mills. Methyl mercaptan occurs naturally in some foods (e.g., onions, radishes, asparagus, some nuts and cheese). It is naturally formed by microflora metabolism of proteins in marine environments and in human and animal gastrointestinal tracts. You might also recognize methyl mercaptan as part of the “boggy” smell that naturally occurs from decaying matter in marshes.