H2S
Hydrogen sulfide, also known as
hydrosulfuric acid,
stink damp or
sewer gas, is a colorless, flammable gas that occurs naturally in
volcanic gases,
crude petroleum and
natural gas. It is also a product of
bacterial breakdown of
organic matter and thus occurs in human and animal
waste. It has a very unpleasant smell, which is commonly recognised as the smell if
rotten eggs;
farts also contain it as the most
olfactorily active ingredient.
In higher concentrations, hydrogen sulfide is toxic, acting as a cellular poison by disturbing the cells' aerobic metabolism in a way quite similar to cyanide poisoning. A single breath of pure hydrogen sulfide is sufficient to kill a human within minutes. Unfortunately (as lj tells me), hydrogen sulfide also rapidly numbs the sense of smell at higher concentrations. Therefore, a toxic concentration of it may in fact not advertise itself through the smell.