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.

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