At low levels, hydrogen sulfide gas has a strong odor similar to rotten eggs. You can smell hydrogen sulfide gas at lower levels than may cause health effects, so smelling the gas does not always mean that it will make you sick. However, at higher levels, your nose can become overwhelmed by the gas and you cannot smell it. At higher levels, hydrogen sulfide gas can make you sick and could be fatal. Hydrogen sulfide gas occurs naturally in crude petroleum, natural gas, volcanic gases and hot springs. It can also result from bacterial breakdown of organic matter and be produced by human and animal wastes.