H2S is hydrogen sulfide. This gas is colorless and extremely dangerous. It occurs during the breakdown of human and animal waste and other organic matter.
Inhaling small concentrations of hydrogen sulfide can lead to eye, nose and throat irritation, and when a person has prolonged exposure to the gas, the symptoms intensify. Dizziness, headaches, nausea, shock, convulsions and death are all possible if the gas is inhaled and fresh air is not obtained.
At low levels, hydrogen sulfide presents an odor resembling rotten eggs. At high concentrations, the brain loses its ability to detect the gas despite the fact that the gas is still present.