Sewer gas is a mixture of gases formed in sewers by the decomposition of organic household or industrial wastes, as well as other gases released from this effluent. When substances are improperly disposed in sanitary sewers or septic systems, their fumes become transient parts of the sewer gases. Sewer gas often has a "rotten eggs" smell, which comes from the hydrogen sulfide it contains.
Traps are used in plumbing to create a water seal that prevents sewer gases from entering buildings. Plumbing vents allow sewer gases to be exhausted outdoors. However, infrequently used plumbing fixtures may not pass enough water to keep their water seals from evaporating with time, especially in dry weather. The result is the most common means of sewage gas entering buildings, and can be remedied by regularly using the fixtures or adding water to their drains.
Exposure to sewer gas can also happen if the gas seeps in via a leaking plumbing drain or vent pipe or even through cracks in a building's foundation. Sewer gas tends to accumulate in basements, but can mix with all the air in a house.