In the early 20th century, sewer gas was greatly feared by homeowners, many of whom believed it could introduce germs into the house that would cause disease. Nowadays people know that sewer gas isn't actually carrying germs, but that doesn't mean it's not hazardous. This noxious gas can become a fire hazard or pose a threat to your health.
Household and industrial waste contains a rich mixture of organic compounds. A variety of microbes thrive in this environment and obtain the nutrition they need by digesting the waste. As they break it down, they release gases and waste byproducts, just as your metabolism releases byproducts like carbon dioxide. Some of the gases produced by these bacteria are harmless, but others can pose a hazard at high concentrations.
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Sewer gas contains a diverse variety of compounds, including methane (CH4), hydrogen sulfide (H2S), ammonia (NH3), carbon dioxide, sulfur dioxide (SO2) and nitrous oxides (N2O). Sometimes other gases are present as well, depending on the types of waste in the sewer system. Carbon dioxide is not especially hazardous except at very high concentrations. Nitrous oxide, better known as laughing gas, has an anesthetic effect. The other gases present, however, are more dangerous.
Methane is a highly flammable gas -- the same gas, in fact, that you use to heat your stove. If a sufficient amount of sewer gas seeps into your house and methane concentrations build up, inadvertently starting a fire or a serious explosion (by lighting a match, for example) is possible. Sulfur dioxide is a noxious gas that acts as an air pollutant. Hydrogen sulfide is the compound that gives rotting eggs their foul smell. Both hydrogen sulfide and ammonia are toxic gases, and at high concentrations they can kill.
Since both hydrogen sulfide and methane are flammable, high levels of sewer gas become a serious fire hazard. Fortunately detecting it is easy; the hideous scent of hydrogen sulfide is obvious even at very low concentrations, so if sewer gas is entering your house, you'll know. Plumbing systems have traps to prevent sewer gas from backing up into the house; adding water to plumbing fixtures on a regular basis prevents the sewer gas from seeping through. Sometimes a blocked vent or a corroded pipe, however, can allow sewer gas to leak into the house. If you have a problem of this kind and detect a rotten-egg odor in your bathroom, kitchen or basement, consult a plumber to figure out what you can do.