Sewer gas is a complex mixture of toxic and non-toxic gasses which can include ammonia, methane, carbon monoxide, sulfur dioxide, hydrogen sulfide and nitrogen oxides. Sewer gas is produced and collected in sewage systems through the decomposition of organic household or industrial wastes.
Sewer gas not only smells bad can be harmful if breathed. Your household drainage system is designed with traps at every fixture that create a water seal which keeps the sewer gas from entering the building. Plumbing vents are run to allow the venting of the gas outdoors.
The main cause of a sewer gas smell inside the home is a dried out p-trap. This commonly happens in a spare bathroom that does not get used often. Also floor drains in laundry areas and near water heaters have traps that tend to dry out.
If you have a spare bathroom make sure the traps are primed every few weeks or so by running your bathtub and sinks. A trap that services a water heater or furnace condensate drain should have a special device called an automatic p-trap primer. This will periodically let some water out to maintain your trap seal. If one is not present you should have one installed by a plumber.
Another cause of sewer gas smells in the home is roof vents that are too low. Sewer gas leaving these vents can be drawn in by an evaporative cooler, an open window, or through a bathroom fan exhaust vent. A plumber can check where your vents terminate and extent them to correct levels if needed.
Sometimes a sewer gas smell is the result of a broken vent pipe inside the wall. These leaks can be found with a smoke test. Once the leak is located the wall is cut open and the vent can be repaired.