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Sewer gas is a mixture of gases formed during the decomposition of household or industrial wastes. These include hydrogen sulfide, ammonia and methane(which are highly toxic), as well as carbon dioxide, sulfur dioxide and nitrogen oxides.

Exposure to sewer gas can happen at home if it seeps in via a floor drain or a leaking or blocked plumbing vent on the roof 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. Individuals who work in sanitation industries or on farms might be exposed on the job if they clean or maintain municipal sewers, manure storage tanks or home septic tanks.

Sewer gas often has a "rotten eggs" smell, which comes from the hydrogen sulfide it contains. Exposure to low levels of this toxic chemical can irritate the eyes, cause a cough or sore throat, shortness of breath and fluid accumulation in the lungs. Prolonged low-level exposure may cause fatigue, loss of appetite, headaches, irritability, poor memory and dizziness. High concentrations of hydrogen sulfide can interfere with the sense of smell so you don't sense the "rotten eggs" smell that warns of its presence. At very high concentrations, hydrogen sulfide can cause loss of consciousness and death.

The other toxic component of sewer gas is methane, which interferes with the oxygen-carrying capacity of blood and can cause suffocation and death when you inhale high concentrations. Exposure to low levels causes headache, nausea and drowsiness.

If you suspect that sewer gas is leaking into your home, contact your local public health department. Try to get an inspection. Alternatively, you can call in a plumber to find and repair a leak. Because sewer gas is highly flammable, don't take any chances. If the smell is strong, call the fire department, avoid striking matches or using appliances that produce flames, and get your family out of the building.

Andrew Weil, M.D.