Septic gas -- also called sewer gas -- is a mixture of mostly hydrogen sulfide and smaller amounts of methane, carbon dioxide, sulfur dioxide, nitrous oxides, chlorine bleaches, industrial solvents, gasoline and other materials. Some of the gases are toxic and some benign. The gas is normally kept away from human contact in underground septic and sewage systems, but these gases can escape if your plumbing or septic system backs up or fails altogether.
Septic gases are a natural byproduct of septic and plumbing system functioning, so any interruption with the normal functioning of the septic system can force gases out of the system. Everything from catastrophic problems such as septic drain field failure to minor problems such as an incorrectly installed toilet gasket can contribute to this problem. If you only smell these gases after a shower, inspect the roof vent pipe. A common cause of septic gas smell is debris, birds' nests, dead animals, ice, leaves or other obstruction blocking the roof vent pipe and causing septic gases to escape into outdoor areas (roof vent pipes are used to expel shower and laundry waste water).
What to Do
When you've discovered a septic gas smell outside your home, first follow the smell of the gas to find its true source. If the roof vent is the source of the problem, clear the debris and obstructions from the vent and run the shower again, checking for any septic gas smells. If you still smell septic gas, contact a plumber immediately as the problem is likely deeper in the plumbing system and will need to be handled by a professional.
Septic Gas Exposure
Exposure to septic gas at high enough levels to cause health problems to humans is unlikely in the case of residential plumbing and septic systems, but it is still important to know the signs of septic gas exposure just to err on the side of caution. Low-level exposure causes eye and respiratory irritation, nervousness, dizziness, nausea, headache and drowsiness. If you or anyone in your home is experiencing these symptoms and they cannot be reasonably attributed to another cause, seek medical attention.