The second cause of a sewer gas leak is more difficult to track down. A crack, or any other damage that causes an opening in a vent pipe (the vertical pipe that extends through the roof), the trap arm (the pipe between the trap and the vent pipe, or the base of a toilet will also allow sewer gas to escape. Also, the failure of drain pipes in the crawl space or under the floor slab can allow gas to rise into the house. If you have filled all the traps around the home and still have a lingering odor, it’s time to call a plumber to locate the pipe damage.
The third stinky-gas cause occurs outside the home where the pipes that exhaust the sewer gas into the atmosphere terminate. They are supposed to extend above the roof and away from any windows or other openings into the home. But if the top of a vent pipe is below the roof and near a window, a breeze blowing in the right direction will push the sewer gas back into the the house. Check outside any rooms with a sewer gas smell for vent pipes nearby. A plumber can usually fix a vent pipe termination problem easily.
So, how dangerous is sewer gas? According to the International Association of Certified Home Inspectors (interNACHI), sewer gas accumulation in a home creates the following risks:
• Hydrogen Sulfide Poisoning. Hydrogen sulfide is an explosive and extremely toxic gas that can impair several different systems in the body at once, most notably the nervous system. So potent that it can be smelled at 0.47 parts per billion by half of human adults, the gas will begin to cause eye irritation at 10 parts per million (ppm) and eye damage at 50 ppm. Other low-level symptoms include nervousness, dizziness, nausea, headache and drowsiness. Exposure to higher concentrations can lead to pulmonary edema, and still higher levels (800 to 1,000 ppm) will cause almost immediate loss of consciousness and death;
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•Asphyxiation. When sewer gases diffuse into household air, they gradually displace oxygen and suffocate occupants. The effects of oxygen deficiency include headache, nausea, dizziness and unconsciousness. At very low oxygen concentrations (less than 12%), unconsciousness and death will occur quickly and without warning. Oxygen will be at its lowest concentrations in the basement, which is where heavy sewer gases, principally methane, are likely to collect.
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•Fire or Explosion. Methane and hydrogen sulfide are explosive components of sewer gas. Vapors from improperly disposed fuel can further increase the risk of fire or explosion.
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•Odor. Hydrogen sulfide is responsible for sewer gas’s characteristic rotten-egg smell, which can be overbearing even at extremely low concentrations. The gas’s odor is a safeguard, however, because it alerts building occupants to the leak long before they’re in any serious danger. It is important to note that at roughly 100 ppm, the olfactory nerve becomes paralyzed, removing the victim’s sense of smell and, subsequently, their awareness of the danger. Another "warning smell" comes from ammonia, which will sear the nostrils and progressively irritate the mucous membranes and respiratory tract. This gas, unlike hydrogen sulfide, is sufficiently irritating that building occupants are likely to vacate before its concentration rises to toxic levels.
If the smell is so intense that you think a high level of sewer gas has accumulated in the home you should evacuate and call the fire department for assistance. Because methane can be explosive, don’t create any spark from an electric appliance, matches or cigarette lighter.
For a lower level of gas, just open the windows while you search for the source. Clearing the air will also help you pinpoint the location of the problem.
While we hope you find this series of articles about home inspection helpful, they should not be considered an alternative to an actual home inspection by a local inspector. Also, construction standards vary in different parts of the country and it is possible that important issues related to your area may not be covered here.
©2015 - McGarry and Madsen Inspection.
