You know the smell: a combination of rotten egg, a little ammonia, and just plain nasty. Low levels of sewer gas are annoying, but higher concentrations can be dangerous. There are five ways that sewer gas gets into a home:

••  A loss of a plumbing fixture’s  water reservoir, called a “trap,” is the first one. Each fixture has a trap in the drain pipe below it (circled under a bathroom sink in the picture at right) or the trap is an integral part of the fixture. Often called a “P-trap,” although it is more of a “U” shape, the water in the trap acts as a seal to keep the gas from rising up through the drain. The trap seal can be lost due to evaporation of all the liquid in it, which is common in floor drains, for example, that haven’t had anything rinsed down them in a while. If you follow your nose to the smell and its emanating from a drain that hasn’t been used recently, just open the tap for a few seconds or, in the case of a floor drain, pour a few cups of water down it.

••  Also, a type of unvented drain configuration called an “S-trap” (shown below) sometimes will suck all the liquid in the trap down the drain as it pulls air from the drain opening to allow the drain cycle to finish. S-traps are no longer code-approved, although we see them occasionally in older homes or a haphazard remodeling. Also, see our blog posts What is an "S-Trap" under my sink? Why is it a problem?

••  The third cause of a sewer gas leak is a loose toilet with a damaged wax seal at the bottom. Loose or corroded bolts that secure the toilet to the floor are one reason. You can test for it by straddling the toilet with your legs while standing, and then gently rocking side-to-side a little to see if the toilet moves with you. Another two reasons are any settlement in the floor under the toilet, and a defective or old wax seal. When the seal is just beginning fail, it will allow gas to leak out into the room but not leak water when flushed.

    The smell can appear when someone is showering, because the water going down the drain intermittently blocks air from getting into the drain pipe from a roof vent. So the air exits through the gap under the toilet. High winds blowing over the roof vent pipe may also create enough negative pressure to pull sewer gas up through the toilet gap.

••  The fourth cause can be 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.

••  And the last 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. In the photo below, the homeowner solved the problem of the stink coming into the adjacent window by sealing the vent pipe, which causes a new problem of sluggish drains.

    See our blog post What are the code requirements for plumbing vent terminations? to learn more about vent terminations. For a more detailed list of places to look for the cause of sewer gas smell, visit our blog post If all the plumbing drains have water in them and you can still smell sewer gas, what's causing the problem?

    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;  
• 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. 
• 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.
• 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.

      An anaerobic bacteria colony in the water heater can cause a smell similar to sewer gas. For details, go to our blog post Why does the water have a rotten-egg smell in some empty houses?

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Here’s links to a collection of our other blog posts about PLUMBING PIPES:

• How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

• Can galvanized steel pipe still be used for new water lines in a house? 

• How can I tell if I have cast iron pipes in my house? 

• Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection? 

• What is the difference between green and white sewer drain pipes?

• Is a washing machine drain hose required to be secured at the standpipe?

• What are the abandoned pipes sticking out of the wall in my house?  

• What are the code requirements for layout of drain piping under sinks?

• What causes a gurgling sound when a bathtub or sink drains? 

• What is a "combination waste and vent" in a plumbing system? 

• What is a building trap?  

• What is a galvanized nipple?

• What are the pipes sticking out near my water valves?

• How do you accurately find a broken water pipe leak under the floor slab?

• What is the difference between water pipe and sewage (waste) pipe? 

• Are plastic pipes (PVC, CPVC, and PEX) safe for drinking water? 

• Is a hot water faucet handle required to be on the left? 

• What is a dielectric union? 

• What's that powdery crust on the pipe connections at the water heater? 

• Can you connect CPVC pipe directly to a gas water heater?  

     Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.