Short definition
Sewer gas is the mixed gas inside your home’s drains and the public sewer beyond — primarily methane, hydrogen sulfide (the rotten-egg smell), carbon dioxide, ammonia, and various decomposition byproducts. Trap seals and the venting system together keep it out of the building. When a homeowner notices a sewer smell, the question isn’t whether the gas exists — it always does — but how it’s getting past the protections that should hold it back.
What it is
Decomposing organic matter in the sewer system produces a constantly-replenished mix of gases. The main components and what they do:
- Methane (CH₄) — flammable at 5–15% concentration in air. Residential indoor sewer-gas levels almost never reach that, but a slab leak in a small enclosed space can.
- Hydrogen sulfide (H₂S) — the rotten-egg smell. Detectable to the human nose at 0.0005 ppm; OSHA’s 8-hour industrial limit is 10 ppm. Toxic at much higher concentrations than residential air ever sees, but the smell alone is unpleasant and a clear signal something has failed.
- Ammonia, CO₂, and volatile organic compounds from decomposition — contribute to the overall odor but don’t drive the safety considerations on their own.
The two-line defense is simple: every fixture has a trap holding water that blocks the gas from coming up the drain, and every drain is vented to the roof so any gas that does escape exits well above doors, windows, and HVAC intakes.
Why it matters to a homeowner
When sewer gas is reaching the indoor air, one of the protections has failed. The good news: in residential settings, the failures are usually fixable in an hour and don’t represent a serious health hazard at the concentrations involved. The diagnostic value of “sewer smell” is that it points at which trap or vent has lost its function.
If a sewer-gas smell coincides with any of these, take it seriously and call a professional: visible cracks in DWV pipe inside walls, a smell that persists after refilling every trap and ventilating the room, or a smell paired with sewage backup at any drain.
How it gets into the home
- Dry trap from evaporation (vacation home, unused floor drain).
- Trap seal loss by self-siphonage (S-trap, long trap arm) or induced siphonage (neighbor fixture).
- Vent obstruction or freeze closure lets pressure events blow trap seals.
- Cracked DWV pipe in a wall cavity — gas leaks into the building airspace directly.
- Roof vent terminal too close to a window or HVAC intake — UPC 906.1 violation.
- Failed AAV (air admittance valve) — diaphragm worn out after 5–7 years.
Common variants / not the same as
- Sewer gas vs. natural gas leak. Both have an unpleasant smell, but they’re distinct: sewer gas is biological/rotten; natural gas (which has mercaptan added as an odorant) is chemically sulfurous in a different way. If you can’t tell which one you smell, evacuate and call your gas utility.
- Sewer gas vs. mold/mildew. Sewer is rotten and sulfurous; mold is musty and earthy.
- Sewer gas vs. disposer odor. Disposer is decaying food trapped in the unit; sewer is the wider methane/H₂S mix from the drain system.