Sewer Gas Smell in the House: Causes and What to Do

First, rule out a gas leak

Natural gas has no smell of its own. Atlanta Gas Light adds an odorant called mercaptan so that leaks smell like rotten eggs, which is also how many people describe sewer gas (Atlanta Gas Light). Propane is odorless too, and propane companies add mercaptan to it as well (Connecticut PURA).

If your home has natural gas or propane and you aren't sure what you're smelling, treat it as a gas leak. Atlanta Gas Light's instructions come down to this (Atlanta Gas Light; safety page):

• leave the area right away, warning others as you go;
• avoid anything that could make a spark, including light switches, phones, lighters, matches, flashlights and vehicles;
• once you're out of the area and in a safe place, call 911 or the gas company;
• stay away until the gas company or emergency crews say it's safe to go back.

Don't go looking for the source. Atlanta Gas Light's sewer-line safety page tells customers not to try to find the source of a suspected leak or stop it themselves. The same page says to act on even a small amount of the odor, and warns that a weak sense of smell or other odors can keep people from noticing it, so don't judge the danger by how strong the smell seems (Atlanta Gas Light). If another company serves your home, its emergency instructions are on its website and bill.

Use the checklist below only after gas is ruled out: your home has no gas service, or the gas company or fire department has checked.

Sewer gas and natural gas compared

Table sources: Wisconsin DHS (sewer gas) and Atlanta Gas Light (natural gas). You can't reliably tell the two apart by smell, so where you notice an odor is a clue, not proof.

Checklist: the usual causes, most likely first

1. A dry drain trap

Every fixture has a trap, a curved section of pipe that holds a plug of water to keep sewer gas out of the room. Georgia's version of the plumbing code requires each plumbing fixture to be separately trapped by a water-seal trap (Georgia amendments to the 2024 IPC, Section 1002.1). In a drain that goes unused for weeks, such as a basement floor drain, a guest shower or a laundry sink, the water evaporates and the seal is gone.

What to do: run water into every rarely used drain for a minute; Wisconsin's health department recommends flushing floor and sink drains with water so traps don't dry out (Wisconsin DHS). If the smell fades within a few hours, you found it. For a floor drain that keeps drying out, ask a plumber about a trap primer.

2. A failed toilet seal

A toilet is sealed to the flange on the drainpipe with a wax ring or a rubber-style gasket. A toilet that rocks, water seeping at the base, or a smell strongest right around the toilet points to a failed seal.

What to do: have a plumber pull the toilet and reset it on a new seal.

3. A loose or missing cleanout cap

A cleanout is a capped opening into the drain line, found indoors or in the yard. A missing, cracked or loose cap gives sewer gas a direct way out.

What to do: replace a missing or cracked cap and snug a loose one, but only when no backup is in progress. The cleanout guide explains how to find each cleanout and check it safely.

4. A blocked or damaged vent

The plumbing vent, usually a pipe through the roof, lets air in and out so that draining water doesn't pull the water out of the traps. Wisconsin's health department lists a leaking or blocked roof vent as one way sewer gas gets inside, and suggests checking the vent occasionally for debris such as leaves or bird nests (Wisconsin DHS).

What to do: if drains gurgle or traps lose water, have a plumber or roofer check the vent rather than climbing up yourself. A plumber can also replace a one-way vent valve (an air admittance valve) that sticks open.

5. Buildup in a drain or sink overflow

Hair, soap scum and slime inside a drain, or in the overflow channel of a bathroom sink, can smell sour or sewage-like even when the trap is full.

What to do: clean the stopper and the overflow opening with a brush and hot, soapy water.

6. Only when the hot water runs

Henry County Water Authority explains that bacteria in a water heater can turn sulfates in the water into hydrogen sulfide, the rotten-egg gas. Its quick test: fill a clean glass from the tap and smell it away from the sink; if the water has no odor, the smell may be coming from the drain (HCWA).

What to do: have a plumber service the water heater; the authority's flushing remedy involves hot water that it warns can cause severe burns quickly.

7. The septic system or its vent

EPA says a septic odor doesn't always mean the system is failing. The roof vent that lets the system's pressure equalize normally releases some odor, and a clogged or frozen vent, or a downdraft, can push it toward the house. Soft, wet or spongy soil around the drainfield when it hasn't rained is a sign the drainfield itself is failing (EPA).

What to do: EPA recommends calling a septic service provider or plumber, since an odor can't be diagnosed remotely. Not sure which system you have? See how to tell whether you're on septic or sewer.

8. A crack or leak you can't see

Wisconsin's health department notes that sewer gas in the soil next to a house can enter through foundation cracks, and that a leak behind a wall may need a plumber to find (Wisconsin DHS).

What to do: call a plumber. If the smell comes with slow or gurgling drains throughout the house, also check the signs of a main sewer line clog.

When the smell is a health concern

Hydrogen sulfide, the rotten-egg part of sewer gas, is noticeable at low concentrations, but OSHA warns that after a while at low levels, or very quickly at high levels, people can no longer smell it. A smell that fades isn't proof the gas is gone. OSHA also notes that hydrogen sulfide is heavier than air and builds up in low-lying and enclosed spaces (OSHA), and Wisconsin's health department says sewer gas is usually most concentrated where it enters a home, often the basement (Wisconsin DHS).

Wisconsin's health department lists headaches, dizziness, drowsiness and nausea among the symptoms of exposure, and says to see a doctor if you have them and suspect sewer gas. If you think gas has built up in an enclosed space, it advises leaving the building, calling the fire department and avoiding anything that could make a spark, since methane and hydrogen sulfide are both flammable (Wisconsin DHS).

Never go into a manhole, a sewer or a septic tank. Wisconsin's health department warns against entering a sewer line or a large tank without proper training and equipment (Wisconsin DHS), and OSHA notes that hydrogen sulfide in sewers and manholes can overcome people almost immediately, including would-be rescuers (OSHA). This is general information, not medical advice: see a doctor if you feel ill after exposure, and call a licensed plumber to find the source.

Bottom line

Rule out gas first: if your home has gas service and you aren't sure what you smell, leave and call 911 or the gas company from outside. Then try the simple fixes, starting with unused drains, and call a plumber if the smell lingers or returns.