Sewer smell in the bathroom or kitchen: where it comes from

A little water in the trap keeps the sewer out.

The answer in 2 lines. A sewer smell almost always comes from a trap: the bend under every drain holds a little water that blocks sewer gas. A shower nobody uses or a basement floor drain dries out, a trap full of hair and gunk smells, a worn toilet wax ring lets gas through, and a blocked vent pipe can suck traps dry. Run water, clean the trap, and if the smell comes back, look at the toilet seal and the venting. If it might be natural gas, get out first and ask later.

The first 3 steps

Run water for a minute in every drain that's rarely used: guest shower, basement floor drain, laundry sink, utility tub.
Sniff drain by drain, stopper open, to find the worst one.
Air out the room while you search.

Stop: danger

It might be natural gas: gas companies add a rotten-egg odor, which is easy to confuse with sewer gas. If in doubt, treat it as gas: don't use switches, phones or anything that could spark inside, leave the house, and from outside call your gas company's emergency number or 911 (Kansas Corporation Commission: natural gas safety).
Strong smell with headache, dizziness or nausea, or a smell in a confined space like a crawl space or sump pit: get everyone out into fresh air. High levels of hydrogen sulfide, a sewer gas, are dangerous, and at high levels it can dull your sense of smell: a smell that suddenly fades is not proof the air is safe (Virginia Department of Health: sewer gas FAQ). Call 911 for serious symptoms.
Never mix drain cleaner, bleach or descaler: some mixtures release toxic gases.

Where is it coming from?

A shower, sink or floor drain that's rarely used: the trap has dried out. Run water regularly.
A sink or shower that drains slowly: a dirty trap (hair, soap, biofilm). Take it off and clean it.
Around the base of the toilet: the wax ring between the toilet and the floor drain is worn, or the toilet rocks. Have it reset with a new ring.
A gurgle when the tub drains, and a nearby sink trap empties: the drain isn't venting properly. The vent pipe on the roof may be blocked (leaves, a nest, ice), or an air admittance valve under a sink may have failed (how drain venting works). Job for a licensed plumber.
Near the washer: the drain hose pushed too deep into the standpipe, or a standpipe without a trap.
Throughout the building, worse in hot weather: a shared stack or vent. Tell building management.
Only from the hot water, on a private well: the smell may come from the water itself, not the drain (a reaction in the water heater). Ask the water heater's maker or a licensed plumber.

Cleaning a trap, step by step

Put a bucket under the trap (the U-shaped bend under the sink).
Loosen the slip nuts by hand, tools only if needed, and empty the trap into the bucket.
Pull out hair and buildup, and scrub it with hot water and a brush.
Check the washers: replace a flattened or cracked one with the same size.
Screw it back hand-tight, run water and check for drips.

What not to do

Pouring a drain cleaner and then opening the trap: risk of chemical burns.
Covering the smell with air freshener: the cause is still there, and so is the gas.

If the smell keeps coming back

Shower drain: lift the grate and clean the hair and slime below it, as far as you can reach.
Sink overflow (the small hole near the top of the basin): it gets slimy too; flush it with hot water and a bottle brush.
Basement floor drain: if you can't run water there often, a trap primer or a drain trap seal insert (sold at hardware stores, sized to the drain) keeps it from drying out.
Vent: drains need to "breathe" through a roof vent or an air admittance valve, or traps get sucked dry. Don't go on the roof yourself; call a licensed plumber.
Long trip: before you leave, pour water into every trap; when you're back, run water everywhere.

Good to know

The water held in the trap is called the trap seal: it's what stops sewer gas from coming back up. Wisconsin's health department explains why a dry trap lets it in (Wisconsin DHS: sewer gas).
Sewer gas smells come mostly from hydrogen sulfide: people notice it at very low levels, well below harmful ones. At home, the usual issue is the nuisance, not poisoning, but a smell that's suddenly very strong deserves attention.

Who pays?

Renters: tell your landlord or building manager in writing. In most states, landlords must keep plumbing in working order, but the details depend on your state, city and lease; cleaning a trap is often left to the tenant. See our guide on repairs between tenant and landlord.
Condo or apartment building: shared stacks and vents are usually the association's or owner's responsibility. Report it to management.

In Denver, Mark smells sewer in the basement bathroom nobody has used since spring. He runs the shower, the sink and the floor drain for a minute each. The next day the smell is gone: the traps had dried out. He adds a monthly reminder on his phone.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on November 1, 2026, for the United States. Confidence: high for safety advice (gas and sewer gas, from state agencies); medium for the list of causes, which describes common cases, not a diagnosis, and for who pays in a rental, which depends on your state and lease.