Why Your Sink Smells Like Sewer Gas

That rotten egg smell drifting up from your sink drain is not normal. It is your plumbing telling you something is broken, dried out, or blocked — and the clock is ticking.

Sewer gas is not a nuisance. Hydrogen sulfide, the compound that makes it smell like rotten eggs, is toxic at moderate concentrations. Methane in sewer gas is explosive. A smelly sink is a safety issue, not a cleanliness issue, and it will not fix itself.

I have walked into houses where a smelly sink was the only symptom of a cracked vent pipe slowly leaking gas into the wall cavity for months. I have also seen a dry P-trap solved with a pitcher of water in thirty seconds. The range is wide. This guide helps you figure out which cause you are dealing with and when to stop troubleshooting and call a plumber.

Sewer gas safety — if the smell is strong, if anyone has a headache or nausea, or if you hear hissing near the drain, open windows. Do not light matches or flip switches. Evacuate if the smell is concentrated and call a licensed plumber from outside.

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What sewer gas actually is

Sewer gas is a mix of gases from decomposing human waste. Hydrogen sulfide (H₂S) gives it that rotten egg smell and is toxic at moderate levels. Methane (CH₄) is odorless and explosive. Ammonia is irritating. Carbon dioxide can displace oxygen in confined spaces.

One critical detail: above 100 ppm, H₂S deadens your sense of smell. You can be in danger without realizing the gas is still there. Symptoms of exposure include headache, nausea, dizziness, eye irritation, and fatigue. If you or anyone in the home has these symptoms with a sewer smell, ventilate and call a plumber immediately.

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Cause #1: Dry P-trap (most common, easiest fix)

Every fixture has a P-trap — the curved pipe under the sink that holds standing water. That water is the only barrier between your indoor air and the sewer system. If it evaporates, gas flows straight up through the drain.

This happens most with guest bathrooms, basement sinks, floor drains in laundry rooms and garages, and seasonal sinks that go weeks without use.

The test: Run the faucet for thirty seconds. If the smell disappears within a minute, your P-trap was dry. Problem solved.

The fix: Pour a gallon of water down every floor drain every two to three months. Run unused sinks for thirty seconds weekly. If you travel, pour mineral oil down rarely used drains — it floats on the water and slows evaporation.

If it persists: The trap is holding water but gas is bypassing it. Move to the next cause.

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Cause #2: Biofilm and bacterial buildup

If the smell is swampy and musty rather than sharp rotten eggs, you are smelling biofilm — bacteria and organic sludge inside your drainpipe. Common in kitchen sinks where food particles and grease feed bacterial growth.

Biofilm vs. sewer gas: Biofilm smells musty/sour, peaks when you first run water, and stays in one sink. Sewer gas smells like sharp rotten eggs, is constant, and can affect multiple fixtures. Biofilm will not give you a headache; sewer gas can.

What to try:

1. Boiling water (metal pipes only). Pour a kettle down the drain slowly. Skip this on PVC — heat softens joints.
2. Baking soda and vinegar. Half a cup baking soda, then a cup vinegar. Let it foam fifteen minutes, flush with hot water.
3. Enzyme drain cleaner. Safe for all pipes, effective long-term.
4. Clean the sink strainer. Pop it out and scrub with dish soap — often the actual source.

What to avoid: Chemical drain cleaners. They damage pipes and make biofilm return faster.

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Cause #3: Blocked or damaged vent pipe

Vent pipes run from drains up through the roof to let sewer gas escape outside and let air into the system. If a vent gets blocked — leaves, bird nests, ice — gas comes back down the nearest open drain.

Signs: Gurgling when the sink drains, slow drainage with the smell, smell worsens when water runs, toilets bubble after flushing, and the smell comes from more than one sink.

The toilet test: Run the sink while flushing the toilet in the same bathroom. If the toilet gurgles and the smell strengthens, the vent is likely blocked.

When to call a pro: A blocked vent needs snaking from the roof or a smoke test. $200 to $500 from a plumber. Not a safe DIY for most homeowners.

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Cause #4: Cracked P-trap

A P-trap can crack from age, chemical drain cleaner damage, or impact from items stored under the sink. A cracked trap holds some water but not enough to seal against gas.

Signs: Moisture on the bottom of the bend, water stains in the cabinet, smell strongest right at the cabinet opening, running water does not help.

The fix: Replacement. A plumber charges $150 to $300. If you are handy with PVC, a new trap assembly costs under $15 at a hardware store.

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Cause #5: Sewer line blockage

If you have ruled out traps, biofilm, and vents and still smell sewer gas, you likely have a partial main sewer line blockage. It creates a vacuum-and-pressure cycle that pushes gas back through the shallowest P-trap.

Signs: Multiple fixtures smell, drains are slow throughout the house, running one fixture makes another gurgle, toilets flush sluggishly, water backs up.

The five-minute test: Run the kitchen sink on full while flushing every toilet. If any gurgles or water rises in a tub or shower, the blockage is in the main line.

What NOT to do: No drain cleaners. No aggressive plunging — you can push water backward into another fixture and cause flooding.

What a plumber will do: Open a cleanout, check for standing water, camera the line to find the cause. Read main sewer line red flags homeowners should know for more.

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Cause #6: Leaking toilet wax ring (bathroom only)

If your bathroom sink smells but checks out fine — water holds in the trap, no gurgling — the source may be your toilet. The wax ring sealing the toilet to the floor flange can crack, letting gas escape around the base. That gas rises into the sink cabinet through the plumbing chase.

How to check: Light a match and hold it near the floor around the toilet base. If the flame flickers or you smell gas at floor level, the wax ring has failed. Also check for water stains or discolored caulk at the base.

I have been called to homes where the owner had cleaned the sink, replaced the trap, and snaked the drain — all unnecessary. The fix was a $15 wax ring and thirty minutes.

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When to call a plumber

Call a licensed plumber if:

• The smell persists after running water for thirty seconds
• More than one fixture smells or gurgles
• You have headache, nausea, or dizziness with the smell
• You see water backing up in any drain
• The smell returned after you tried to fix it

Call a plumber immediately if:

• The smell triggers headache or nausea
• Sewage is backing up
• Someone has trouble breathing or confusion
• You hear hissing near a drain or smell gas near a combustion appliance

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Frequently asked questions

Q: Why does my sink smell like rotten eggs even after I clean it?

The smell is likely coming from somewhere other than the drainpipe. Common hidden sources: a dry P-trap in a nearby floor drain or unused sink, a leaking toilet wax ring, or a blocked vent pipe. Run every drain in the house, including floor drains. If the smell persists, call a plumber for a smoke test.

Q: How can I tell biofilm from real sewer gas?

Biofilm smells musty and sour, peaks when water first runs, and stays in one sink. Sewer gas smells like sharp rotten eggs, is constant, and can affect multiple fixtures. Biofilm will not make you sick; sewer gas can cause headache and nausea.

Q: Is it safe to pour bleach down the sink to kill the smell?

No. Bleach reacts with organic matter to create chloramine gases that are toxic and irritating to your lungs. It also damages rubber gaskets. Use baking soda and vinegar or enzyme cleaners for biofilm. If the smell is real sewer gas, flush with water and call a plumber.

Q: Can a garbage disposal cause sewer gas smell?

A dirty disposal smells like food rot, not sewer gas. If you smell rotten eggs rather than sour food, the gas is coming from the drain beyond the disposal. Clean the disposal with ice and citrus. If the egg smell remains, the problem is in the trap or drain. Read garbage disposal problems you can safely check for more.

Q: Why does the smell come and go?

Intermittent sewer gas usually means a partial blockage or vent problem. Water flow temporarily pushes gas through the shallowest P-trap. If the smell happens only when water runs, focus on the vent and main line. If random, check for dry floor drains.

Q: How much does it cost to fix a sewer gas smell?

Dry P-trap: free. Cracked trap replacement: $150–$300. Vent blockage: $200–$500. Main line snaking: $250–$600. Camera inspection: $200–$500. Read how to read a plumbing estimate before authorizing work.

Q: Can sewer gas make you sick?

Yes. Low-level exposure causes headache, nausea, eye irritation, and fatigue. Higher concentrations can cause loss of consciousness and respiratory failure. If anyone has symptoms with a sewer smell, ventilate and call a plumber or emergency services.

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Related reading

This article is part of our drains cluster:

• Slow Drain or Main Sewer Problem? — fixture clog vs. main line diagnosis
• Main Sewer Line Red Flags Homeowners Should Know — what to watch for
• Kitchen Sink Clogs: What to Try and What to Avoid — step-by-step troubleshooting
• Sewer Camera Inspection Guide
• Toilet Wax Ring Failure Signs — hidden bathroom source
• Garbage Disposal Problems You Can Safely Check
• How to Read a Plumbing Estimate — before getting quotes

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Bottom line

A sink that smells like sewer gas is a mechanical failure of one of the barriers between your indoor air and the sewer system. It is not something to live with or mask.

Start simple: run every drain in the house, including floor drains. If the smell goes away, you had a dry trap. If not, work through each cause. Sewer gas is toxic, flammable, and will not go away on its own.

The right fix for most causes costs a few hundred dollars. The wrong response — ignoring it, using bleach, or pouring drain cleaner down — can turn a $300 problem into a $5,000 one.

When in doubt, call a plumber. We would rather spend ten minutes telling you to pour water down a floor drain than get called at 10 p.m. on a Saturday for a sewage backup that started three months ago as a funny smell.