Yes, sewer gas can make you sick, but usually only at high concentrations

Sewer gas is a mixture of gases produced by decomposing organic matter in sewers and septic systems. The main components are methane, hydrogen sulfide, ammonia, and carbon dioxide. At the low levels most people encounter in their homes, sewer gas causes mild symptoms like nausea, headaches, and eye irritation. At higher concentrations—which happen when ventilation fails or a trap dries out—it can cause more serious problems including dizziness, respiratory distress, and in rare cases, loss of consciousness.

The gas that causes the most noticeable health effect is hydrogen sulfide, which smells like rotten eggs. Even small amounts irritate mucous membranes in your nose, throat, and eyes. Most people notice the smell long before the gas reaches dangerous levels, which actually works in your favor—your nose is an early warning system.

Key Takeaways

  • Sewer gas at typical household levels causes temporary irritation like headaches and nausea, not serious illness.
  • Hydrogen sulfide, the component that smells like rotten eggs, irritates your eyes, nose, and throat at concentrations well below those that cause systemic harm.
  • A dried-out P-trap under a sink or toilet is the most common source of sewer gas entering your home, and refilling it with water stops the problem when ready.
  • Prolonged exposure to high concentrations can cause respiratory problems and dizziness, but this requires a significant plumbing failure or a sealed space like a crawlspace.

Symptoms of sewer gas exposure in your home

When sewer gas enters your living space, the first sign is usually smell—a strong, unpleasant odor like rotten eggs or a backed-up toilet. If you continue to breathe the gas, you may experience headache, nausea, dizziness, or a sore throat within minutes to hours. Your eyes may water or feel irritated. These symptoms typically fade once you leave the area or the source is fixed.

More serious symptoms appear only at higher concentrations or with longer exposure. These include difficulty breathing, chest tightness, and confusion. People with asthma or chronic respiratory conditions are more sensitive and may experience symptoms at lower levels than others. If you have these symptoms and suspect sewer gas, open windows when ready and move to fresh air.

Why a dried P-trap is the most common culprit

The P-trap is the U-shaped bend in the pipe under your sink, toilet, or shower. It holds standing water that acts as a seal, blocking sewer gases from rising into your home. When a drain sits unused for weeks or months—a guest bathroom, a seasonal cabin, or a rarely used sink—the water evaporates. Once the seal breaks, sewer gas flows directly into your home.

This is the easiest problem to fix. Pour a quart of water down the drain to refill the trap. The smell should disappear within minutes. If you have a fixture you use infrequently, pour water down it once a month to keep the trap full. This single step prevents most sewer gas problems in homes.

Plumbing failures that let sewer gas in

Beyond dried traps, sewer gas enters homes through cracks in drain lines, loose toilet seals, or a backed-up main sewer line. A cracked drain pipe underground may not cause obvious water damage but will release gas into your crawlspace or basement. A toilet that rocks slightly when you sit on it may have a broken wax ring, the seal between the toilet and the floor flange—this allows gas to seep around the base.

A backed-up main line is more serious. If multiple drains in your home are slow or gurgling, or if you smell sewer gas throughout the house rather than from one location, the main line may be blocked or damaged. This requires a plumber to inspect with a camera and determine whether the line needs cleaning or repair. Do not ignore these signs, because a backed-up line can eventually cause sewage to back into your home.

Ventilation problems in older homes

Sewer systems are designed to vent gases safely through vent stacks—pipes that run from your drains up through the roof. If a vent stack is blocked by debris, ice, or a bird's nest, gases cannot escape and build up in your drain system. When pressure builds, gas can be forced back into your home through drains and fixtures.

Blocked vents are more common in older homes or after heavy storms. You may notice slow drains throughout the house, gurgling sounds from drains, or sewer smell in multiple rooms. A plumber can clear the vent from the roof or use a plumbing snake to remove the blockage from inside. Once the vent is clear, pressure equalizes and the smell stops.

When sewer gas is a sign of a serious problem

Occasional sewer smell from a single drain is usually a dried trap. But persistent smell from multiple drains, smell in your basement or crawlspace, or smell that does not improve after you refill a trap points to a larger issue. A backed-up line, a cracked drain, or a failed vent system all require professional inspection.

If you also notice sewage backing up into your shower or tub, or if you see wet spots in your yard or basement, stop using the drains and call a plumber when ready. These are signs of a main line failure, which can contaminate your property and pose a health risk beyond sewer gas irritation.

How to reduce sewer gas in your home right now

Start with the simplest fixes. Pour water down any drain you have not used in more than a week—especially guest bathrooms, basement sinks, or outdoor drains. Check that all toilet seats fit snugly and do not rock. Make sure your washing machine drain hose is properly connected and does not have cracks.

Open windows to ventilate the area where you smell the gas. If the smell is strong and does not fade after you open windows and refill a trap, do not assume it will resolve on its own. Call a plumber to inspect your drains and vents. A camera inspection of your main line costs less than an emergency repair after sewage backs up into your home.

Frequently Asked Questions

Can sewer gas cause permanent damage to my lungs?

Occasional exposure to sewer gas at home levels does not cause permanent lung damage. Chronic exposure to very high concentrations—which would require a severe plumbing failure in an enclosed space—can irritate airways over time, but this is rare in residential settings. If you have asthma or another respiratory condition, even low-level exposure may trigger symptoms, so fix the source promptly.

Is sewer gas toxic to pets?

Pets are affected similarly to humans—they may experience nausea, irritation, or respiratory discomfort at high concentrations. Cats and small dogs are more sensitive because of their smaller body size. If your pet seems unwell and you smell sewer gas, move them to fresh air and fix the source. Pets cannot tell you they are uncomfortable, so take any sewer smell seriously if you have them.

What if I smell sewer gas but cannot find the source?

Check every drain in your home, including basement floor drains, outdoor drains, and rarely used sinks. Pour water down each one. If the smell persists, the source may be underground or in your vent system. A plumber with a camera can inspect your main line and vent stack to locate the problem. Do not wait—a hidden leak or blockage usually gets worse.

Does sewer gas smell go away on its own?

It depends on the cause. If a trap dried out, refilling it stops the smell within minutes. If a vent is blocked or a line is cracked, the smell will not go away without repair. Some people become used to a mild smell and stop noticing it, but the gas is still present. If you smell it, fix it rather than waiting.