Sewer Gas: What It Is, Health Dangers, and Detectors

Sewer gas is the mix of gases released when waste breaks down in your drains and sewer line, and its main hazards are hydrogen sulfide (H2S) and methane (CH4). Hydrogen sulfide gives the rotten-egg smell and is toxic at high levels. Methane is explosive. Small whiffs are a warning sign, but a strong, steady sewage odor indoors calls for fast action.

What is sewer gas made of?

Sewer gas is not one chemical. It is a blend produced by bacteria digesting organic waste, dominated by hydrogen sulfide and methane, with ammonia, carbon dioxide, and traces of carbon monoxide and sulfur dioxide. The exact mix shifts with what is decomposing and how much oxygen is present. Two components drive the danger: H2S is toxic, and methane is flammable.

Is sewer gas dangerous?

Sewer gas is dangerous at high concentrations, mainly because of hydrogen sulfide. It is the second most common cause of fatal gas inhalation on the job after carbon monoxide. In homes, the usual outcome from a small leak is discomfort: headaches, nausea, and irritated eyes. The real risk comes with a large leak in a sealed, poorly ventilated space, where levels can climb fast.

The odor itself is a poor gauge of danger. You can smell H2S at 0.0005 ppm (0.5 ppb), thousands of times below a harmful dose. That means a faint smell is a warning, not proof of poisoning. The trap is olfactory fatigue: at roughly 100 ppm, H2S deadens your sense of smell within 2 to 15 minutes, so the gas seems to fade while the concentration stays deadly.

Sewer gas symptoms by concentration (ppm)

Sewer gas symptoms track the hydrogen sulfide concentration, measured in parts per million (ppm). Low levels irritate; high levels can kill in minutes. The table below pairs H2S levels with documented effects, using OSHA and NIOSH exposure limits. Note that pulmonary edema and other serious effects may be delayed up to 72 hours, so get evaluated even if you feel better after leaving.

If anyone shows severe symptoms, collapses, or the smell is overpowering, get everyone out and call 911. Do not re-enter to help until responders arrive, because a single breath at high concentration can drop a rescuer too.

Can sewer gas explode?

Yes. The methane in sewer gas is flammable, and hydrogen sulfide burns as well. Methane is explosive in air between about 5 percent and 15 percent by volume, its lower and upper explosive limits. In a confined space with a spark, pilot light, or electrical arc, a heavy buildup can ignite. This is uncommon in a normal house but real in sealed basements, crawl spaces, and manholes.

If you walk into a strong sewage smell, do not flip light switches, light a flame, or run electrical appliances. Open windows and doors, leave the area, and ventilate before doing anything that could create a spark.

Where sewer gas gets into your house

Sewer gas enters through breaks in the barrier that normally keeps drain air out of living space. The most common failures are a dry or siphoned P-trap, a failed toilet wax ring, and vent problems. Each has a specific fix. Diagnosing the exact odor source is a job in itself, covered on our guide to track down and fix the source of a sewer smell.

• Dry P-trap: Every drain has a U-shaped trap that holds water to block gas. In a rarely used sink, floor drain, or guest bath, that water evaporates and the seal breaks. Understanding how a P-trap holds its water seal explains why simply running water often fixes the smell.
• Failed wax ring: The wax seal under a toilet can dry out, crack, or shift if the toilet rocks, letting gas escape at the base.
• Vent issues: Vent pipes carry sewer gas above the roof and let drains breathe. A blocked, cracked, or missing vent pushes gas back indoors. See how plumbing vents are supposed to route gas out the roof.
• Cracked or broken drain lines: Corroded pipes, tree-root damage, or a break in the buried sewer line releases gas into walls, crawl spaces, or the yard.

Do sewer gas detectors work?

Yes, but you need the right device. A standard carbon monoxide or smoke alarm will not sense sewer gas, because it does not detect hydrogen sulfide or methane. A dedicated H2S monitor or a combustible-gas detector measures these gases directly and can alarm before levels get dangerous. Plumbers and utility crews use handheld multi-gas meters for the same reason.

Home H2S monitors are useful near basement floor drains, sump pits, and in homes on septic or with a history of odor. Place H2S sensors low, since the gas is heavier than air, and combustible-gas sensors higher, since methane rises. Detectors warn you; they do not fix the leak.

What to do if you smell sewer gas

Act in order: make the air safe first, then find and fix the cause. A faint, occasional smell is usually a dry trap you can solve in minutes. A strong, constant odor, or one with symptoms, means stop and ventilate before anything else. The steps below move from safety to repair.

1. Open windows and doors to ventilate. If the smell is strong or anyone feels dizzy or nauseated, get everyone outside and call your gas utility or 911.
2. Avoid sparks and flames until the air clears. Do not light stoves, candles, or cigarettes, and hold off on switching electrical devices in a heavily fouled space.
3. Run water for a minute in every sink, tub, shower, and floor drain to refill dry P-traps. This resolves a large share of household cases.
4. Check the toilets for rocking or moisture at the base, which points to a failed wax ring.
5. If the smell persists after refilling traps, call a licensed plumber to inspect vents, seals, and the sewer line. Building trap and vent checks into your routine helps; use our seasonal plumbing maintenance checklist to stay ahead of it.

See a doctor if headaches, nausea, or eye and throat irritation continue after you leave the area, since some hydrogen sulfide effects can appear or worsen hours later.

Frequently asked questions

Is it dangerous to smell sewer gas?

A faint rotten-egg whiff is a warning, not usually an emergency, because hydrogen sulfide is detectable at 0.0005 ppm, far below harmful levels. A strong, persistent smell is different. It signals higher concentrations that can cause headaches, nausea, and eye irritation. Ventilate the space, leave if the odor is intense, and find the source.

Can sewer gas make you sick?

Yes. Low-level sewer gas exposure causes headaches, dizziness, fatigue, nausea, and eye or throat irritation. Symptoms often ease once you get to fresh air. Higher hydrogen sulfide levels, above roughly 100 ppm, can deaden your sense of smell and cause pulmonary edema, loss of consciousness, and death. See a doctor if symptoms persist after you leave.

Can sewer gas kill you in your sleep?

In rare cases, yes. A large leak in a small, closed bedroom can build hydrogen sulfide to dangerous levels, and at over 500 ppm it can cause loss of consciousness before you wake. Methane can also displace oxygen. This is uncommon in homes, but treat a strong overnight sewage smell as urgent and ventilate immediately.

Will a carbon monoxide detector detect sewer gas?

No. Carbon monoxide detectors sense only CO, not the hydrogen sulfide or methane that make up sewer gas. You need a dedicated hydrogen sulfide (H2S) monitor or a combustible-gas detector to measure sewer gas. Do not rely on your smoke or CO alarm to warn you about a sewage leak inside the home.

How do you get rid of a sewer gas smell?

Fix the entry point, do not mask it. Run water in unused drains to refill dry P-traps, which solves most cases. Check for a failed toilet wax ring, a blocked or cracked vent, and damaged drain lines. Persistent odor after refilling traps means a hidden fault that a plumber should locate and repair.

Does sewer gas rise or fall?

It does both, depending on the gas. Hydrogen sulfide is slightly heavier than air and pools low, in basements and crawl spaces. Methane is lighter than air and collects near ceilings and in attics. Because the mix behaves differently at different heights, ventilate the whole room, not just one level, when you smell sewage.

Last reviewed: September 2026.