What are the first signs of sewer gas poisoning?

Sewer gas is what forms when waste breaks down. Wisconsin's health department says it is a mixture that includes hydrogen sulfide, ammonia, carbon dioxide, methane and nitrous oxides. Hydrogen sulfide is the part you smell. The Agency for Toxic Substances and Disease Registry (ATSDR) calls it a flammable, colorless gas that smells like rotten eggs, and notes it is heavier than air, so the risk is greater at ground level and in enclosed spaces. That matters in a home, because gas that leaks in low tends to stay low, in a utility closet, a laundry room or a shower stall. Carbon monoxide is a different hazard with no smell at all, which our page on whether carbon monoxide smells like rotten eggs explains.

Symptoms track how much gas you are breathing and for how long. OSHA lists them by concentration, in parts per million (ppm).

1. 1The smell. OSHA lists the odor threshold at 0.01 to 1.5 ppm, so some people notice it at very low levels.
2. 2Irritation and low-grade symptoms. At 2 to 5 ppm, OSHA says prolonged exposure may cause nausea, tearing of the eyes, headaches or loss of sleep. Wisconsin lists headaches, dizziness, drowsiness, nausea, nervousness and eye and breathing irritation.
3. 3Fatigue and fuzziness. Around 20 ppm, OSHA lists fatigue, loss of appetite, headache, irritability, poor memory and dizziness.
4. 4Coughing and losing the smell. Around 100 ppm, OSHA lists coughing, eye irritation and loss of smell after 2 to 15 minutes.

ATSDR adds that lower, longer exposure, around 10 ppm or less, can cause eye irritation, headache and fatigue. If several people in the home feel off in the same room, and it clears when they leave, take that as a clue.

### The higher levels

The National Institute for Occupational Safety and Health (NIOSH) sets the level considered immediately dangerous to life or health at 100 ppm.

The smell is the first warning, and it can also be the first thing to fail. When you breathe about 100 ppm, your sense of smell can shut off within 2 to 15 minutes. ATSDR calls the odor an ineffective hazard warning for exactly that reason. A room that smelled awful ten minutes ago and now smells fine is not necessarily better. Do not judge safety by whether you can still smell it.

Long exposure changes the picture too. ATSDR says eye, nose or throat irritation can occur from long-term exposure, a year or more, at 5 to 50 ppm, and some people with asthma may have difficulty breathing, headaches, nausea and vomiting. That is a reason to chase a persistent low-level smell rather than get used to it.

For context, ATSDR ties the highest exposures to places like wastewater treatment plants, manure storage pits and landfills. A leaky trap in a spare bathroom is a different situation. Even so, a strong sewer smell indoors is a reason to act, not to wait it out.

1. 1Leave the area and get fresh air. Open doors and windows as you go.
2. 2Leave and call 911 if the smell is strong, you feel faint or someone is sick. Wisconsin says to evacuate and call the fire department if you suspect dangerous levels in an enclosed space.
3. 3See a doctor if symptoms continue after you are out of the smell, and tell them what you were exposed to.
4. 4If you cannot tell sewer gas from natural gas, treat it as gas. Leave and call. Our guide on a house that smells like gas or rotten eggs covers how to tell them apart.
5. 5Never go into a manhole, tank or pit to look for the source. Wisconsin warns against entering one without training and equipment.

A commercial building adds its own wrinkles, such as floor drains and grease traps. See sewer gas smell in a commercial building and gas smell in a restaurant: leak or sewer gas.

### Two examples of how it plays out

These are made-up examples to show the decision, not real events.

A faint smell near a guest bathroom. Nobody has used the bathroom in weeks. You catch a mild rotten-egg smell in the hallway, and one person has a slight headache. Step outside for air, open a window, run water in the guest bathroom sink, tub and floor drain to refill the traps, and check whether the smell fades. If it does and the headache clears outside, a dry trap is the likely cause.

A strong smell that does not fade. The smell is heavy in a closet or lower level, the eyes sting, someone feels dizzy or nauseated, or you cannot find where it is coming from. Do not stay to investigate. Get everyone out, call 911, and let the fire department check the air. Once the space is cleared, a plumber can trace the leak.

The difference between the two is the strength of the smell, the symptoms, and whether it clears when you get fresh air. When any of those tips toward the second case, go with the second case.

Wisconsin gives two simple checks. First, run water in floor drains and little-used sinks so the traps refill, since a dry trap opens a path for gas. Second, check roof vents for leaves or nests that block them. Both are quick to check.

Note when the smell shows up, because timing helps a plumber. Does it come after a shower, when the washer drains, or after rain? Is it in one room or the whole house? Write it down before the visit. If the smell is still there, common causes include:

• A dry trap in a guest bath, laundry floor drain or garage drain. Water fixes it.
• A blocked or damaged vent on the roof or in the wall.
• A failed seal at a toilet or a loose cleanout cap.
• A crack or break in the drain or sewer line. Roots, age and shifting ground can all open one. See signs of tree roots in a sewer line.

For a bathroom-by-bathroom walk-through, read why a bathroom drain smells like sewer. Wisconsin's advice on a hidden leak is short: if you find the source but cannot fix it, or it is hidden, hire a plumber. To lower the odds of a recurrence, use the routine in how to maintain your sewer line to prevent backups.

HQ Plumbing offers 24/7 emergency service and free estimates. If you smell gas of any kind and are unsure, leave first and call second.