Quick answer
A sewer gas smell in a bathroom most often comes from a dried-out P-trap, a failed toilet seal, a blocked vent or a damaged drain line. Run water in every drain to test the trap theory. If the smell is a strong rotten-egg odor that could be natural gas, leave the building and call 911 or your gas utility.
Start here: is it sewer gas or natural gas?
Both can smell like rotten eggs, so rule out natural gas first. Natural gas has no odor of its own. Puget Sound Energy says it adds a sulfur-based scent, often described as rotten egg, so that leaks are easier to notice.
If you think you smell natural gas, do not investigate. Leave the building and take everyone with you. Do not turn light switches on or off, and do not use a landline inside. From outside, call 911 or your gas utility. If you are not sure who your gas utility is, check your gas bill. The utility says it sends a technician to check at no charge.
Other signs of a gas leak include a hissing or roaring sound, or dead plants where they would not normally be. Carbon monoxide is separate again: it has no added scent, so you cannot detect it by smell.
Pyper Plumbing does not work on gas lines. A suspected gas leak is a job for your gas utility and emergency responders first.
What sewer gas is
Sewer gas is a mix of gases produced as waste breaks down. One article from a building-industry publication lists hydrogen sulfide, ammonia, carbon dioxide, methane, nitrogen and hydrogen. Hydrogen sulfide and ammonia are the main sources of the smell, and some of the gases have no odor at all.
Under normal conditions, plumbing keeps these gases out of living space. Each fixture drain has a trap, a U-shaped bend that holds water as a seal. A vent system releases gas outside and balances air pressure so the traps stay full. When any of these parts fails, odor can enter the bathroom.
Common causes of a sewer smell in the bathroom
These are the usual suspects, roughly in order of how often they turn out to be the cause.
- Dry P-trap. The trap water evaporates and leaves a clear path for gas. A building-industry article calls this the most common cause. It is likely in a guest bath, a seldom-used shower or a basement floor drain.
- Failed toilet seal. A wax ring seals the toilet base to the drain flange and keeps water and sewer gas from escaping. Signs include a rocking toilet, water around the base, or staining or warping of nearby flooring. An odor alone does not prove the ring is at fault.
- Blocked or damaged vent. Gurgling drains can point to a vent problem. A vent can be blocked by debris or nests, and one trade source notes that ice and snow can seal vent openings in cold regions.
- Cracked drain or loose cleanout cap. A break underground or in a wall can let gas into the soil and the building. A smell in several rooms at once can point to the main line. A loose or cracked cleanout cap can also leak gas.
- Floor drains. Floor drains lose their trap seal easily when unused. Sioux Chief, a plumbing products manufacturer, notes that trap seals can be lost to evaporation, infrequent use or pressure changes, and that codes often call for trap primers on floor drains in remote areas.
Safe checks you can do yourself
These steps use water and your nose only. Stop and ventilate the room if the smell is strong or you feel unwell.
Work through them in order and note what changes.
- Open the window or run the bathroom fan.
- Run water in every sink, tub, shower and floor drain for about a minute, including ones you rarely use. If the smell fades, a dry trap was probably the cause.
- Check the toilet. Look for water at the base and gently test whether it rocks. Watch whether the bowl water level drops between uses.
- Pour about two cups of water into any floor drain, then check again in a day or two. One building-industry source describes this as a temporary fix.
- Listen for gurgling when large amounts of water drain. Gurgling can suggest a vent issue.
- Look at any outdoor or crawl space cleanout cap for cracks or a loose fit.
- Note which rooms smell. One room points to a local cause. Several rooms point toward the main line.
What not to do
Leave roof vent work to a professional. Vent clearing needs roof access and specialized equipment, and a deep blockage cannot be cleared from the ground.
Do not pour chemical drain cleaner into drains when you suspect a sewer line or vent problem. It will not fix a cracked pipe or a dry trap, and it adds a chemical hazard to a space that already smells. Never mix cleaning products.
Do not stay in a home with a sewage backup. Bob Vila's guide advises that people and pets stay elsewhere until the problem is resolved.
Do not use open flame to hunt for a smell, and do not light matches near a suspected gas odor.
When it is a health issue
The U.S. Agency for Toxic Substances and Disease Registry describes hydrogen sulfide as a flammable gas with a rotten-egg odor. People can usually detect it at very low levels, well below those that cause harm. Long-term exposure to roughly 10 ppm or less can cause eye irritation, headache and fatigue, and some people with asthma may have trouble breathing.
The agency also warns that smell is not a reliable alarm at high levels. Around 100 ppm, people can lose their sense of smell within minutes, and much higher levels can cause unconsciousness or death. One building-industry article says odors in buildings are usually more bothersome than harmful, but the gas is heavier than air and can collect in low or enclosed spaces, so take any strong smell seriously.
Treat the situation as urgent if you notice headache, nausea, dizziness or eye irritation that eases when you go outdoors, if the smell is very strong, or if a sewage backup is present. Leave the area and ventilate. Seek medical help for serious symptoms. Methane, also found in sewer gas, is flammable, so avoid sparks and flames near a strong odor.
Common fixes once the cause is found
A dry trap is fixed by refilling it, and by running the fixture regularly. A failed wax ring means shutting off the water, removing the toilet, cleaning the flange and resetting the toilet with a new seal. A blocked vent may need clearing from the roof. A cracked buried line usually needs a camera inspection to locate the damage before repair.
If the smell comes back after you have refilled traps, the cause is probably not the trap.
When to call Pyper Plumbing
Call a plumber if the smell returns after you refill traps, if you cannot find the source, if the toilet rocks or leaks at the base, if drains gurgle, or if several rooms smell. Pyper Plumbing handles drain cleaning, sewer line repair, toilets and leak detection, and can help find where the odor is coming from.
If sewage is backing up into the house, treat it as an emergency, keep people and pets away from the affected area, and call right away.
Pyper does not service septic systems. If your home is on septic and you smell sewage with slow drains, a septic professional is the right call. For any possible natural gas odor, leave and call 911 or your gas utility first.
Common questions
Can sewer gas make you sick?
It can. Hydrogen sulfide, one of the gases in sewer gas, can irritate the eyes, nose and throat and cause headache and fatigue. Higher concentrations are dangerous. Ventilate, leave if you feel unwell and have the source found.
How do I tell sewer gas from natural gas?
You often cannot by smell alone, because utilities add a sulfur-based scent to natural gas. If there is any chance it is natural gas, leave the building and call 911 or your gas utility from outside.
Why does only one bathroom smell?
A single smelly bathroom often means a dry trap, a toilet seal or a local vent problem. Smells in several rooms point more toward the main drain line.
Does pouring water down the drain really help?
If a trap dried out, refilling it restores the seal and the smell should fade. If the smell returns, something else is the cause.
Sources
Facts in this guide come from the following references. Local rules and manufacturer instructions can differ, so follow your utility and your equipment manual.
- PSE: Natural gas leaks (detect a leak)
- ATSDR: Hydrogen Sulfide ToxFAQs
- Buildings: 5 Things You Need to Know About Sewer Odor in Buildings
- Sioux Chief: Trap Primers
- HomeServe: Signs it is time to replace your toilet wax ring
- Mr. Rooter: What causes sewer gas smell in your home
- Bob Vila: Sewer smell in bathroom
This guide is general information, not a substitute for an on-site inspection. If you smell natural gas, leave the building and call 911 or your gas utility. Pyper does not perform gas line work.
