To test for sewer gas, first check for a rotten egg smell near drains and make sure every P-trap holds water. Then use a peppermint test, a smoke test, or a sewer gas detector to find where the gas is getting in.
This guide is for homeowners who notice a sewer smell and want to find the source before calling a plumber. It covers what sewer gas is, the warning signs, common causes, four ways to test, and when to get professional help.
What Is Sewer Gas?
Sewer gas is a mix of gases produced as wastewater and organic waste begins to break down in sewers and septic tanks. Its main components are hydrogen sulfide, methane, ammonia, and carbon dioxide.
Hydrogen sulfide causes the rotten egg smell. It irritates the eyes and throat at low levels. Worker exposure limits are about 10–20 ppm, and around 100 ppm is considered immediately dangerous; severe exposure can occur quickly and result in death at very high concentrations. At high levels it also numbs your sense of smell.
Methane is odorless and becomes flammable at about 5% in air.
Ammonia irritates the eyes, nose, and lungs.
Carbon dioxide can displace oxygen in enclosed spaces.
In most homes, sewer gas levels are low. Even so, any ongoing sewer smell should be found and fixed, and prompt detection is important because harmful effects can occur in humans within minutes in extreme cases.
Signs of Sewer Gas Smells in Your Home
The most common sign is a rotten egg smell that’s strongest near a drain, toilet, kitchen sink, bathroom, floor drain, or basement. Other warning signs that can help you suspect where the odor is coming from include:
Gurgling drains or toilets
A toilet bowl water level that drops on its own
Slow drains or a sewage backup
Headaches, nausea, dizziness, fatigue, or irritated eyes and throat that improve when you leave the house
Common Causes of a Sewer Smell
Dry P-traps. A P-trap must hold water to keep its seal, and that water helps protect living spaces by blocking sewer gas. A trap in an unused drain, such as a guest shower or basement floor drain, can dry out in 3 to 5 months, and a house with a lot of drains has more places this problem can start.
Blocked or missing vents. Every plumbing fixture should have a properly vented P-trap. Proper venting makes the plumbing system work correctly and can help locate the real problem when a clogged roof vent siphons water out of traps, which often causes gurgling.
A failed toilet wax ring. A loose or rocking toilet can let sewer gas escape at the base.
Cracked or corroded pipes. Old drain lines can crack behind walls or under floors.
Sewer line problems. A blockage or backflow can push sewer gas back into the house, and if nearby homes share a line, you may need to raise concerns with neighbors if the issue seems to affect multiple properties.
Negative air pressure. Strong exhaust fans can pull sewer gas through weak trap seals.
How to Test for Sewer Gas: 4 Methods
Start with the simplest check and work up. Never use a match, lighter, or candle to look for sewer gas, because methane is flammable.
1. Check Your Drains and P-Traps
This free check is an easy first step for a common sewer smell problem. Run water for about a minute in every drain, especially ones you rarely use: guest bathrooms, a bathroom sink or shower, basement floor drains, and utility sinks. Check that each toilet is solid on the floor. If the smell fades within a day, a dry trap was likely the cause. Time matters here because refilling a dry trap may not remove odors immediately.
2. Do a Peppermint Test
A peppermint test finds leaks in the drain and vent pipes by sending a strong scent through the system. You’ll need a helper, about 2 ounces of peppermint oil, and a gallon of hot water.
Close the windows and doors, and stay inside.
Have your helper pour the peppermint oil down the main roof vent, followed by the hot water. Use a ladder safely, or hire a pro for steep roofs.
Your helper should stay outside, since the scent will cling to their hands and clothes.
Walk room by room. A peppermint smell near a fixture or wall points to a leak in that area.
Not smelling peppermint doesn’t guarantee there’s no leak, because small leaks can be missed.
3. Have a Smoke Test Done
In a plumbing smoke test, the kind of procedure often explained in a plumbing blog, a plumber blows non-toxic colored smoke into the drain system through a cleanout or vent. The smoke escapes wherever there’s a crack or bad seal, making it an effective way to find hidden leaks. It also helps homeowners identify issues with sewage lines or piping. That reveals hidden leaks behind walls and under floors that other tests miss. Some smoke tests also leave a non-toxic UV residue that helps plumbers confirm leak points.
4. Use a Sewer Gas Detector
Electronic gas detectors monitor methane and hydrogen sulfide levels from decomposing waste. A hydrogen sulfide detector shows levels in ppm and sounds an alarm at a set limit. A combustible gas detector is a portable device with visual and audible alarms for gas presence. Choose one with the right sensitivity and good battery life. These detectors are essential for safety in confined spaces. Place it near basements, bathrooms, or wherever the smell is strongest.
When to Call a Plumber
If the smell is strong or anyone feels sick, it’s important to leave the house immediately rather than keep testing. Ventilation helps clear sewer gas odors, but it doesn’t fix the source. Call a plumber if:
The smell comes back after you refill the traps
Drains gurgle, run slowly, or back up
A peppermint test or detector can help locate the source if you suspect an ongoing leak
People in the home have symptoms that ease when they leave
The technicians at Hawthorne Plumbing, Heating and Cooling use sewer gas detectors, smoke testing, and camera inspections to find where the odor is coming from and fix the underlying problem. They can then repair cracked pipes, failed seals, and blocked vents.
Frequently Asked Questions
Yes. Low levels of sewer gas in a home usually cause mild symptoms such as headaches, nausea, dizziness, fatigue, and irritated eyes or throat. These symptoms often improve when you leave the house, which is a useful clue. Higher levels are more dangerous, because hydrogen sulfide is toxic and can numb your sense of smell, so you may stop noticing it. At high concentrations, symptoms can occur fast, and dangerous exposure can result in death. That risk is why prompt action is important for humans in enclosed areas. A strong odor, or anyone feeling unwell, means you should air out the home, leave, and call a plumber instead of continuing to search for the source.
Most sewer gas smells can be prevented with simple habits that help protect the home. Run water in every drain at least once a month, including guest bathrooms, basement floor drains, and utility sinks, so the P-traps stay full; this is an easy step, and some unused traps can dry out over time. Keep roof vents clear of leaves, nests, and debris. Fix slow drains early, and replace the toilet wax ring if the toilet rocks or leaks at the base. If you have a septic tank, have it pumped on schedule. A plumber’s inspection every few years helps catch cracks and blockages early.
Both can smell like rotten eggs, because gas utilities add a sulfur-based odorant called mercaptan to natural gas. Sewer gas is usually strongest near drains, toilets, or floor drains, and it often comes and goes with water use. A natural gas leak is usually near the stove, water heater, furnace, or gas meter. If you cannot tell where the odor is coming from, treat it as a gas emergency and do not spend a lot of time investigating indoors. Leave the house without using switches or phones, then call your gas company or 911 from outside, and talk with the gas company or a plumber about sorting out the source safely over time.
