A sewer gas smell goes away once the source is fixed and fresh air replaces the gas already inside, so how long it takes depends on airflow. Open windows and run exhaust fans. A smell that keeps coming back means gas is still getting in. After a sewage backup, it lasts until soaked materials are cleaned or removed.
Why does the smell fade once the source is fixed?
Because nothing new is coming in. Sewer gas is a mix of gases that forms when waste decays in the drain and sewer system. Wisconsin's health department says it can enter a home "through a floor drain, from a leaking or blocked plumbing roof vent," or through foundation cracks when the gas is in the soil. Seal that entry point and the house stops refilling.
Ventilation does the rest. Oatey, the plumbing products maker, says ventilation helps "by diluting the concentration of gases in your home and replacing contaminated air with fresh outdoor air," and that it "accelerates odor dissipation." How long that takes depends on how much gas built up and how fast you move air. No official source puts a number of hours on it, so judge by your nose over the next day, not by a clock.
### Can a fading smell fool you?
Yes, and that's the safety catch. Sewer gas isn't one gas. Wisconsin's health department notes that hydrogen sulfide smells like rotten eggs, but that "At high concentrations, hydrogen sulfide can interfere with the sense of smell, making it harder to use the gas's distinct odor as a warning sign." Methane, another part of the mix, has no smell at all, and both are flammable.
So a smell that fades fast in a closed room isn't proof the air is clean. The same agency says that if you believe there are high concentrations of sewer gas in an enclosed space, "evacuate the building and call the fire department for help," and avoid creating sparks. If you have symptoms and suspect sewer gas, it says to see a doctor. Our guides on when a sewer smell means get out and the first signs of sewer gas exposure go deeper.
A rotten egg smell can also be a natural gas leak. If you're not sure which one you're smelling, treat it as gas: see what to do when the house smells like gas.
How do you air out the house?
Get fresh air moving through the rooms that smell, and give it time:
- 1Confirm the fix first. Pour water into the drain that smelled, or check that the repair is done. Airing out a house that's still leaking gas just buys a break.
- 2Open windows and doors on opposite sides of the house so air flows through, not just in.
- 3Run exhaust fans. Oatey says to "use exhaust fans in bathrooms, kitchens, and laundry rooms to vent any lingering smells outdoors" and to run them "for at least 15-20 minutes after identifying sewer gas odor." Longer doesn't hurt.
- 4Use an odor absorber for the leftovers. Oatey suggests charcoal-based absorbers or baking soda near the source or in the affected rooms.
Skip the air freshener. It covers the smell you're using to tell whether the fix worked.
Why does the smell keep coming back?
A returning smell means gas is still getting in. The usual suspects:
| Cause | What you'll notice | Fix |
|---|---|---|
| Dry trap in a drain nobody uses | Smell near a guest shower, floor drain or laundry sink | Run water; it returns when the trap dries again |
| Blocked roof vent | Smell comes and goes, gurgling drains | Clear the vent |
| Toilet wax ring leaking | Smell at the base of one toilet | Reset the toilet on a new seal |
| Cracked drain pipe in a wall or floor | Smell with no obvious drain nearby | Find and repair the pipe |
| Sulfur smell only from hot water | Rotten eggs when the hot tap runs | A water heater issue, not sewer gas |
Dry traps cause most repeat visits. Every drain has a trap that holds water as a seal. Phoenix's plumbing code requires each fixture trap to hold a liquid seal "of not less than 2 inches (51 mm) and not more than 4 inches (102 mm)." Water in a trap evaporates, and Oatey says a fixture left unused "for an extended period (typically 2-4 weeks)" can lose its seal completely. That's why the smell often greets snowbirds and anyone back from a long trip. The code requires trap seals "subject to evaporation" to be protected by one of several listed methods, such as a trap seal primer. For a house that will sit empty, Oatey suggests adding "a small amount of mineral oil (1-2 tablespoons) to P-traps before extended absences," since it floats on the water and slows evaporation.
If one drain is the culprit, our page on why a bathroom drain smells like sewer covers the fixes. If the smell is from the hot water only, it's likely the water heater, covered in what causes smelly hot water.
How long does the smell last after a sewage backup?
Much longer, because the odor is coming from what got wet, not from the air. Vermont's environmental agency says drying out "can take several weeks in an enclosed area such as a basement or crawl space," and microorganisms keep growing "as long as the humidity remains high." If the area isn't cleaned and dried properly, it warns that "a musty odor, signifying the continued growth of microorganisms, can remain long after the sewage overflow."
The same guidance says contaminated mattresses, pillows, foam rubber items, upholstered furniture, books and most paper products "should generally be discarded because they soak up contamination and are difficult to disinfect." If carpet got wet, it adds that the foam padding will likely have to be replaced. Airing out won't fix any of that. The smell leaves when the soaked material does.
Drying the space is the other half. Vermont's guidance says to lower the humidity by opening up the house and removing standing wastewater, open closets and cabinet doors, and use fans and dehumidifiers. It says whole-house air conditioners or furnace blowers should be used "only if the air ducts were not impacted by standing wastewater," and it recommends a professional cleaning company for furnishings worth saving. Fix the pipe that backed up at the same time, or the cycle starts over.
When is it time to call a plumber?
Call when water in the trap doesn't stop it, when it comes back after you air out, or when you can't find the source. Wisconsin's health department says that if the leak is behind a wall, "you may need to hire a plumber to find the source and fix it." Plumbers can test the drain and vent system to find where gas escapes, including with a smoke test.
HQ Plumbing & Air tracks down sewer odors, from dry traps to cracked drain lines, across the Phoenix area, with 24/7 emergency service. Fix the source, air it out, and the smell goes with it.