Is sewer gas in my basement dangerous?

Sewer gas is a mix, and the part that matters most is hydrogen sulfide, which is what gives it the rotten-egg smell. At low levels it's mainly an unpleasant nuisance that shouldn't be in your home, and ongoing exposure can cause headaches and nausea. Here's the important caveat people don't know: at high concentrations hydrogen sulfide can actually deaden your sense of smell, so a strong odor that suddenly seems to fade is not a good sign. Treat a strong or persistent smell seriously. Ventilate the area, find and fix the source promptly, and if you ever feel symptoms or suspect a real gas problem, leave the space and call the appropriate professional.

I poured water down the drain and the smell is gone. Am I done?

Probably, if it was a dried-out trap, which is the cause we find most. Keep that drain topped up every month or two and add a little mineral oil to slow evaporation, especially in winter. If the smell comes back fast or never fully cleared, the source is something else, like a vent, a bad seal, or the line itself, and it's worth digging into before it gets worse.

Do you use ozone to clear the smell out of my basement?

No. Ozone can knock down odor, but it's a toxic gas that's unsafe to breathe, so a space has to be emptied of people, pets, and plants while it runs. We don't use it, and we don't use chlorine gas either. Our methods are green and non-toxic: source removal and cleaning, enzymatic and oxidizing treatments, thermal fogging, hydroxyl generators that are safe in occupied spaces, HEPA and activated-carbon air scrubbing, and sealing. That gets set-in sewer odor out at the molecular level without filling your home with something harsh, and it's safe around kids, pets, and electronics.

Can you remove the smell if it soaked into the concrete and drywall after a backup?

Yes, that's exactly what our process is built for. Once the contamination source is handled, set-in sewer odor in porous material is the problem we target. We clean and remove what's too far gone, then use enzymatic and oxidizing treatments, hydroxyl generators, activated-carbon air scrubbing, thermal fogging, and sealing to eliminate the odor at the molecular level instead of masking it. It's all green, non-toxic, never ozone or chlorine, and backed by our 100% guarantee across Greater Cleveland and Northeast Ohio.

Sewage Smell in the Basement: 6 Causes and How to Kill the Odor

Specialty · By Odor Removal Experts · Updated June 2026 · Cleveland and Northeast Ohio

A sewage smell in the basement is not something to live with, and an air freshener won't touch it. That rotten-egg or sewer odor almost always means gas is getting in somewhere it shouldn't, and spraying over it just hides the warning while the real source keeps venting. The good news is that on most jobs the cause is something simple we can point to in a few minutes. Once the source is handled, the smell that's left behind can be removed for good instead of perfumed over.

We get called for this constantly, and after enough basements you start to see the same handful of culprits over and over. Below are the six reasons a basement starts to smell like sewer, what each one actually looks and smells like, and how to fix it. I'll also be straight about when it's a quick DIY job and when the odor has soaked deep enough into the space that you need professional, source-level removal.

The short answer

A sewage smell in the basement almost always means sewer gas is getting in, usually through a dried-out drain trap, a cracked or loose trap, a blocked vent, a failing sewer line, or a bad ejector-pit seal. Fix the source first, often just by refilling a floor drain, then have any set-in odor removed at the molecular level instead of masked.

A dried-out floor drain trap is the cause we find most, so refill it first.
Sewer gas carries hydrogen sulfide, so treat a strong or lasting smell seriously.
Fix the plumbing source before you treat the leftover odor.
Odor soaked into concrete after a backup needs source-level removal, never masking or ozone.

1. A dried-out floor drain trap (the one we find most)

Almost every basement has a floor drain with a U-shaped trap under it. That trap holds a small pool of water, and that little water seal is the only thing blocking sewer gas from rising into the room. In a drain that rarely gets used, the water just evaporates over weeks, and once the seal is gone the gas comes straight up. This is the number one thing we find, and it's the answer to 'why does my basement smell like sewer' more often than any other cause.

The spot people miss is that the drain looks bone dry and totally innocent, so they walk right past it. The fix is easy.

Pour in water. Send about a gallon of water down the floor drain to refill the U-shaped trap and rebuild the water seal.
Add mineral oil. Float a couple tablespoons of mineral oil on top to slow down future evaporation.
Confirm the source. If the smell fades within a day, the dry trap was your problem.
Top it off. Refill drains you don't use every month or two, especially through dry winter heating season when evaporation speeds up.

2. Dry or cracked traps under sinks, tubs and laundry

The same drying-out problem hits any fixture you barely touch: a basement utility sink, a laundry standpipe, a rarely used basement toilet, or a shower in a guest bath. If one specific drain smells and the rest of the basement is fine, a dry P-trap on that fixture is usually it. Run water through every one of them, including the ones you forget exist, and flush any seldom-used toilet to refill the bowl trap.

Traps don't only dry out, they also crack or work loose at the slip joints, and that's a steadier smell that water won't cure. If running water doesn't fix it, get under the fixture with a flashlight and look for moisture, white corrosion crust, or a joint you can wiggle by hand. A plastic P-trap is a cheap, fair DIY swap. If it's old corroded metal or buried behind finished walls, that's where a plumber saves you a mess.

3. Blocked, frozen or missing plumbing vents

Your drain system has vent pipes that run up through the roof to let air in and keep pressure balanced. When a vent clogs with leaves, a bird's nest, or winter ice (which we see plenty of in Cleveland), draining water can actually siphon the water right out of your traps. The tell is a smell that comes and goes with use, strongest right after a big drain event.

If your basement only stinks after someone runs the washer or drains a tub upstairs, suspect the vent. Clearing a roof vent means getting on a ladder and roof safely, so this is an honest call-a-pro item for most people. A licensed plumber can clear it or confirm a vent was never run correctly in the first place, which is more common in older homes and bad additions than you'd think.

4. Cracked, clogged or failing sewer lines

A strong, steady sewer odor that none of the water tricks solve can point to a damaged drain line under or near the slab. Old cast iron rots from the inside, clay pipe cracks and takes on tree roots, and gas, sometimes moisture too, seeps into the soil and works up through the floor or a slab crack. A partial clog can also push gas back into the house instead of out the vent. This odor tends to be heavier and more constant, not the on-and-off pattern of a vent issue.

This is where a plumber with a sewer camera earns the fee. Scoping the line shows whether you've got a break, a root intrusion, or a blockage, and tells you if it's a simple cleanout or a real repair.

Scope before you dig

Don't let anyone quote a dig over the phone before the line has been scoped. Get the camera first, then decide.

5. A sump pit or ejector pump issue

If your basement has a sewage ejector pump for a below-grade bathroom or laundry, or a sump pit, the lid and seal are doing more work than people realize. An ejector basin needs a sealed, vented lid, period. A cracked lid, a dried gasket, or a vent line that's come loose lets raw sewer gas pour straight into the room, and on these the smell is usually right on top of the pit. Sump pits that collect organic gunk can also go sour and musty on their own.

Here's what to check on the basin:

The lid is intact and bolted down with a good gasket.
The vent line is actually connected.
The basin isn't sitting full of sludge that needs cleaning.

A new gasket or lid is a reasonable DIY. Opening up an ejector basin you've never serviced is messier, so gloves, ventilation, and a strong stomach, or just call someone.

6. Backups, past flooding and contaminated materials

Sometimes the pipe is already fixed but the smell won't leave. After a sewer backup or a basement flood, contaminated water wicks into concrete, drywall, baseboards, carpet pad, framing, and whatever was stored down there. At that point sewer-gas removal isn't a plumbing problem anymore. It's odor compounds locked into porous material, the deep, set-in kind of smell that bleach and air fresheners can't reach because they sit on the surface while the odor lives inside the pore.

Here's the honest part on cost. A dry trap costs you a gallon of water. A cracked main line can run into serious money. Professional odor remediation after a backup lands in between, and it depends on how much material absorbed the contamination, the square footage, how long it sat, and whether the space needs cleaning, treating, sealing, or some demolition first. We quote after we see the basement, not over the phone, because guessing helps nobody.

How sewage smell in basement gets removed at the source, not masked

Once the leak or backup is fixed, the leftover odor has to be neutralized, not covered. We work at the molecular source and pick the tool to fit the material. First we physically clean and haul out whatever is too contaminated to save, because no treatment beats getting the source out of the building. Then enzymatic treatments go on organic, protein-based residue to break it down, and oxidizing treatments handle the harder sulfur compounds behind that rotten-egg note. HEPA and activated-carbon air scrubbers run the whole time to pull particles and gas out of the air. We use thermal fogging on purpose here, because the warm fog travels the same cracks and crevices the sewer gas traveled, so it reaches odor where it actually settled instead of just freshening the open room. We seal porous surfaces where sealing is what finally locks it down.

Our approach is deliberately green and non-toxic, so we never reach for ozone or chlorine gas. We run hydroxyl generators, which break down odor molecules and are safe to operate in occupied spaces, so you, your kids, and your pets can keep living in the home while we treat it. Paired with our enzymatic and oxidizing treatments, carbon scrubbing, fogging, and sealing, that gets the smell out without putting anything harsh into your air. Our products are green, eco-friendly, and safe for kids, pets, and electronics, and the work is done by discreet, uniformed, fully insured technicians. Most jobs wrap in one to four hours with same- or next-day service across Greater Cleveland and Northeast Ohio. Every job carries our 100% guarantee: if it's not gone, we're not done. Contact us once the plumbing side is squared away and we'll make sure the smell stays gone.