If your sump pump or sump pit is putting out a noticeable smell, your first question is probably whether it’s actually dangerous. The honest answer is: it depends on what the smell is. Some sump pump odors are mild nuisances. A few are genuine health hazards. One in particular is a fire risk. This guide walks through the most common sump pump smells, what each one means, the actual health risk level, and exactly what to do. If your sump pump is smelling like burning plastic or you smell natural gas, skip the article and call (844) 247-7668 for immediate dispatch. For everything else, read on.
The 5 Most Common Sump Pump Smells and Their Toxicity Levels
Different smells point to different causes and different risk levels. Here’s the breakdown.
Smell 1: Rotten Egg or Sewer Gas (Moderate Risk, Address Promptly)
What it smells like: rotten eggs, sulfur, sewage, septic.
What’s causing it: hydrogen sulfide gas. This comes from one of three sources:
- Bacterial activity in the standing water of the sump pit (organic matter decomposing without enough oxygen)
- A dry P-trap on a nearby floor drain (sewer gas backing up into the basement through a drain that has lost its water seal)
- A sewer line cross-connection to the sump system (a plumbing code violation, sewage is actually entering the sump pit from a connected sewer line)
Health risk: hydrogen sulfide is genuinely toxic at high concentrations. The good news is that you can smell it at concentrations far below dangerous levels (parts per billion). Symptoms at low concentrations include headaches, nausea, eye irritation, and respiratory irritation. According to the CDC’s hydrogen sulfide fact sheet, exposure becomes dangerous at higher concentrations, but typical residential sump pit smells are well below those levels. People with asthma, respiratory conditions, or chemical sensitivities should treat any rotten egg smell as more serious.
What to do:
- Ventilate the basement (open windows, run a fan)
- Pour a gallon of water into any floor drains in the basement to refill dry P-traps
- Clean the sump pit (remove debris, scrub interior, treat with a sump pump deodorizer)
- If smell persists more than 48 hours after these steps, call us at (844) 247-7668 for professional sump pump service and confirm there’s no sewer cross-connection
For the full diagnostic and prevention process, read our complete guide on reasons sump pump smells and how to prevent them.
Smell 2: Musty or Mildew (Low Risk, but Address It)
What it smells like: damp basement, old gym bag, wet cardboard, mildew.
What’s causing it: mold and bacteria growth in the standing water of the sump pit. Common when the pit has had sediment buildup, organic debris, or has been sitting with stagnant water for a long time.
Health risk: low for most people. People with mold allergies, asthma, immunocompromised conditions, or young children should take it more seriously. Long-term exposure to musty environments can worsen respiratory issues even in healthy adults.
What to do:
- Clean the sump pit (scrub with a mild bleach solution, 1 cup bleach to 1 gallon of water)
- Treat with a sump pump deodorizer (foaming cleaners and enzyme tablets work best)
- Run a dehumidifier in the basement to reduce overall humidity
- Verify the pump is functioning so water isn’t standing for long periods
If musty smells return within a few weeks of cleaning, there may be an underlying issue (excess inflow, failed check valve, slow-leaking pipe nearby). Worth a professional sump pump service call to diagnose.
Smell 3: Burning Plastic or Hot Motor (HIGH RISK, Act Immediately)
What it smells like: melting plastic, burning rubber, hot electrical components, burned wires.
What’s causing it: the sump pump motor itself. The pump is failing electrically or mechanically. Common causes:
- Motor overheating from continuous running (failed float switch, undersized pump)
- Worn bearings creating friction heat
- Burning motor windings (terminal failure)
- Jammed impeller forcing the motor to strain
- Failed wiring at the pump connection
Health risk: HIGH on two fronts. First, fumes from burning plastic and electrical components are toxic when inhaled. Second, and more urgently, the pump is at risk of catching fire. A burning sump pump can ignite combustible items in the basement.
What to do RIGHT NOW:
- Turn off power to the sump pump at the circuit breaker immediately. Do not unplug it directly if you can avoid touching the unit.
- Open the basement windows to ventilate
- Do not attempt to run the pump again
- Move flammable items away from the pit
- Call (844) 247-7668 for emergency sump pump replacement (typically same-day in our service area)
This is not a “wait until tomorrow” smell. A failing sump pump motor is hours to days from total failure, and an active fire risk in the meantime.
Smell 4: Natural Gas (EMERGENCY, Evacuate Immediately)
What it smells like: rotten eggs, sulfur. Yes, the same as sewer gas, which is why this smell needs careful interpretation.
What’s causing it: natural gas is intentionally given a sulfur-smelling odorant (mercaptan) so leaks are detectable. The smell is similar to hydrogen sulfide from sewer gas. The difference: natural gas leaks usually have no other accompanying smells (no musty, no sewage), and the smell is often noticed throughout the home rather than just near the sump pit.
How to tell the difference:
- Sewer gas smell is concentrated near drains, the sump pit, or low areas of the basement
- Natural gas smell is throughout the home, near gas appliances, or near the gas meter outside
- Sewer gas smell often comes with humidity or musty undertones
- Natural gas smell is pure sulfur with no other notes
If you suspect natural gas:
- Leave the home immediately. Do not flip any electrical switches on the way out (a spark can ignite the gas).
- Don’t use any device that could create a spark (phones, garage door opener, light switches)
- Once outside, call your gas utility’s emergency line
- Do not re-enter the home until the gas company has cleared it
Bolingbrook gas utility emergency line: Nicor Gas, 888-642-6748.
Health risk: EXTREME. Natural gas is both a fire/explosion hazard and an asphyxiation risk at high concentrations.
Smell 5: Stale or Stagnant Water (Low Risk)
What it smells like: old water in a vase, swampy, stale, mildly fishy.
What’s causing it: water sitting in the sump pit for extended periods without movement or treatment. Common in homes with low water tables where the pump rarely activates. The water itself isn’t usually dangerous, but it can host bacteria and become a nuisance odor over time.
Health risk: low.
What to do:
- Clean the pit (remove debris, scrub interior)
- Add a deodorizer tablet
- Pour a half-cup of bleach or enzyme cleaner into the pit monthly
- If your pump rarely activates, manually test it monthly (pour 5 gallons of water in) to keep water flowing through the system
Quick Toxicity Reference
Low risk (clean and treat at home): musty/mildew, stale/stagnant water.
Moderate risk (address promptly, may need professional help): rotten egg/sewer gas.
High risk (turn off power, call us today): burning plastic, hot motor, electrical smell.
Extreme risk (evacuate immediately): natural gas (sulfur smell throughout the home, not just at the pit).
When Sump Pump Smells Become a Health Issue
For most people, ordinary sump pump smells are unpleasant but not directly harmful. The exceptions:
- People with asthma, COPD, or chronic respiratory conditions: any persistent smell from the sump pit can trigger flare-ups, even at concentrations harmless to healthy adults
- Pregnant women: should avoid extended exposure to any chemical fumes including hydrogen sulfide and burning electrical smells
- Young children: spend more time near floor level where heavier-than-air gases concentrate; treat sump-related smells more cautiously around kids
- Elderly residents: reduced lung function and tolerance for irritants means earlier intervention is warranted
- Immunocompromised individuals: more susceptible to mold and bacterial contamination from sump-related sources
- Pets: same considerations as small children, plus pets often investigate the source of smells with their face
If anyone in your household falls into these categories and you have a persistent sump pump smell that doesn’t resolve with basic cleaning, schedule professional service sooner rather than later.
How to Prevent Toxic Sump Pump Smells
The smells that pose actual health risks (burning plastic, hydrogen sulfide at high concentration) are largely preventable with basic maintenance:
- Annual sump pump maintenance: cleaning, motor test, float switch test, check valve inspection. Catches problems before they become smells. Cost: $125 to $225 per visit through our sump pump service.
- Replace pumps at end of life: most last 7 to 10 years. After year 7, schedule proactive replacement before motor failure. Cost: $475 to $900 installed.
- Keep the pit clean: quarterly visual inspection, scrub interior annually, remove any debris or sediment.
- Treat regularly: monthly enzyme or deodorizer treatment prevents bacterial buildup.
- Maintain floor drain traps: pour a quart of water into any rarely-used floor drains every 2 to 3 months to keep traps wet and sealed.
- Install a battery backup: prevents the pump from running on a failing battery (which can produce hot battery smells).
When to Call a Professional
Call us at (844) 247-7668 for professional sump pump service if:
- Burning plastic or hot motor smell (call today, do not run the pump)
- Rotten egg or sewer smell that persists more than 48 hours after cleaning and refilling P-traps
- Visible sewage in the sump pit (indicates a cross-connection, code violation)
- Sump pump that has been running constantly with associated burning or hot smells
- Anyone in the household is experiencing symptoms (headache, nausea, respiratory irritation) that correlate with time spent near the pit
- Pump is over 7 years old and starting to smell odd, even mildly
For natural gas smell throughout the home, do not call us first. Evacuate and call Nicor Gas at 888-642-6748.
Frequently Asked Questions
Q: Is the smell from my sump pump dangerous to breathe?
A: Most sump pump smells (musty, stale, mildly sulfuric) are unpleasant but not directly dangerous at typical residential concentrations. Burning plastic or electrical smells from the pump motor are dangerous and require immediate action. Natural gas smells (often confused with sewer gas) are a true emergency. Always ventilate and identify the cause before assuming a smell is harmless.
Q: What does it mean if my sump pump smells like rotten eggs?
A: Rotten egg smell from a sump pump is caused by hydrogen sulfide gas, usually from bacterial activity in the standing pit water, a dry P-trap on a nearby drain, or a sewer cross-connection. At typical residential levels, it’s irritating but not directly dangerous. If the smell persists after cleaning the pit and refilling floor drain traps, call a professional to check for a cross-connection.
Q: My sump pump smells like burning plastic. Is it on fire?
A: Not actively on fire, but at risk. A burning plastic smell from a sump pump means the motor is overheating, the bearings are failing, or the windings are burning. The pump can ignite combustible material nearby. Turn off power at the circuit breaker immediately, ventilate the area, and call (844) 247-7668 for emergency replacement.
Q: Can sump pump smells make me sick?
A: Generally only with prolonged exposure to high concentrations, which is uncommon in residential settings. People with asthma, respiratory conditions, mold allergies, or chemical sensitivities are more susceptible. Symptoms to watch for include headaches, nausea, eye/throat irritation, and worsening respiratory symptoms. If anyone in the home experiences these symptoms in correlation with sump area exposure, address it promptly.
Q: How do I tell sewer gas from natural gas in my basement?
A: Sewer gas is concentrated near drains, the sump pit, and low areas of the basement. It often has musty or sewage undertones. Natural gas smells throughout the home, especially near gas appliances, and is a pure sulfur smell with no other notes. If you can’t tell with confidence, treat it as natural gas: evacuate and call your gas utility from outside.
Q: Why does my sump pump smell worse in summer?
A: Three reasons. First, bacterial activity in the standing pit water accelerates in warm temperatures. Second, basement humidity rises in summer, which spreads any pit odors more readily through the air. Third, dry P-traps on rarely-used floor drains evaporate faster in summer, allowing more sewer gas back into the basement. Annual summer maintenance addresses all three.
Q: Will a sump pump deodorizer cover up the smell?
A: Some deodorizers cover the smell, others actually address the bacterial source. Enzyme-based treatments work by consuming the organic matter that causes the smell, so they’re the right long-term choice. Heavy-fragrance products just mask the smell temporarily and can interfere with diagnosis of the real cause. We recommend enzyme treatments for ongoing maintenance.
Q: How often should I clean my sump pump pit?
A: At minimum, annually as part of a complete pump service. If you have a high water table or your pump activates frequently, every 6 months. If you’ve had odor issues, quarterly. Cleaning takes about 30 minutes if you’re comfortable doing it yourself; otherwise it’s included in a professional annual maintenance visit.
When to Call RootBusters
If you have a sump pump smell that you can’t identify, that doesn’t resolve with basic cleaning, or that’s accompanied by any of the higher-risk indicators above, call us at (844) 247-7668. RootBusters Plumbing, Sewer and Drains, Inc. provides honest sump pump diagnostics and service across Bolingbrook, Joliet, Naperville, Romeoville, and the surrounding Chicagoland area. Same-day service, transparent pricing, and a 100% satisfaction guarantee. Request service online or read our full guide on reasons sump pump smells and how to prevent them.
