The essentials in 30 seconds
Natural gas has no smell of its own: utilities add an odorant called mercaptan, which contains sulfur, so leaks smell like rotten eggs. That makes a gas leak the first thing to rule out. If the smell is throughout a room or near a gas appliance or meter, or you hear hissing, leave and call from outside. If it comes only from a drain, floor drain or toilet, think sewer gas from a dry trap, a failed seal or a blocked vent. If it appears only when you run water, it is hydrogen sulfide in the water: hot water only points to the water heater, hot and cold to the supply or well. A burning smell near outlets or the panel is an electrical warning, and a sweet, rotting smell that grows over days is often a dead animal.
- When in doubt, treat it as gas: get out first, investigate later.
- Pour water into floor drains and unused sinks to refill dry traps.
- Rotten egg water is usually a nuisance rather than a health risk, but it needs the right fix.
Gas leak, sewer gas or smelly water: how to tell
Start by noticing three things: where the smell is strongest, when it appears, and whether it is getting stronger.
| What you notice | Most likely source | First step |
|---|---|---|
| Smell through a room, near the stove, furnace, water heater, dryer or meter; possibly hissing | Natural gas or propane leak | Leave, then call the utility or 911 from outside |
| Smell outside near the meter or the street, dead patches of grass, bubbling in puddles | Leak on a gas line outdoors | Move away, call the utility or 911 |
| Smell near a floor drain, a rarely used sink, shower or basement drain | Dry trap letting sewer gas in | Run water into the drain |
| Smell around the base of a toilet, sometimes with water on the floor | Failed wax ring or toilet seal | Check the toilet for rocking or leaks |
| Smell from several drains at once, gurgling, slow draining | Blocked plumbing vent or sewer problem | Plumber to check the vent and sewer line |
| Smell only when hot water runs | Water heater anode reacting with the water | Water heater flush and anode change |
| Smell from both hot and cold taps, often on well water | Hydrogen sulfide in the water supply | Test the water; contact the well service or water utility |
| Burning plastic or rubbery smell near outlets, switches or the panel | Overheating wiring or device | Switch off the circuit; electrician |
| Sweet, rotting smell in one wall, attic or crawl space that grows for days | Dead animal | Locate and remove it safely |
| Smell from the sump pit, septic area or yard | Sump pit or septic system | Check the sump lid and septic tank |
Natural gas or propane leak: the smell that comes first
The Kansas Corporation Commission, which regulates gas pipelines in that state, explains that pure natural gas is odorless and that gas companies add mercaptan, a sulfur compound, so leaks can be noticed. Propane is usually odorized for the same reason. Other signs of a leak include a hissing or blowing sound, dead or discolored vegetation in an otherwise green area, bubbling in wet or flooded ground and dust blowing from a hole in the ground.
What to do, in this order:
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01
Get out
Leave the house by the nearest door, without switching anything on or off.
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02
Keep sparks away
Leave phones, light switches, the garage door opener and the car alone until you are well away.
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03
Call from outside
Phone the gas utility's emergency line or 911 from a safe distance. In an apartment, warn neighbors on the way out if you can do so quickly.
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04
Wait for the all-clear
Stay out until the gas company says it is safe. Let them check with their instruments even if the smell fades.
One more point: noses tire of a smell after long exposure, so a gas odor can seem to fade while it is still there. If a visitor notices it when you no longer can, take it seriously.
A gas leak is a different danger from carbon monoxide, which has no smell at all. You need a working CO alarm as well; see our carbon monoxide detector guide. If your furnace is acting up at the same time, see furnace not working.
Sewer gas smell from drains and toilets
Sewer gas also smells of rotten eggs because it contains hydrogen sulfide. Every sink, shower, tub and floor drain has a trap, a bend that holds water and blocks gas from the sewer. When that water dries out, the gas comes straight through. The City of Minneapolis notes that badly maintained basement floor drains are the most common cause of sewer gas odor in basements.
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01
Refill dry traps
Pour a bucket of water into floor drains, and run water for a minute in guest bathrooms, basement showers and laundry sinks that are rarely used. Repeat every few weeks.
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02
Clean slow drains
Hair, soap and grease in a drain can rot and smell. Clean the trap and stopper; see how to unblock a sink. Tiny flies around the drain point to the same problem: see our drain flies guide.
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03
Check the toilet seal
A toilet that rocks, or water and smell around its base, points to a failed wax ring. See toilet leaking at the base.
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04
Check the vent
Plumbing vents on the roof let sewer gas out and air in. If several drains gurgle or smell at once, leaves, a nest, snow or ice may be blocking the vent stack; Minneapolis advises keeping roof vents clear. Roof work is for a plumber or roofer.
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05
Check the cleanout and sump
A loose cleanout cap or an unsealed sump pit can let gas in. See our guides to the sewer cleanout and sump pump.
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06
On septic
A strong smell in the yard or house can mean the tank needs attention; see septic tank pumping.
Rotten egg smell from water: hot only, or hot and cold
When the smell appears only as water runs, the cause is hydrogen sulfide dissolved in the water. Penn State Extension says most people can smell it at concentrations below 0.5 mg/L, and that the water usually becomes unpleasant to drink long before it could harm health. It can still corrode plumbing and leave yellow or black stains on fixtures.
Run a simple test: fill a glass from the cold tap, then one from the hot tap, and smell each one away from the sink.
| Smell from | Likely cause | What to do |
|---|---|---|
| Hot water only | The water heater's magnesium anode rod reacting with sulfate in the water | Flush the heater; ask a plumber about replacing the anode with an aluminum one |
| Hot and cold, private well | Sulfur-reducing bacteria or sulfur in the groundwater | Test the water; ask a well contractor about treatment |
| Hot and cold, city water | Rare from the supply itself | Call your water utility; also check for a drain smell near the sink |
| Only from one sink | Smell rising from the drain, not the water | Smell the water in a glass away from the sink |
| Only after a water softener was added | Bacteria can occasionally thrive in a softener | Service the softener; see our water softener guide |
Hot water only: the water heater
Penn State Extension explains that the magnesium rod in many tank water heaters can turn sulfates in the water into hydrogen sulfide. Removing the rod often stops the smell but can shorten the tank's life and void the warranty, so the better option is replacing it with an aluminum rod, which still protects the tank. Have a plumber do it, especially on a gas water heater. Our electric water heater guide covers the other symptoms worth checking at the same visit.
Hot and cold: the well or supply
On a private well, sulfur-reducing bacteria thrive in the low-oxygen conditions of the well and plumbing. Penn State notes that shock chlorination usually gives only temporary relief, because the bacteria and smell tend to return, and that lasting fixes depend on the concentration: activated carbon for low levels, oxidizing filters or aeration for moderate levels, and continuous chlorination or similar oxidation for high levels. A water test tells you which one fits, and hydrogen sulfide needs to be tested in the home or with a sample stabilized for the lab. Washington State Department of Health advises well owners to contact the company or person who services their well. On city water, call the utility: its number is on your bill.
Burning smells, dead animals and other sources
Electrical burning smell
Overheating wires and devices smell of hot plastic, rubber or insulation rather than rotten eggs, but in a worrying moment people mix them up. CPSC lists the odor of hot insulation, warm or discolored outlets and switch plates, buzzing and sparks as warning signs in home wiring. Switch off that circuit at the panel if you can do so safely and call an electrician; if you see smoke, get out and call 911. See our circuit breaker tripping guide.
Dead animal
A mouse or rat that dies inside a wall, attic or crawl space gives off a sweet, rotting smell that builds over several days, often with flies appearing in one spot. The smell is strongest in one area and does not follow the plumbing. Locate it by following the smell, remove it with gloves, and seal entry points; our guides to mice, rats and mouse droppings explain safe cleanup.
Other everyday sources
Spoiled food, a garbage disposal with trapped scraps or a washing machine seal can also smell foul.
Who to call for each cause
| Cause | Who to call |
|---|---|
| Gas or propane smell | Gas utility or propane supplier, or 911, from outside |
| Sewer gas that persists after refilling traps | Licensed plumber |
| Smelly hot water | Plumber for the water heater anode |
| Smelly well water | Water testing lab, then a well or water treatment contractor |
| Burning or electrical smell | Licensed electrician; 911 if there is smoke |
| Dead animal you cannot reach | Wildlife or pest control service |
Rotten egg smell: frequently asked questions
Why does my house smell like rotten eggs?
The usual causes are a gas leak, sewer gas from a dry drain trap or failed toilet seal, or hydrogen sulfide in the water. Rule out gas first by leaving and calling the utility if the smell is near gas appliances.
Does a gas leak smell like rotten eggs?
Yes. Utilities add mercaptan, a sulfur compound, to otherwise odorless natural gas so you can notice a leak.
Can sewer gas make you sick?
At the low levels from a household drain it can irritate the eyes, nose and throat and cause headaches in some people. Ventilate, refill traps and get a plumber if it persists.
Why does my hot water smell like rotten eggs?
Usually the magnesium anode rod in the water heater reacts with sulfate in the water. A plumber can flush the tank and fit an aluminum anode.
Is it safe to shower in water that smells like rotten eggs?
Penn State Extension notes that the water usually becomes unpleasant long before hydrogen sulfide reaches harmful levels. Have the water tested to find the cause.
Why does my bathroom smell like sewage?
Often a dry trap in a rarely used shower or sink, a failed toilet wax ring or a blocked vent. Run water into every drain first.
What should I do if I smell gas but find no leak?
Call the gas utility anyway from outside. Leaks can be small or intermittent, and the utility can check with instruments.
Sources
Official safety, health and extension guidance, checked on 27 October 2026.
- Kansas Corporation Commission · Natural gas safety ↗Mercaptan and the rotten egg smell, signs of a leak, what to do and avoid if you smell gas.
- Penn State Extension · Hydrogen sulfide (rotten egg odor) in water wells ↗Odor threshold, sulfur-reducing bacteria, water heater magnesium rod and aluminum replacement, staining, testing and treatment options.
- Washington State Department of Health · Hydrogen sulfide ↗Health effects at low and high levels, loss of smell at high levels, advice for well owners.
- City of Minneapolis · Home sewer problems ↗Floor drains as the most common source of basement sewer gas, refilling traps, keeping roof vents clear.
- US CPSC · Guide to home wiring hazards (publication 518) ↗Odor of hot insulation and other electrical warning signs.
- NHS · Carbon monoxide poisoning ↗UK gas emergency number, 0800 111 999.
Smell keeps coming back?
Note where it is strongest, when it appears and whether it comes with the water before you call a plumber.