Smells in the House and What Each One Might Mean
Why this matters
Your nose is a good early warning system for a house, and a poor diagnostic tool. It tells you something has changed. It cannot tell you for certain what, and two of the most important smells in a home, natural gas and sewer gas, both smell like rotten eggs. This card gives you the likely meaning of each common smell, the one or two things you can safely check yourself, and the smells where you do not investigate at all: you leave and call from outside.
The two smells you never investigate
Rotten eggs or sulfur anywhere in the air. Gas utilities add a sulfur-smelling odorant to natural gas and propane so a leak can be noticed. If you smell it, do not try to find the source, do not switch lights or appliances on or off, do not unplug anything, and do not use a phone or light anything inside. Get everyone out, leave the door open, and call the gas utility's emergency line or 911 once you are outside and well away from the house. The gas utility will come and check, usually at no charge for a suspected leak, and it is their job to tell you whether it was gas. See How to Respond If You Smell Gas in Your Home.
The reason this rule has no exceptions: sewer gas and a few other harmless sources can smell almost the same, and nobody standing in the hallway can reliably tell them apart by nose. The cost of being wrong in one direction is an embarrassed phone call. In the other direction it is a fire or an explosion.
Hot plastic, burning, or a fishy smell near an outlet, switch, light or the electrical panel. Overheating wiring insulation and plastic electrical parts often give off a sharp, fishy or acrid smell before there is any smoke. If you see smoke, flame, or a scorch mark, or the plate is hot to the touch, get out and call 911. If there is no smoke and you can reach the panel on dry floor, switch off the breaker for that area, stop using the outlet or fixture, and call a licensed electrician. The breaker stays off until the electrician has found the cause. Do not unplug a cord whose plug is hot or melted. See Reading Your Electrical Panel and Its Breakers.
Carbon monoxide has no smell at all. You cannot rely on your nose for it. If your CO alarm sounds, follow How to Respond When Your Carbon Monoxide Alarm Goes Off, even if the house smells perfectly normal.
The smell key
| Smell | Most likely source | What you can check | When it becomes a call |
|---|---|---|---|
| Rotten eggs in the air | Gas leak, until proven otherwise | Nothing. Leave and call from outside. | Always. |
| Rotten eggs from hot water only, at the faucet | Bacteria in the water reacting with the water heater's internal rod | Run cold water at the same faucet. If cold smells fine and the smell is only in the hot water, never in the room air, it is the water. | When it bothers you; a plumber can change the rod. |
| Sewer smell in one room | A drain trap that has dried out | Run water into every drain in that room (see below). | If it returns within a few days, or several rooms smell. |
| Dusty, burning dust | Dust burning off the furnace or heater the first time it runs each fall | Open a window and let it run. | If it lasts more than a few hours of run time, or turns sharp or plastic-like. |
| Hot plastic or fishy near electrical | Overheating wiring or a device | Breaker off from dry floor, if no smoke. | Always, electrician. |
| Musty, damp, earthy | Moisture: a slow leak, high humidity, or mold growth | Look for stains, check the basement and under sinks; note the humidity if you have a gauge. | If you find water, staining, or visible mold growth. |
| Sweet, chemical | Recent pest treatment, cleaning products, new carpet or paint | Ventilate as the applicator or label instructs. | If anyone feels ill: leave the area and call Poison Control. |
| Dead animal, sweet-rotten | An animal in a wall, attic or duct | Notice whether it is strongest at one wall or one vent. | When it does not fade in a week or two, or you hear activity. |
| Exhaust, like a car | A vent problem on a gas appliance, or a running engine in an attached garage | Make sure no car or generator is running in the garage. | Always. Check your CO alarm and call the HVAC or gas utility. |
| Burning rubber or hot metal from the dryer area | Lint buildup, a slipping belt | Stop the dryer. | Before you use it again. See Your Dryer Vent and Why It Matters. |
Why a dried-out trap smells, and the check you can do
Every drain has a trap: the U-shaped bend under a sink, built into a toilet, or under a floor drain or shower. It holds a few inches of water, and that water plug is what stops sewer gas from the pipes below from coming up into the room. When a drain is not used for weeks (a guest shower, a basement floor drain, a laundry sink), the water evaporates, the plug disappears, and the room starts to smell like a sewer. See The Drains in Your House and What Not to Put Down Them.
The check. Pour or run water into every drain in the room where the smell is strongest: about a quart into a floor drain, and run a sink or shower for half a minute. Do not forget the floor drain behind the furnace or water heater, which often never gets water. If the smell fades within an hour or so, you have found it. For a drain that is rarely used, refill it every few weeks.
When it is not the trap. A sewer smell that returns within a day or two after refilling, a smell around the base of a toilet (often a failed seal under it), or a smell in several rooms at once points to a drain or vent problem that a plumber should look at.
The October basement case, worked through
What happened. On the first cool evening of October, the homeowner notices a rotten-egg smell in the basement, near the room with the furnace and water heater. It is faint, but it is there.
The first move. Even though they suspected the old floor drain, they did not go and sniff around to decide. They took everyone outside, left the side door open, and called the gas utility's emergency line from the driveway. That was the right move, and it remains the right move even though the story ends happily.
What the utility found. The gas utility technician checked the meter, the piping and every gas appliance with a gas detector and found no gas at any point. They looked at the room and pointed to the floor drain in front of the water heater: the trap was dry, and the smell was strongest right over it. Sewer gas carries hydrogen sulfide, which is the same rotten-egg smell.
The fix and the confirmation. The homeowner poured a quart of water into the floor drain. By the time they had made dinner, about an hour later, the basement smelled normal. They marked a calendar reminder to pour in another quart on the first of each month.
The number that mattered later. Three weeks after that, the smell came back, faintly, even though the monthly refill was not yet due. A trap that dries out in three weeks in a heated basement is losing water faster than evaporation would usually explain. That shortened time was the clue that something more was going on, and it was worth a plumber's visit: in this case the floor drain's trap was cracked and leaking. If the smell had stayed away for the full month, the refill routine alone would have been the answer.
What would have been wrong. The tempting alternative is to stand in the basement trying to decide if the smell is "really" gas. Smell intensity is not a measure of danger: a small amount of odorant smells strong, and people who live with a smell for a while stop noticing it. That is why the decision is made by the rule, not by your nose.
The first-heat smell, and when it is something else
When a furnace, heat pump backup heat or space heater runs for the first time each season, dust that settled on the hot surfaces over the summer burns off. It smells like warm, burnt dust, and it usually fades over the first few hours of run time. It helps to change the filter before the first heat (see How to Change Your Furnace Filter).
It is not the dust if the smell is sharp, chemical or plastic, if it gets stronger instead of fading, if you see smoke, or if your CO alarm sounds. Turn the system off at the thermostat and call, and leave it off until the technician has found the cause. See Normal and Not-Normal Sounds from Your Heating and Cooling for the noises that sometimes come with it.
Musty and damp: a slower problem worth chasing
A musty smell is moisture, and moisture always has a source. Where to look first, in order of how often it turns out to be the answer: under sinks and around toilets; the basement or crawlspace after rain; around the water heater; closets on outside walls; the air conditioner's drain pan in summer. A humidity gauge, which is inexpensive, is worth having. Indoor humidity that stays above about 60 percent for long stretches lets mold and dust mites thrive; EPA guidance and many others suggest keeping it below that, and ideally somewhere around 30 to 50 percent.
Photograph what you find and note when the smell is worst (after rain points outside the house; after showers or laundry points inside). If you find active water, see How to Handle the First Ten Minutes of a Water Leak. For a stain on a ceiling, see What a Ceiling Stain Can and Cannot Tell You.
Your part, and where it ends
Your part: notice, ventilate, refill drain traps, change the furnace filter, check your CO and smoke alarms, photograph and note when and where, and leave the house and call for gas or electrical smells. The professionals' part: finding and fixing gas leaks (the gas utility and a licensed gas fitter), anything behind an outlet or in the panel (a licensed electrician), cracked traps and failed toilet seals (a plumber), and mold beyond a small surface spot (a remediation contractor). See When to Stop and Call a Licensed Pro Around the House.
References
- See related: How to Respond If You Smell Gas in Your Home
- See related: When to Stop and Call a Licensed Pro Around the House
- See related: Carbon Monoxide Testing + Response Reference
- See related: The Drains in Your House and What Not to Put Down Them
- EPA guidance on indoor humidity and moisture control in the home