You think you smell fish—even though you haven’t cooked anything resembling salmon or swordfish in days. Or maybe it’s the smell of your fireplace—even though you haven’t set fire to a log in weeks. Or the odor resembles ammonia, rotten eggs or something else entirely.

Such household odors can serve as clues that there are hidden dangers in the home—problems that could be pricier to repair and potentially hazardous to your health if not found and fixed fast.

Here’s what home owners need to know about eight home odors

Fishy smell that’s not fish. Some people refer to this as a urine or burning rubber smell. This could be something electrical in the home overheating and melting its insulating plastic or ­rubber—which could lead to a fire.

The most likely culprit is an appliance, such as a dishwasher, washing machine or an air conditioner. Sniff near each of these while they are running to try to identify the source. When you think you have found the source, either call in a repair professional to investigate…or replace the appliance if it is approaching the end of its useful life.

Warning: If the odor seems to be emanating from a wall switch, outlet or some other part of your home’s electrical wiring, not an appliance, switch off the circuit breaker and call an ­electrician.

Ammonia smell that’s not ammonia. Some people describe this as the smell of death, and they’re right—a mouse or some other small animal likely has died inside your home. One solution is to find and remove the corpse, and that’s often easier said than done—there’s a good chance that it is in a hard-to-reach spot inside your walls, ceiling or floor.

Use your nose to find where the smell is strongest, and search everywhere you can in that area. Consider buying and using a small snaking digital inspection camera to peek into walls, behind cabinets and appliances and into other tight spots. Ridgid Hand-Held inspection cameras are a good choice, starting at less than $150.

Unfortunately, even with a snaking camera, you won’t be able to see everywhere without drilling holes in walls, and that usually causes more problems than it solves. Of course, you could just wait for the smell to go away, which typically takes a few weeks.

Meanwhile, inspect the perimeter of your home for gaps where rodents can enter, and seal these to prevent further invasions. Also, deploy rodent traps—not rodent poison—in the home. If you use poison, additional rodents might die in their hard-to-reach nests inside your home, creating more bad smell.

Damp, musty smell. This usually signals mold or mildew, which could become a big problem for both your home and your health if not quickly remedied. Use a digital ­hygrometer to check the relative humidity of each room in the house—you can find these for less than $10 online or at home-improvement stores. If you get readings above 50%, run a dehumidifier. Apply an antimicrobial spray to carpets, curtains and fabric-covered furniture in any room that has high humidity and/or a musty odor.

Meanwhile, search these rooms for water leaks. Look behind refrigerators and under sinks for wet spots. Visit the room during the next hard rain to look for visible leaks. Go down to the basement, crawl space or room directly beneath this musty room to look for evidence of water leaks there, too—those could point to leaks that are hard to spot in the room above.

Musty/smoky odor from a fireplace that’s not in use. First, make sure that the chimney dampers are closed. Chimney smells can be drawn into the house when these are left open.

If that’s not the problem, vacuum and then scrub the “firebox”—the area that contains the fire at the base of the chimney. If the smell persists after the firebox has dried following this cleaning, call in a chimney sweep to clean and inspect your chimney. Mention that you suspect water might be getting in. The problem might be as simple as a dislodged chimney cap, or there might be cracks in the masonry.

Make sure the chimney sweep checks whether the damper is sealing ­sufficiently. You can purchase and use an inflatable damper, sometimes called a fireplace draft stopper. These are available for less than $100.

Dusty burning smell when you turn on your heat for the first time in months. Burning smells understandably trigger home owner anxieties, but this one usually is not a problem—the dust that settled on the unit over the summer is simply burning away. This smell should disappear on its own within a few hours of turning on the heat. Do replace your HVAC system’s filter if you haven’t done so recently because it might be allowing excessive dust to collect on heating components.

Warning: Turn off your heating system, and call in an HVAC repair pro if you see smoke coming from vents and/or the burning smell occurs at any time other than the heating system’s first few uses of the season.

Chemical smell from new furniture, carpet or paint. A fresh coat of paint, a new piece of pressed-board furniture or a new carpet can off-gas volatile organic compounds (VOCs) that are not just unpleasant to smell but also unhealthy to breathe. When possible, remove new pressed-wood furniture and carpeting from its packaging and let it air out in your garage for at least an hour, and preferably overnight, before bringing it into your home. If that isn’t feasible, open all the windows in the room and run fans to circulate the air for at least an hour and as long as a full day. That method also works when you are painting interior walls.

Sewage smell. This sometimes is confused with the rotten-egg smell described in the box on page four, but it’s less “eggy” and more like the smell of an outhouse or a portable toilet. It probably is coming from a drain. Use your nose to determine which one—it could be the drain of a toilet, sink, tub or shower.

One possibility is that there isn’t enough water in this drain’s “P trap.” The P trap is a curved piece of pipe below the drain that should be full of water—the water blocks offensive odors from your sewer line or septic tank from wafting back into the house. If this is the case, running the water for a few seconds (or flushing the toilet) should solve the problem. You also will have to air out the house to get rid of the odor that’s already there. This is especially likely if the toilet, sink, tub or shower has gone unused for months—the water in the P trap might have evaporated.

If that does not solve the problem—or if you see that the water is not draining properly—the odor probably is coming from material clogged in the drain line. Put on rubber gloves, and use a flexible 16-to-18-inch drain-cleaning brush to clear out any gunk from the drain. Next, let the hot water run for a few minutes (or flush the toilet) to confirm that it is now draining properly. Then shut off the water and pour one cup of bleach into the drain. Let the bleach sit for 30 to 60 minutes, then run the hot water (or flush the toilet) again. If the odor persists, you may need to rent or purchase a plumber’s auger or snake to dislodge the clog.

Rotten-Egg Smell Could Be Two Things

A rotten-egg smell might be a potentially dangerous natural gas leak. Get everyone out of the house—leaving the door and windows open so that some gas can escape—and call your gas company immediately to report the problem and request further instructions. Natural gas itself is odorless, but a sulfurlike rotten-egg smell is added so that leaks will be detected.

If a faint rotten-egg smell occurs only when your hot water is running, however, it’s probably not a gas leak at all but rather a small amount of odorous sulfur in the water. Sometimes you or a plumber can solve this problem by shutting off the water line leading to the water heater, using a hose to drain the tank, then refilling the tank.

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