An unexplained smell in your home is almost always trying to tell you something. It could be harmless, like off-gassing from new furniture, or it could signal a gas leak, electrical problem, or hidden mold. The key is matching the specific odor to its most likely source so you know whether to open a window or call for help.
Rotten Eggs: Gas Leak or Sewer Gas
A rotten egg smell is one of the most important odors to identify correctly because it can point to two very different problems. Natural gas is actually odorless. Utility companies add a chemical called mercaptan specifically so you can detect leaks by smell. If the rotten egg odor is strongest near a stove, furnace, or gas line, treat it as a potential gas leak: leave the house, avoid flipping light switches, and call your gas company from outside.
The other common source is hydrogen sulfide, the same compound responsible for sewer gas. Your nose can detect it at concentrations as low as 0.5 parts per billion, making it one of the easiest gases to notice early. At low levels it’s just unpleasant, but it becomes dangerous quickly. Concentrations above 50 ppm irritate the nose and throat. At 100 ppm, your nose actually stops being able to smell it (a phenomenon called olfactory fatigue), which is what makes it treacherous. The CDC considers 100 ppm immediately dangerous to life or health. If you smell sewer gas near drains, it usually means a dried-out P-trap. Running water for a few seconds refills the trap and blocks the gas.
Rotten egg smell from your tap water is a separate issue. Hydrogen sulfide can occur naturally in well water from decay and chemical reactions in soil, or from sulfur bacteria living in the well or plumbing system. Water heaters are a common culprit, especially if the smell is only in hot water. You may also notice white, grey, or reddish-brown bacterial slime around fixtures.
Fishy or Burning Plastic: Electrical Problem
A fishy smell with no fish in sight is a classic sign of an overheating electrical component. When circuits overheat, the plastics and heat-resistant chemicals in outlets, circuit breakers, and wiring insulation release compounds that smell distinctly like fish or urine. This is not a smell to ignore. Check your outlets for discoloration, warmth, or scorch marks. If you can’t identify the source, shut off the circuit and have an electrician inspect it. Electrical fires often smolder inside walls before they become visible.
Musty or Earthy: Hidden Mold
That damp, earthy, “old basement” smell is almost always mold. As mold colonies grow, they release compounds called microbial volatile organic compounds, which produce the characteristic musty odor even before you can see any visible growth. The EPA notes that exposure to these compounds has been linked to headaches, nasal irritation, dizziness, fatigue, and nausea, though the full health effects are still being studied.
The smell is often strongest in areas with poor ventilation and moisture: behind walls, under sinks, around window frames, in crawl spaces. If you notice a musty smell that intensifies when your HVAC system runs, the mold may be inside the ductwork. Eliminating the moisture source is the only lasting fix. Cleaning visible mold without addressing the water problem behind it just delays the return.
Chemical or “New” Smell: Off-Gassing
New furniture, mattresses, carpets, and flooring all release volatile organic compounds into your air. This is called off-gassing, and it’s responsible for that distinct “new car” or “new carpet” smell. Common sources include pressed-wood products like plywood and particleboard, vinyl and laminate flooring, paints, varnishes, and adhesive-backed materials.
The timeline varies by product. New furniture and mattresses typically off-gas heavily for several days to weeks, though some materials continue releasing compounds for months. Freshly installed flooring or carpet off-gasses most intensely in the first 72 hours, but some emissions can linger for years at lower levels. Even after the noticeable “new smell” fades, VOCs and formaldehyde may still be present in your indoor air. Ventilating the room, running fans, and keeping windows open during the first few days makes the biggest difference.
Sweet or Decaying: Something Died
A thick, sweet, sickeningly organic smell that gets worse over days usually means a dead animal in a wall, attic, ceiling, or crawl space. Mice, rats, squirrels, and birds are the usual culprits. During decomposition, anaerobic bacteria break down soft tissue and produce a cocktail of gases: hydrogen sulfide, ammonia, and methane. As decay progresses, protein and fat break down further into compounds like putrescine and cadaverine, which are responsible for the intensely foul, unmistakable odor of rotting flesh.
The smell typically peaks during the “bloated” stage, when internal gas pressure builds and fluids begin to seep out. For a small rodent, the worst of the odor usually lasts one to three weeks depending on temperature and humidity. Warmer, more humid conditions speed up decomposition (and intensify the smell) but also shorten the overall duration. If you can’t locate or remove the carcass, sealing the area and increasing ventilation are your main options while waiting it out.
Fruity, Ammonia, or Metallic Breath
Sometimes the mystery smell is coming from you or someone near you, and certain body odors can actually signal a medical problem. Breath that smells fruity or like rotten apples can be a sign of uncontrolled diabetes, where the body starts burning fat for fuel and produces compounds called ketones. Breath with an ammonia or urine-like smell can indicate kidney problems, since the kidneys are no longer filtering waste efficiently. Serious liver disease can produce breath that smells musty or like a mix of garlic and rotten eggs.
These aren’t subtle changes you’d notice after eating garlic. They’re persistent, unusual odors that don’t go away with brushing or mouthwash, and they usually accompany other symptoms.
Smelling Things That Aren’t There
If you’re smelling smoke, chemicals, or something burning and nobody else around you can detect it, you may be experiencing phantosmia, the perception of an odor without any actual source. This is more common than most people realize and has a wide range of causes: upper respiratory infections (including COVID-19), head injuries, sinus problems, migraines, aging, and certain medications.
The mechanism works in two ways. Peripheral phantosmia happens when the smell-detecting tissue inside your nose is damaged and then heals incorrectly, sending garbled signals to the brain. Central phantosmia originates in the brain itself, where the signals are misinterpreted. It has been described in conditions ranging from sinusitis and hypothyroidism to temporal lobe seizures and Parkinson’s disease.
A related condition, parosmia, distorts real smells rather than creating imaginary ones. After a viral infection, olfactory neurons can regenerate incompletely, picking up only some of the molecular components of an odor. Coffee, meat, eggs, garlic, and onions are among the most common triggers, and they’re consistently described as smelling fecal or rotten. Research suggests this happens because the nose detects only the most potent, unpleasant molecular components of the odor without the balancing pleasant ones. For most people, parosmia after a viral infection gradually improves over months as nerve regeneration continues.