Real — but it fades
Formaldehyde — off-gassing from pressed wood
The classic "new furniture / new cabinets" smell. It off-gasses from the glues in pressed-wood products, and it's a recognized carcinogen — but indoor levels drop as materials cure, and ventilation cuts them fast. The concern is real; the panic usually isn't.
What matters: air out new furniture and flooring before daily use, ventilate, and keep humidity and heat moderate — both raise off-gassing.
Evidence: Peer-reviewed IARC Group 1 · Regulatory emission limits
Read the full Formaldehyde page
Real — from specific sources
Benzene — mostly tracked in, not built in
A known human carcinogen — but indoors it usually comes from identifiable sources rather than the building itself: an attached garage, tobacco smoke, stored fuels and solvents. Which is good news, because sources can be removed.
What matters: don't idle a car in an attached garage, don't smoke indoors, store fuels and solvents outside the living space, and ventilate.
Evidence: Peer-reviewed IARC Group 1 · Regulatory
Read the full Benzene page
Manageable — solvent chemistry
Toluene — paints, adhesives, some products
A common solvent that off-gasses from fresh paint, adhesives, and some consumer products. Short spikes during and after use are the main exposure; it's very responsive to ventilation and fades as things cure.
What matters: ventilate hard during and after painting, crafting, or gluing, and let freshly finished materials off-gas before heavy use.
Evidence: Peer-reviewed · Regulatory occupational limits
Read the full Toluene page
Largely avoidable
Naphthalene — check for mothballs
Indoors, the standout avoidable source is mothballs and some deodorizer blocks; it also comes from combustion and tobacco smoke. The exposure that matters is usually one you can simply stop creating.
What matters: skip mothballs and naphthalene deodorizer blocks (cedar and sealed storage do the job), and ventilate combustion sources.
Evidence: Peer-reviewed IARC 2B · Regulatory
Read the full Naphthalene page
Real — the one to actually test for
Radon — the invisible one that matters most
If you do one thing on this page, do this. Radon is a radioactive gas that seeps from the soil into homes, and it is the second-leading cause of lung cancer after smoking. It's colorless, odorless, and completely undetectable without a test — and levels vary house to house, so a neighbor's result tells you nothing about yours.
What matters: buy a cheap radon test kit (or a monitor). If you're above the EPA action level (4 pCi/L), mitigation is a well-established, affordable fix. This is the highest-leverage indoor-air action there is.
Evidence: Peer-reviewed IARC Group 1 · Regulatory EPA action level
Read the full Radon page
The hidden driver — don't add more
Ozone — and the chemistry it sets off
Ozone drifts indoors from outside and is also produced by some "air purifiers" and ionizers. It matters less for its own toxicity than for what it does: it reacts with skin oils, cleaning-product scents, and other compounds to create new irritants and ultrafine particles right in your breathing zone.
What matters: never use an ozone-generating "air purifier" or ionizer. On high-outdoor-ozone days, ventilate in the cooler morning or evening rather than the smoggy afternoon.
Evidence: Peer-reviewed secondary chemistry · Regulatory EPA advises against ozone generators
Read the full Ozone page
The acute one — get an alarm
Carbon monoxide — the same-day killer
Everything else on this page is a slow, cumulative worry. Carbon monoxide is not — it's an acute asphyxiant that can kill a household in a single night, most often during sleep. It's colorless, odorless, and comes from any fuel-burning appliance, a generator, or a car in an attached garage. It's the deadliest thing in your air, and the cheapest to defend against.
What matters: put a CO alarm on every level and near every bedroom, test it monthly, and replace it on schedule. Never run a generator, grill, or car in an enclosed space.
Evidence: Regulatory EPA standards · ~400 US deaths/yr (CDC)
Read the full Carbon Monoxide page
Real — mostly your gas stove
Nitrogen dioxide — the gas-stove question, calibrated
A gas cooktop is the biggest indoor source of nitrogen dioxide, and the link to childhood asthma is real — one analysis attributes about an eighth of US childhood asthma to gas stoves. That's a population signal, not a verdict on your kitchen. The fix isn't panic; it's ventilation.
What matters: use a range hood that vents outside every time you cook on gas (many just recirculate — check yours). When you replace the range, induction removes the source entirely.
Evidence: Peer-reviewed childhood-asthma link · Regulatory NAAQS
Read the full Nitrogen Dioxide page
The biggest — and the fixable one
Fine particulates (PM2.5) — cooking, candles, wildfire smoke
By total health cost, fine particle pollution tops the list — small enough to reach your bloodstream, tied to heart and lung disease. Indoors it comes from cooking, candles, smoke, and wildfire haze drifting in. And here's the good news this hub has been building to: this is the one a HEPA purifier genuinely fixes.
What matters: run a true-HEPA purifier sized for the room, in the bedroom. Vent cooking outside. On smoky days, close windows and let the filter work. Skip ozone-generating "ionic" purifiers.
Evidence: Peer-reviewed mortality link · Regulatory IARC Group 1
Read the full PM2.5 page
Every verdict above links to its full, cited compound page, and carries an evidence tier — how strong the science behind the verdict actually is. That tagging is the whole method. Detection of a chemical is not proof of harm; we say so, every time.