Several materials found in everyday homes release pollutants into indoor air, but volatile organic compounds (VOCs) from household products and building materials are among the most widespread. Formaldehyde from pressed wood furniture, benzene from stored fuels, and particulate matter from cooking and candles all contribute to indoor air that can be two to five times more polluted than outdoor air. The full list of common indoor air pollutants spans gases, particles, and biological contaminants, many of which come from materials you wouldn’t suspect.
Volatile Organic Compounds From Household Products
VOCs are organic chemicals that evaporate at room temperature, releasing gases from an enormous range of household materials. Paints, varnishes, wax, cleaning supplies, disinfectants, aerosol sprays, air fresheners, moth repellents, hobby supplies, and cosmetics all contain organic solvents that off-gas both during use and while sitting in storage. Even dry-cleaned clothing releases perchloroethylene, the chemical most commonly used in the dry cleaning process, into your home’s air for days after you bring it inside.
Benzene is one of the more dangerous VOCs found indoors. Its main sources are tobacco smoke, stored fuels, paint supplies, and automobile exhaust that drifts in from attached garages. Methylene chloride, another high-risk compound, shows up in paint strippers, adhesive removers, and aerosol spray paints. These chemicals don’t just appear while you’re actively using the products. Storage alone allows a slow, continuous release into your living space.
What makes VOCs particularly tricky is that some react with other gases already in your home to form entirely new pollutants. Cleaning products containing limonene (the compound that gives citrus-scented cleaners their smell) and pine-derived chemicals react with ozone that seeps in from outdoors. These reactions generate formaldehyde, ultrafine particles, and other irritating compounds as byproducts. Cleaning in the afternoon produces higher levels of these secondary pollutants because outdoor ozone concentrations peak later in the day, pushing more ozone indoors.
Formaldehyde From Building Materials
Formaldehyde deserves its own discussion because it’s one of the most measurable and pervasive indoor air pollutants. It off-gases from pressed wood products like particleboard, medium-density fiberboard (MDF), and hardwood plywood paneling, all of which use urea-formaldehyde resin as a binding agent. The emission rates vary dramatically by product: fiberboard averages 1.5 milligrams per square meter per hour, particleboard averages 0.30, and plywood paneling averages 0.17. That means a single shelf made of MDF can release roughly five times more formaldehyde than the same shelf made of plywood paneling.
These emissions are highest when the product is new and gradually decrease over months to years, but they never fully stop. Heat and humidity accelerate the release. If your home has particleboard subflooring, laminate cabinets, or flat-pack furniture, formaldehyde is almost certainly present in your indoor air. Ensuring good ventilation, especially in the first few months after installing new furniture or cabinetry, helps keep concentrations lower.
Particulate Matter From Combustion
Tiny airborne particles, collectively called particulate matter (PM), are generated indoors by cooking, burning candles, using fireplaces, and operating unvented space heaters or kerosene heaters. Tobacco smoke is one of the most significant sources. Burning or combusting almost anything produces PM, and the smallest particles (PM2.5, those under 2.5 micrometers) penetrate deep into the lungs.
Wood-burning stoves and fireplaces are major contributors unless they meet current emission standards. Using dry, seasoned wood and keeping stove doors closed reduces particle output significantly. For gas, coal, oil, and wood-burning appliances, venting exhaust directly to the outdoors is the single most effective way to limit indoor particulate exposure.
Gas Stoves and Nitrogen Dioxide
Gas and propane stoves release nitrogen dioxide every time a burner ignites. Typical use increases a household’s nitrogen dioxide exposure by about 4 parts per billion averaged over a year, which is three-quarters of the level the World Health Organization considers unsafe for outdoor air. What surprises most people is how far the gas travels: concentrations frequently exceed health benchmarks in bedrooms far from the kitchen, and they can stay elevated for hours after the stove is turned off.
A Stanford study found that pollutant levels breach health limits in bedrooms within an hour of gas stove use. If you cook with gas, running a range hood that vents outdoors (not one that recirculates) makes a measurable difference. Opening a window near the kitchen while cooking also helps dilute concentrations before they spread through the house.
Carbon Monoxide From Fuel-Burning Appliances
Carbon monoxide is an odorless, colorless gas produced by any appliance that burns fuel: furnaces, water heaters, gas dryers, fireplaces, and even incense. WHO guidelines set exposure limits at 35 milligrams per cubic meter for one hour and 10 milligrams per cubic meter over eight hours. Research has shown that incense burning alone can produce peak concentrations of about 9.6 milligrams per cubic meter, approaching the eight-hour safety limit depending on room size and ventilation. A poorly maintained or unvented gas appliance can push levels far higher.
Radon From Soil
Radon is a naturally occurring radioactive gas that seeps into buildings from the ground. The primary pathway is soil vapor intrusion, where radon released by decaying uranium in rock and soil migrates upward through foundation cracks, gaps around pipes, and sump pits. It can also enter through well water and, less commonly, from building materials that contain trace amounts of radium.
Unlike other indoor pollutants, radon is invisible, odorless, and impossible to detect without testing. It’s the second leading cause of lung cancer after smoking. The EPA recommends taking action if your home tests at or above 4 picocuries per liter. Testing is inexpensive, and short-term test kits are available at most hardware stores. If levels are high, a mitigation system that vents soil gas from beneath the foundation to the outdoors typically reduces concentrations by up to 99 percent.
Lead Paint in Older Homes
Roughly three-quarters of homes built before 1978 in the United States contain some lead-based paint. The paint itself isn’t dangerous when it’s intact and in good condition. The risk comes when it deteriorates: peeling, chipping, or chalking paint releases lead dust that settles on floors, windowsills, and toys. Renovation work like sanding or scraping old paint without proper containment creates especially high concentrations of airborne lead particles. Children under six are most vulnerable because they absorb lead more readily and are more likely to ingest dust from contaminated surfaces.
Mold and Biological Contaminants
Mold grows on almost any surface that stays damp, and it thrives when indoor relative humidity climbs above 60 percent. The EPA recommends keeping humidity between 30 and 50 percent to prevent growth. Common problem spots include areas around leaky pipes, poorly ventilated bathrooms, damp basements, and any material that absorbs moisture. Particleboard and pressed board are more susceptible to mold colonization than solid wood. Visible signs of a mold-friendly environment include rippling wall coverings, cracked drywall tape, and peeling paint.
Humidifiers themselves can become a source of biological pollutants if filters aren’t cleaned or replaced regularly, as mold colonizes the wet filter material and disperses spores directly into the air stream.
Flame Retardants in Furniture
Since the 1970s, chemical flame retardants have been added to foam in upholstered furniture sold in the United States, with some furniture foam containing flame retardant chemicals at up to five percent of its total weight. These additives aren’t chemically bonded to the foam, so they gradually migrate out of cushions and settle into household dust. You then inhale or ingest them through normal hand-to-mouth contact. The foam itself is the primary source; fabric coverings contain far lower levels. Although a 2013 regulatory change reduced the use of these chemicals in new furniture, older couches, chairs, and mattresses manufactured before that date can continue releasing flame retardants into your home for years.