Indoor air pollution comes from a surprisingly wide range of sources, many of them ordinary household items you use every day. Cooking, cleaning products, building materials, and even the soil beneath your foundation all contribute. A large study published in the Proceedings of the National Academy of Sciences found that about half of the fine particulate matter inside a typical home originates from outdoor air seeping in, while the other half comes from indoor activities like cooking and persistent sources like off-gassing furniture. That means the air inside your home is shaped by both what you bring into it and what drifts in from outside.
Gas Stoves and Combustion Appliances
Any device that burns fuel inside your home releases pollutants into the air. Gas stoves, unvented kerosene heaters, gas space heaters, wood stoves, and fireplaces all produce carbon monoxide, nitrogen dioxide, and fine particles. Of these, gas stoves have drawn the most attention in recent years because of how sharply they spike nitrogen dioxide levels during routine cooking.
In controlled measurements at homes in California, nitrogen dioxide concentrations in kitchens jumped from a background level of about 18 parts per billion (ppb) to an average of 197 ppb within minutes of turning on a gas burner. In some cases, levels exceeded 400 ppb, which is four times the EPA’s one-hour outdoor exposure limit of 100 ppb. These spikes happen in a confined space, often without adequate ventilation, and can linger well after the stove is turned off. A range hood that vents to the outside (not one that just recirculates air) makes a significant difference.
Wood-burning stoves and fireplaces produce heavy loads of fine particulate matter. Cracked furnace heat exchangers and improperly maintained chimneys or flues can also funnel combustion gases back into the living space instead of venting them outdoors.
Household Chemicals and VOCs
Volatile organic compounds, or VOCs, are gases released by a huge number of everyday products. Paints, varnishes, cleaning sprays, disinfectants, air fresheners, moth repellents, aerosol sprays, wood preservatives, hobby supplies, glues, adhesives, and permanent markers all emit VOCs both during use and while sitting in storage. Even dry-cleaned clothing releases perchloroethylene, the solvent most commonly used in the dry cleaning process.
Benzene is one of the more concerning VOCs found indoors. Its main indoor sources are tobacco smoke, stored fuels, paint supplies, and car exhaust that migrates from an attached garage into the house. Methylene chloride, another harmful compound, shows up in paint strippers, adhesive removers, and aerosol spray paints. These chemicals don’t require high concentrations to cause irritation. Chronic low-level exposure over months or years is the pattern that affects most people.
Building Materials and Furniture
Formaldehyde is one of the most well-known indoor pollutants, and it comes primarily from pressed wood products held together with certain adhesive resins. Particleboard (common in shelving and cabinetry), hardwood plywood paneling, and medium-density fiberboard (used in drawer fronts and furniture tops) all off-gas formaldehyde, sometimes for years after installation. New furniture, new cabinetry, and recent renovations tend to produce the highest levels, which gradually decrease over time.
Homes built before 1978 may also contain lead-based paint. The older the home, the higher the likelihood. Lead dust becomes a problem during renovations, sanding, or any activity that disturbs painted surfaces. Asbestos is another legacy pollutant found in older homes, typically in pipe insulation, floor tiles, ceiling panels, textured paints, and shingles. Asbestos is generally not dangerous when it’s intact and undisturbed, but cutting, drilling, or crumbling these materials releases microscopic fibers into the air.
Mold, Dust Mites, and Biological Pollutants
Living organisms and their byproducts form an entire category of indoor air pollution. Mold and mildew, dust mites, pet dander, cockroach debris, bacteria, viruses, and pollen all circulate through indoor air. Central air handling systems that aren’t properly maintained can become breeding grounds for mold and bacteria, distributing spores throughout the house every time the system runs.
Moisture is the key driver for biological pollutants. The EPA recommends keeping indoor relative humidity between 30% and 50%. Anything above 60% creates conditions where condensation forms on surfaces, and mold growth becomes likely. Bathrooms without exhaust fans, basements with poor drainage, and homes with unresolved leaks are the most common trouble spots. Controlling moisture is more effective than trying to clean up mold after it appears.
Radon From the Ground
Radon is an invisible, odorless radioactive gas that seeps into homes from naturally occurring uranium in the soil and rock beneath the foundation. It enters through dirt floors, cracks in concrete walls and floors, floor drains, and sump pits. Radon is the second leading cause of lung cancer after smoking, and because you can’t see or smell it, testing is the only way to know whether it’s present.
The EPA recommends taking action if your home tests at 4 picocuries per liter (pCi/L) or higher, and suggests considering mitigation even at levels between 2 and 4 pCi/L, because there is no known safe level of exposure. Inexpensive test kits are available at most hardware stores, and professional mitigation systems (which vent the gas from beneath the foundation to outside) are effective and relatively straightforward to install.
Outdoor Air That Seeps Inside
Your home isn’t sealed off from the outside world. Outdoor fine particulate matter (PM2.5) from traffic, industry, and wildfire smoke penetrates through gaps in the building envelope, open windows, and mechanical ventilation systems. How much gets in depends on how tightly your home is constructed, wind conditions, and whether you run any filtration.
Research using crowdsourced air sensors across U.S. homes found that outdoor sources accounted for a median of 52% of total indoor PM2.5, while cooking contributed about 28% and persistent indoor sources made up the remaining 20%. In western states, prior studies have estimated that 50% to 80% of indoor fine particles originate outdoors. The infiltration factor (the fraction of outdoor particles that make it inside and stay suspended) averaged about 0.25 to 0.28, meaning roughly a quarter of outdoor PM2.5 ends up in your indoor air.
Reducing Indoor Air Pollution
Source control is the most effective first step. Venting gas stoves and combustion appliances to the outdoors, choosing low-VOC paints and cleaning products, fixing leaks promptly, and storing chemicals in well-ventilated areas all reduce what enters your air in the first place. Ventilation matters too: opening windows when outdoor air quality is good, running exhaust fans while cooking or showering, and ensuring your HVAC system is properly maintained all help dilute pollutants.
Filtration picks up where source control and ventilation leave off. The EPA recommends using a filter rated at least MERV 13 in your HVAC system, which captures at least 50% of particles in the 0.3 to 1.0 micron range and 85% or more of particles between 1.0 and 3.0 microns. HEPA filters go further, capturing 99.97% of particles at 0.3 microns. Portable air purifiers with HEPA filters are a practical option for individual rooms, especially bedrooms.
Consumer-grade air quality monitors can now detect PM2.5, carbon dioxide, carbon monoxide, radon, formaldehyde, total VOCs, temperature, and humidity. They won’t identify every pollutant, but they can flag cooking-related spikes, poor ventilation (rising CO2), and humidity levels creeping into the mold danger zone. For radon specifically, a dedicated radon detector or test kit is the right tool.