The four major indoor air pollutants are radon, carbon monoxide, volatile organic compounds (VOCs), and particulate matter. Each comes from different sources, affects your body through different mechanisms, and requires different strategies to control. Indoor air can be two to five times more polluted than outdoor air, largely because these four pollutants accumulate in enclosed, poorly ventilated spaces where people spend most of their time.
Radon
Radon is a colorless, odorless radioactive gas that seeps into buildings from the natural decay of uranium in the soil and rock beneath them. It has a half-life of about 3.8 days, which gives it enough time to migrate through cracks in foundations and accumulate indoors before it breaks down. Radon itself isn’t the main threat. As it decays, it produces radioactive particles (primarily two forms of polonium) that are solid rather than gaseous. These particles settle on the lining of your airways when inhaled.
Once lodged in lung tissue, these decay products emit alpha radiation, a type that releases intense energy over a very short distance. That concentrated energy is far more damaging to DNA than other forms of radiation. It causes double-strand DNA breaks, chromosomal rearrangements, and mutations that disrupt the cell cycle and promote cancer development. Radon exposure accounts for roughly 21,000 cancer deaths per year in Europe alone, and it is the second leading cause of lung cancer after smoking.
The EPA recommends taking action if your home’s radon level reaches 4 picocuries per liter (pCi/L) or higher. Because no level of radon exposure is considered safe, the EPA also suggests considering mitigation for levels between 2 and 4 pCi/L. Testing is straightforward: inexpensive short-term kits are available at most hardware stores, and long-term detectors give a more accurate picture of year-round exposure. If levels are high, a radon mitigation system (essentially a vent pipe and fan that pulls gas from beneath the foundation and releases it outside) typically brings concentrations well below the action level.
Carbon Monoxide
Carbon monoxide (CO) is produced whenever fuel burns incompletely. Gas stoves, furnaces, water heaters, fireplaces, space heaters, and attached garages with running vehicles are the most common indoor sources. Like radon, it’s invisible and odorless, which makes it particularly dangerous because high concentrations can build up before anyone notices symptoms.
CO does its damage by binding to hemoglobin in your red blood cells far more aggressively than oxygen does. When CO attaches, it forms carboxyhemoglobin (COHb), which blocks oxygen from reaching your tissues. At COHb levels of 10 to 20 percent, most people experience only a mild headache. At 35 percent, manual dexterity drops noticeably. At 40 percent, mental confusion and severe coordination problems set in. Higher levels can cause seizures, loss of consciousness, and death.
Symptoms at lower concentrations often mimic the flu: headache, dizziness, nausea, and fatigue. The key difference is that CO poisoning doesn’t cause a fever, and symptoms tend to improve when you leave the building. CO detectors on every level of your home, especially near bedrooms and fuel-burning appliances, are the most reliable protection. Annual maintenance of furnaces, water heaters, and chimneys prevents the malfunctions that most often lead to dangerous CO buildup indoors.
Volatile Organic Compounds
Volatile organic compounds are a broad class of chemicals that evaporate easily at room temperature, releasing gases into your indoor air. The list of household sources is long: paints, paint strippers, aerosol sprays, disinfectants, air fresheners, moth repellents, stored fuels, dry-cleaned clothing, glues, permanent markers, and even new furniture and building materials. Concentrations of many VOCs are consistently two to five times higher indoors than outdoors, and levels spike dramatically during and immediately after activities like painting or using cleaning products.
Formaldehyde is one of the most well-known VOCs and one of the few that can be readily measured in homes. It off-gases from pressed wood products, insulation, and some textiles. Short-term exposure to high levels of VOCs causes eye, nose, and throat irritation, headaches, dizziness, and nausea. Longer exposure can damage the liver, kidneys, and central nervous system. Some VOCs, including formaldehyde and benzene, are classified as known or suspected human carcinogens.
Reducing VOC exposure is largely about source control. Choose low-VOC or zero-VOC paints and finishes. Ventilate well when using cleaning products, adhesives, or hobby supplies. Store solvents, fuels, and pesticides outside the living space when possible. Allow new furniture and building materials to off-gas in a well-ventilated area before bringing them into closed rooms. For ongoing filtration, activated carbon filters are the most effective option because they adsorb gaseous chemicals and odors that standard particle filters miss entirely.
Particulate Matter
Indoor particulate matter, especially fine particles smaller than 2.5 micrometers (PM2.5), comes from cooking, burning candles or incense, tobacco smoke, wood-burning stoves, and particles that drift in from outdoor air. These particles are small enough to bypass your nose and throat and penetrate deep into the lungs. Ultrafine particles can even cross from the lungs into the bloodstream.
The health effects are significant at both high and low concentrations. For every 10 microgram per cubic meter increase in coarser particulate matter (PM10), day-to-day death rates rise approximately 4 percent for respiratory disease and 2 percent for cardiovascular disease. PM2.5 triggers inflammation and oxidative stress in lung tissue, which over time promotes atherosclerosis and increases heart attack risk. Ultrafine particles that enter the bloodstream can constrict blood vessels and raise blood pressure directly. On days with elevated particulate levels, hospitals see corresponding spikes in heart failure admissions, with measurable increases in pressure inside the heart’s right ventricle.
The respiratory effects are equally concerning. PM2.5 exposure disrupts normal lung development in children and accelerates the decline of lung function with age. Studies show a consistent link between elevated indoor PM2.5 and asthma flare-ups, increased use of rescue inhalers, and worsening COPD symptoms. Even at relatively low indoor levels around 11 micrograms per cubic meter, researchers found increases in severe COPD exacerbations among former smokers. During warm months, indoor heat combined with PM2.5 creates a synergistic effect that worsens respiratory symptoms beyond what either factor causes alone. People with higher body mass also tend to deposit more particulate matter in their lungs, which may explain why obesity is linked to a greater adverse response to particle pollution.
The World Health Organization tightened its recommended annual PM2.5 exposure limit in 2021, cutting it in half from 10 to 5 micrograms per cubic meter. HEPA air purifiers are effective at capturing fine particles, trapping 99.97 percent of particles as small as 0.3 microns, including dust, pollen, mold spores, and bacteria. However, HEPA filters do not remove gases or odors, so they won’t help with VOCs or carbon monoxide.
Biological Pollutants and Moisture
While not always included in the “big four” list, biological pollutants deserve mention because they are among the most common triggers of indoor respiratory problems. Mold, dust mites, pet dander, cockroach allergens, and bacteria thrive in damp, poorly ventilated indoor environments. The World Health Organization identifies persistent dampness and microbial growth on interior surfaces as the primary driver of increased respiratory symptoms, allergies, asthma, and immune system disruption in buildings.
Controlling moisture is the single most effective strategy. Fix leaks promptly, keep indoor humidity between 30 and 50 percent, use exhaust fans in kitchens and bathrooms, and ensure crawl spaces and basements have adequate ventilation. HEPA filters capture mold spores and allergen particles effectively, making them a useful complement to moisture control in homes where biological pollutants are a concern.
Matching the Right Filter to the Right Pollutant
No single technology addresses all four major pollutants, which is why understanding what you’re dealing with matters. HEPA filters excel at trapping particles: dust, pollen, pet dander, mold spores, and fine particulate matter from cooking or smoke. They won’t touch gaseous pollutants. Activated carbon filters work the opposite way, adsorbing VOCs, chemical fumes, and odors but offering little protection against larger particles. Many air purifiers combine both filter types, which covers a wider range of pollutants in a single unit.
Neither filter type addresses radon or carbon monoxide. Radon requires a dedicated mitigation system that vents gas from below your home’s foundation. Carbon monoxide requires properly maintained combustion appliances, adequate ventilation, and battery-backed CO detectors. Ventilation itself is one of the most universally effective tools: simply opening windows or running exhaust fans dilutes concentrations of all four pollutants, though it’s less practical in extreme weather or areas with poor outdoor air quality.