Indoor air quality refers to the condition of the air inside buildings and structures, especially as it relates to the health and comfort of the people in them. Since most people spend the vast majority of their time indoors, the air you breathe at home, at work, and at school has a surprisingly large effect on your health. In many cases, indoor air can be more polluted than outdoor air, even in cities.
What Determines Indoor Air Quality
Three main factors shape the air inside any building: pollution sources, ventilation, and moisture. Products and materials inside a building constantly release gases or tiny particles into the air. If ventilation is poor, meaning not enough fresh outdoor air circulates in, those pollutants build up. High temperature and humidity make the problem worse by increasing the concentration of certain contaminants.
This means indoor air quality isn’t a fixed trait of a building. It shifts throughout the day and across seasons, depending on what’s happening inside (cooking, cleaning, running a space heater) and how much fresh air is flowing through.
The Most Common Indoor Pollutants
The EPA identifies more than a dozen common indoor pollutants. Some are invisible gases, others are microscopic particles, and a few are living organisms. Here are the ones most likely to affect your home or workplace:
- Volatile organic compounds (VOCs): Gases released by paints, cleaning products, new furniture, and building materials. Formaldehyde, one of the most studied VOCs, comes primarily from composite wood products like particleboard and plywood. Benzene, another concern, enters indoor air through tobacco smoke, stored fuels, paint supplies, and car exhaust from attached garages.
- Particulate matter: Tiny particles from cooking, burning candles, wood stoves, and tobacco smoke. These particles can be small enough to penetrate deep into the lungs.
- Carbon monoxide (CO): A colorless, odorless gas produced by fuel-burning appliances like gas stoves, furnaces, and fireplaces. Short-term exposure to elevated levels indoors can be lethal.
- Radon: A naturally occurring radioactive gas that seeps into buildings through cracks in the foundation. It is the second leading cause of lung cancer.
- Biological contaminants: Mold, dust mites, pet dander, cockroach allergens, and bacteria. These thrive in damp or poorly maintained environments.
- Nitrogen dioxide (NO2): Released by gas stoves, kerosene heaters, and other combustion sources.
- Secondhand smoke and aerosols: Tobacco smoke contains thousands of chemicals, many of which are known carcinogens.
How Poor Air Quality Affects Your Health
The health effects of indoor air pollution range from mild annoyances to life-threatening conditions. In the short term, exposure to common indoor pollutants can cause eye, nose, and throat irritation, along with headaches, dizziness, and fatigue. These symptoms sometimes appear quickly and resolve once you leave the building, a pattern known as “sick building syndrome,” where occupants of a particular building all experience similar symptoms that fade once they go home.
Long-term exposure carries more serious risks. Chronic exposure to indoor pollutants is linked to respiratory diseases, heart disease, and cancer. Radon alone accounts for roughly 21,000 lung cancer deaths per year in the United States. Poorly maintained heating and cooling systems can harbor Legionella bacteria, which causes Legionnaires’ disease, a severe form of pneumonia.
For people with asthma, indoor pollutants are a constant threat. Dust mites, mold, pet dander, tobacco smoke, cockroach allergens, and particulate matter are all established asthma triggers that can provoke attacks in sensitive individuals.
Carbon Dioxide and Cognitive Performance
One pollutant that rarely gets attention is carbon dioxide. CO2 builds up naturally in any occupied room as people breathe, and levels climb quickly in poorly ventilated spaces like classrooms, meeting rooms, and bedrooms. A 2023 meta-analysis found that CO2 concentrations below 5,000 ppm, well within the range of a stuffy office, impaired cognitive performance. Complex tasks like decision-making and strategic thinking were hit hardest, with significant declines starting at concentrations between 1,000 and 1,500 ppm.
For context, outdoor air typically contains about 420 ppm of CO2. A well-ventilated office might sit around 600 to 800 ppm. A crowded conference room or classroom with the doors closed can easily exceed 1,500 ppm within an hour or two. The research supports keeping workplace CO2 below 1,000 ppm, particularly in jobs that require focused thinking, and limiting time in high-CO2 spaces to no more than two hours at a stretch.
Key Thresholds to Know
Indoor air quality involves a handful of specific numbers worth remembering:
- Radon: The EPA recommends fixing your home if radon levels reach 4 pCi/L (picocuries per liter) or higher. Testing is inexpensive and widely available through hardware stores.
- Humidity: The EPA recommends keeping indoor relative humidity between 30 and 50 percent, and no higher than 60 percent. Below 30 percent, air becomes dry enough to irritate skin and airways. Above 50 to 60 percent, mold and dust mites thrive.
- Carbon dioxide: Below 1,000 ppm is the recommended target for spaces where people need to think clearly.
- Fine particles (PM2.5): The World Health Organization updated its guidelines in 2021, lowering the recommended annual exposure limit to 5 micrograms per cubic meter, a level that very few places currently meet.
Practical Ways to Improve Indoor Air
The most effective strategy is source control: reducing or removing the things that pollute your air in the first place. Choosing low-VOC paints and finishes, avoiding air fresheners and scented candles, running your gas stove with an exhaust fan, and fixing water leaks before mold takes hold all make a measurable difference. If you have an attached garage, keeping the door to the house sealed and not idling your car inside prevents benzene and CO from migrating into living spaces.
Ventilation is the second line of defense. Opening windows when weather allows, using bathroom and kitchen exhaust fans, and maintaining your HVAC system all help dilute indoor pollutants. In tightly sealed modern buildings, mechanical ventilation systems are often the only way fresh air gets in, so keeping filters clean and systems serviced matters more than it might seem.
Air filtration adds another layer of protection. HEPA filters capture at least 99.97 percent of particles as small as 0.3 microns, which includes most dust, pollen, mold spores, and bacteria. Portable HEPA air purifiers work well for individual rooms, while whole-house filtration systems can be integrated into existing HVAC systems. Keep in mind that standard air purifiers primarily remove particles, not gases. VOCs and radon require different solutions: ventilation for VOCs, and a specialized mitigation system for radon.
A simple, inexpensive CO2 monitor can give you real-time feedback on ventilation quality. If levels climb above 1,000 ppm, it’s a clear signal to open a window or adjust your ventilation. Similarly, a hygrometer (humidity gauge) helps you stay within the 30 to 50 percent sweet spot, using a dehumidifier in damp climates or a humidifier in dry ones as needed.