Indoor air quality refers to how clean or polluted the air is inside buildings where you live, work, and spend time. It matters more than most people realize: concentrations of many common pollutants are consistently up to ten times higher indoors than outdoors, and the average person spends roughly 90% of their time inside. Poor indoor air quality has been linked to respiratory disease, heart disease, cognitive problems, and cancer.
What Makes Indoor Air Unhealthy
Indoor air pollution comes from a surprisingly wide range of sources, and most of them are things you already have in your home. The major categories are chemical pollutants, biological contaminants, and combustion byproducts.
Volatile organic compounds (VOCs) are gases released by thousands of everyday products: paints, cleaning sprays, disinfectants, air fresheners, glues, permanent markers, new furniture, and even dry-cleaned clothing. Formaldehyde, one of the most common VOCs, off-gases from pressed wood products, insulation, and some fabrics. Benzene, a known human carcinogen, enters homes through tobacco smoke, stored fuels, paint supplies, and car exhaust drifting in from attached garages. These chemicals don’t just smell bad. They actively degrade the air you breathe, and many continue releasing gases even when stored and not in use.
Biological pollutants include mold spores, dust mites, pet dander, pollen, and bacteria. Mold thrives when indoor relative humidity climbs above 60%, and dust mites flourish in similar conditions. The EPA recommends keeping indoor humidity between 30% and 50% to discourage both mold growth and pest activity.
Combustion sources round out the picture. Gas stoves, fireplaces, unvented space heaters, and tobacco smoke all release particulate matter, nitrogen dioxide, and carbon monoxide directly into your living space. Radon, a naturally occurring radioactive gas that seeps up through foundations, is the second leading cause of lung cancer after smoking.
How Poor Air Quality Affects Your Body
Short-term exposure to indoor pollutants typically causes symptoms that feel a lot like allergies or a mild cold: headaches, eye and throat irritation, dry cough, dizziness, fatigue, and difficulty concentrating. These effects can show up within hours of exposure and usually fade once you leave the space. In healthy adults, even short-term exposure to indoor ozone has been linked to increased blood pressure and elevated blood platelet levels, a risk factor for clotting.
The cognitive effects are striking. NIEHS-funded research from Harvard found that office air quality directly affects employees’ response times, ability to focus, and productivity. Studies in schools have found that some classrooms exceed WHO guidelines for pollutants like nitric oxide, which can impair verbal abilities and executive functioning in children.
Long-term exposure raises the stakes considerably. Chronic contact with radon, secondhand smoke, asbestos, and formaldehyde increases the risk of lung cancer. Indoor air pollution during pregnancy has been associated with impaired lung function in newborns, which may increase their risk of pneumonia in the first year of life. Early-life mold exposure appears to increase the prevalence and severity of asthma, and reducing that exposure can have lasting protective effects.
Carbon Dioxide and Ventilation
Carbon dioxide builds up in any occupied room simply because people exhale it. Outdoor CO2 levels sit around 380 to 500 ppm. Indoors, levels climb higher, and in crowded spaces like conference rooms, classrooms, and airplane cabins, they can reach several thousand ppm. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends a maximum indoor CO2 level of 1,000 ppm as a marker of adequate ventilation.
That threshold isn’t arbitrary. Researchers found a moderate decrease in decision-making performance across six of nine measured categories at 1,000 ppm, and a more substantial decline in seven of nine categories at 2,500 ppm. If you’ve ever felt foggy or sluggish in a packed meeting room, rising CO2 is a likely culprit. The fix is straightforward: better ventilation. ASHRAE recommends homes receive at least 0.35 air changes per hour, or a minimum of 15 cubic feet per minute of fresh air per person.
Sick Building Syndrome
Sick building syndrome describes a pattern where people in a particular building develop symptoms like headaches, nausea, dry skin, throat irritation, and difficulty concentrating, but no specific illness or cause can be pinpointed. The key indicator is that symptoms improve or disappear shortly after leaving the building. It most often occurs in offices, schools, and commercial buildings where HVAC systems are poorly maintained or not distributing air effectively. Many buildings are simply not operated to deliver the ventilation rates they were originally designed for, and routine failures like dirty filters or blocked ducts can quietly degrade air quality for months.
How to Measure Indoor Air Quality
You can’t assess air quality by smell alone. Many of the most harmful pollutants, including radon and carbon monoxide, are odorless. Consumer-grade air quality monitors can track particulate matter, CO2 levels, humidity, and sometimes VOCs in real time, giving you a useful baseline picture of your home’s air. Carbon monoxide alarms and smoke detectors are essential safety devices that alert you when concentrations reach immediately dangerous levels. Radon test kits, available at most hardware stores, are the only way to know whether your home has a radon problem.
Practical Ways to Improve Your Air
The three main strategies are source control, ventilation, and filtration. Source control is the most effective: if you can eliminate or reduce a pollutant at its origin, you don’t have to filter it out later. That means choosing low-VOC paints and cleaners, storing chemicals in sealed containers outside living areas, fixing water leaks promptly to prevent mold, and avoiding air fresheners that simply layer fragrance over pollutants.
Ventilation brings fresh outdoor air in and pushes stale air out. Opening windows when weather allows is the simplest approach. Exhaust fans in kitchens and bathrooms remove moisture and combustion byproducts at the source. In tightly sealed newer homes, mechanical ventilation systems may be necessary to meet that 0.35 air changes per hour target.
Filtration captures particles that are already airborne. If you have a forced-air HVAC system, the filter’s MERV rating tells you how effectively it traps particles. Filters rated MERV 1 through 4 capture less than 20% of large particles and do very little for air quality. A MERV 13 filter captures at least 50% of the smallest particles (0.3 to 1.0 microns), which includes many allergens and fine dust. Standalone HEPA air purifiers go further, trapping 99.97% of particles at 0.3 microns, making them particularly useful in bedrooms or rooms where someone has allergies or asthma. One practical note: higher-rated filters restrict more airflow, so check that your HVAC system can handle a higher MERV rating before upgrading.
Humidity control ties everything together. A dehumidifier or properly sized air conditioner that keeps relative humidity in the 30% to 50% range discourages mold, dust mites, and cockroaches simultaneously, addressing some of the most common biological triggers of respiratory symptoms.