IAQ stands for indoor air quality, and it refers to the air quality within and around buildings and structures, especially as it relates to the health and comfort of the people inside. Since most people spend roughly 90% of their time indoors, IAQ affects daily well-being more than outdoor air pollution does for the majority of the population. It applies to homes, offices, schools, hospitals, and any other enclosed space where people live or work.
What Determines Indoor Air Quality
Three primary factors shape the air you breathe indoors. The first is the outdoor air quality in your area, since air from outside constantly enters through windows, doors, and ventilation systems. The second is human activity inside the building: cooking, cleaning, smoking, and even just breathing all release substances into the air. The third is the building itself, including construction materials, furniture, and equipment that can slowly emit chemicals over time.
These factors interact constantly. A well-sealed energy-efficient home might keep outdoor pollution out but trap indoor pollutants at higher concentrations. A drafty older building might have better natural ventilation but let in pollen, wildfire smoke, or vehicle exhaust from a nearby road.
Common Indoor Pollutants
Indoor air contains a surprisingly wide range of contaminants. The EPA identifies several major categories:
- Volatile organic compounds (VOCs): Gases released by paints, varnishes, cleaning products, air fresheners, glues, permanent markers, moth repellents, and even new furniture and building materials. These chemicals are released during use and, to a lesser degree, during storage.
- Particulate matter: Tiny airborne particles from cooking, candles, wood-burning stoves, and dust. The smallest particles (called PM2.5, measuring less than 2.5 microns across) can penetrate deep into the lungs.
- Biological contaminants: Mold, dust mites, pet dander, pollen, and bacteria that thrive in damp or poorly ventilated spaces.
- Combustion gases: Carbon monoxide and nitrogen dioxide from gas stoves, furnaces, fireplaces, and attached garages.
- Radon: A naturally occurring radioactive gas that seeps into buildings from the ground and is the second leading cause of lung cancer.
- Secondhand smoke and aerosols: Tobacco smoke and emissions from e-cigarettes.
How Poor IAQ Affects Your Health
Short-term exposure to indoor pollutants can cause headaches, dizziness, eye and throat irritation, dry cough, fatigue, and difficulty concentrating. These symptoms often improve once you leave the building. Long-term exposure is more serious: respiratory diseases, heart disease, cognitive deficits, and cancer are all linked to chronic indoor air pollution. Radon, secondhand smoke, asbestos, and arsenic are among the indoor substances that raise lung cancer risk over time.
Children are especially vulnerable. Mold exposure in early life increases the prevalence and severity of asthma, and airborne allergens in schools have been tied to decreased lung function in asthmatic children. Exposure during pregnancy can impair lung development in the fetus, which may then increase the risk of pneumonia in the first year of life.
Adults are not spared from subtler effects. Research led by the Harvard T.H. Chan School of Public Health found that office air quality affects employees’ cognitive function, including response times, ability to focus, and overall productivity. Some inner-city schools that exceeded WHO air quality guidelines showed measurable impacts on children’s verbal abilities and executive functioning.
Sick Building Syndrome
When occupants of a particular building experience a cluster of health complaints that improve after leaving, it is often called sick building syndrome. There is no single identifiable cause, which is what distinguishes it from a specific illness like carbon monoxide poisoning. Symptoms include headaches, nausea, dry or itching skin, hoarseness, sensitivity to odors, cold or flu-like symptoms, and personality changes. The condition typically points to a combination of poor ventilation, chemical off-gassing, and biological contaminants acting together.
Key Numbers to Know
A few measurable thresholds help you gauge whether your indoor air is healthy.
Carbon dioxide is a useful proxy for ventilation. Outdoor air sits around 380 to 500 ppm. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends keeping indoor CO2 below 1,000 ppm. In crowded conference rooms, classrooms, and airplane cabins, levels can climb to several thousand ppm. One study found moderate declines in decision-making performance at 1,000 ppm and substantial declines at 2,500 ppm across most measures tested.
For fine particulate matter, ASHRAE sets an indoor PM2.5 limit of 15 micrograms per cubic meter. The WHO’s long-term guideline for annual average exposure is 10 micrograms per cubic meter, with a 24-hour limit of 25 micrograms per cubic meter. These are aggressive targets, and many indoor environments exceed them during cooking or in homes with wood-burning stoves.
Relative humidity should stay between 40% and 60%. Below 40%, air becomes dry enough to irritate airways and skin and may reduce your body’s defenses against respiratory viruses. Above 60%, mold growth becomes more likely, and dust mite populations increase. Above 75% to 85%, some viruses, including the one that causes COVID-19, may actually survive longer in airborne droplets.
Air Filtration Basics
Filters are rated using the MERV scale (Minimum Efficiency Reporting Value), which runs from 1 to 16. The higher the rating, the smaller the particles a filter can capture. A MERV 8 filter, common in residential HVAC systems, catches at least 70% of particles in the 3 to 10 micron range and about 20% of particles between 1 and 3 microns. A MERV 13 filter captures at least 50% of particles as small as 0.3 microns and 85% or more of particles between 1 and 3 microns, making it a popular upgrade for homes and offices concerned about fine particulate matter and airborne pathogens.
HEPA filters sit above the MERV scale entirely. They capture 99.97% of particles at 0.3 microns, the size that is hardest to trap, and perform even better on particles that are larger or smaller. HEPA filters are standard in portable air purifiers but generally cannot be used in residential HVAC systems because they restrict airflow too much for most home furnaces and air handlers.
Practical Ways to Improve IAQ
Ventilation is the single most effective lever. Opening windows when outdoor air quality is good, running kitchen and bathroom exhaust fans, and maintaining your HVAC system all help dilute indoor pollutants. If your home has a forced-air system, upgrading to a MERV 13 filter is one of the simplest improvements you can make, though you should confirm your system can handle the added resistance.
Source control matters just as much. Choosing low-VOC paints and cleaning products, fixing water leaks promptly to prevent mold, testing your home for radon (inexpensive kits are available at hardware stores), and avoiding smoking indoors all reduce pollutants before they accumulate. Cooking on a gas stove without running an exhaust fan can spike nitrogen dioxide levels to concentrations that exceed outdoor air quality standards within minutes.
Portable air purifiers with HEPA filters are effective for individual rooms, particularly bedrooms where you spend hours at a stretch. A standalone CO2 monitor, available for around $100 to $200, can help you judge whether a room is adequately ventilated. If readings regularly climb above 1,000 ppm, it is a clear signal to increase fresh air flow.