Indoor air is not automatically clean, and outdoor air is not automatically the problem. The right action depends on which pollutant is present, where its source is, and whether outdoor conditions are suitable for ventilation.
The U.S. EPA organizes indoor-air decisions around three tools: source control, ventilation, and filtration. They complement one another, but they do different jobs.
Outdoor and indoor readings answer different questions
An outdoor AQI describes ambient air for a reporting location. It does not measure the air in your bedroom, kitchen, classroom, or office.
Outdoor air enters buildings through open doors and windows, mechanical ventilation, exhaust-driven replacement air, and leakage around the building envelope. How much enters varies with the building, weather, pressure, HVAC operation, and occupant behavior. There is no responsible universal rule such as “indoors is always 50% cleaner.”
Indoor sources can also dominate. Cooking, smoking, vaping, candles, incense, aerosol products, attached garages, fuel-burning appliances, renovation work, and some cleaning or hobby activities can add particles or gases. Moisture can support mold growth. Radon can enter from the ground. These hazards are not represented by an outdoor PM2.5 AQI.
EPA notes that people spend about 90% of their time indoors and that concentrations of some pollutants are often several times higher indoors than outdoors. See the agency's 2026 indoor-air-quality factsheet.
The three-part strategy
1. Source control: stop adding pollution
EPA calls source control the most effective approach for many indoor-air problems. Examples include:
- do not smoke or vape indoors;
- avoid candles and incense when particle levels are a concern;
- use a range hood that exhausts outdoors while cooking, when outdoor conditions allow;
- vent combustion appliances correctly and maintain them;
- store solvents and fuels safely and follow product instructions;
- repair moisture problems rather than trying to filter away mold growth; and
- test for radon with an appropriate test rather than assuming an air purifier removes it.
Filtration cannot compensate for an active carbon-monoxide source, gas leak, or unsafe appliance. Install and maintain carbon-monoxide alarms as recommended by local authorities. If an alarm sounds or you suspect combustion poisoning, leave the area and follow emergency guidance.
2. Ventilation: dilute indoor pollutants with suitable outdoor air
Opening windows or bringing in mechanical outdoor air can dilute indoor-generated pollutants. Kitchen and bathroom exhaust fans that vent outdoors remove pollution near its source.
Ventilation is not always the right immediate move. Before opening windows, check:
- outdoor AQI and the dominant pollutant;
- smoke, dust, pollen, temperature, and humidity;
- whether the outdoor air is actually cleaner than indoor air; and
- whether the indoor activity can be paused instead.
During a wildfire-smoke episode, bringing in more outdoor air may increase indoor PM2.5. EPA advises temporarily limiting smoke entry, filtering recirculated air, and ventilating again when outdoor air improves. During painting, solvent use, or a combustion problem, removing the source and exhausting contaminated air may be more urgent than keeping smoke out. Indoor-air decisions are pollutant-specific.
3. Filtration: remove airborne particles that pass through
Portable air cleaners and compatible HVAC filters can reduce airborne particles. EPA describes filtration as a supplement to source control and ventilation, not a complete replacement for them. Particle filters do not automatically remove gases, radon, or carbon monoxide.
Choose a portable unit by clean air delivery rate (CADR) and room size, not by vague “coverage” marketing. For a target air-change rate:
Required CADR (cfm) = room volume (ft³) × target ACH ÷ 60
A 150 ft² room with an 8-foot ceiling has a volume of 1,200 ft³. At 5 air changes per hour, the theoretical clean-air requirement is:
1,200 × 5 ÷ 60 = 100 cfm
Real rooms have leakage, furniture, open doors, noise constraints, and variable fan speeds, so treat this as a sizing estimate. Use the Air Purifier Size Calculator for consistent arithmetic and review the EPA guide to air cleaners and filters. Avoid devices that intentionally produce ozone.
Four common scenarios
Outdoor smoke, few indoor sources
Close windows and doors without compromising safe exit. Set HVAC or vehicle systems to recirculate if that is appropriate for the equipment. Run correctly sized particle filtration and avoid adding smoke or cooking particles indoors. Check both outdoor conditions and, if available, an indoor PM2.5 monitor.
When outdoor air improves, air out the home and use exhaust fans as appropriate. EPA's wildfire indoor-air guidance emphasizes that smoke can enter even with windows closed, so “sealed” does not mean smoke-free.
Clean outdoor air, high indoor particles or odors
Stop the source first. Use local exhaust and outdoor air when weather and outdoor quality permit. Filtration may help with particles, but an odor can come from gases that a particle filter does not capture.
Do not use incense, candles, or smoke to “test” airflow or leaks. That creates the pollutant you are trying to remove.
Wildfire smoke and extreme heat at the same time
Heat can be immediately dangerous. Keeping every window closed is not a safe universal recommendation for a home that is overheating.
EPA advises using air conditioning when available and planning access to an air-conditioned location or cooling center when it is not. If outdoor air is hotter than indoor air, closing windows and using shades may slow heating; when outdoor air becomes cooler, ventilation may help. Outdoor smoke still matters, so choose the safest available balance and consider a cleaner-air, air-conditioned public space. See EPA guidance on extreme heat and indoor air quality.
Never run a fuel-powered generator, grill, or other combustion device indoors or in a garage to power cooling or filtration.
A tightly closed room occupied for many hours
Particle filtration does not remove exhaled carbon dioxide. A rising indoor CO2 reading can indicate that ventilation is limited, but CO2 is not a complete measure of all indoor-air risks. Balance smoke control with ventilation when outdoor conditions allow, and do not interpret a low PM2.5 value as proof that the room is healthy in every respect.
Measuring indoor PM2.5 responsibly
A consumer PM2.5 monitor can show trends and help compare rooms, filtration settings, or indoor and outdoor periods. It is not a regulatory monitor.
For more useful readings:
- place it away from a purifier outlet, open window, stove, humidifier, and direct draft;
- allow time for conditions to stabilize;
- compare trends rather than reacting to one brief spike;
- check whether high humidity may affect an optical sensor; and
- remember that it usually does not measure ozone, CO, NO2, radon, or every VOC.
Do not convert an indoor PM2.5 concentration into an “indoor AQI” and then assume it represents all indoor pollutants. If you want a rough cigarette-equivalent comparison for measured PM2.5, enter the concentration directly in the PM2.5 Cigarette Calculator.
A simple decision sequence
- Identify the pollutant or source. Outdoor smoke, indoor cooking particles, carbon monoxide, radon, moisture, and VOCs require different responses.
- Stop controllable indoor sources. Source control usually comes first.
- Check outdoor air and weather. Ventilate only when the incoming air improves the situation and conditions are safe.
- Filter particles. Use appropriately sized filtration and maintain filters.
- Measure the outcome. Recheck indoor PM2.5 trends, temperature, and relevant safety alarms.
- Escalate when needed. Follow emergency instructions for fire, gas, CO, severe heat, or medical symptoms.
Bottom line
The useful question is not “Is indoor or outdoor air better?” It is “What is the source, and which control works for this pollutant right now?”
Use source control for emissions you can stop, ventilation when outdoor air and weather permit, and filtration for airborne particles. During smoke, heat, or combustion hazards, safety boundaries override a simplistic windows-open or windows-closed rule.