Indoor vs Outdoor Air Quality: Sources, Infiltration and Control

At a glance

What matters most

01
Outdoor AQI describes ambient conditions; it is not a measurement inside a home, school or workplace.
02
Outdoor particles enter through open windows, doors, ventilation systems and gaps in the building envelope.
03
Cooking, smoking, candles, heaters, cleaning activities and other indoor sources can raise pollution independently of outdoor AQI.
04
Source control, appropriate ventilation and filtration play different but complementary roles.

Indoor and outdoor air are connected—but not identical

Factor	Outdoor air	Indoor air
Major influences	Traffic, industry, fires, dust, weather and regional transport	Outdoor infiltration plus cooking, combustion, products, moisture and occupant activity
Typical measurement	Regulatory monitor, sensor network or model grid	A monitor located in the specific room or building
Ventilation effect	Disperses and transports pollution	Can dilute indoor sources or bring outdoor pollution inside
Filtration effect	Not controlled by an individual building	Can reduce some particles in recirculated or incoming air

How outdoor pollution gets indoors

Outdoor air enters through open windows and doors, mechanical ventilation and infiltration through cracks and gaps. The amount reaching a room changes with building tightness, wind, temperature differences, ventilation settings and filter performance.

Indoor PM2.5 can follow outdoor episodes with a delay and at a different concentration. A closed building is not perfectly sealed, while an open window can rapidly increase exchange with outdoor air.

Important indoor pollution sources

Cooking, especially frying, grilling and other high-temperature methods
Tobacco smoke, candles, incense, fireplaces and unvented combustion
Cleaning, renovation, hobbies and consumer products
Moisture, mould, pests, allergens and biological material
Attached garages, generators or malfunctioning fuel-burning appliances

Source control, ventilation and filtration

EPA identifies source control, improved ventilation and air cleaning or filtration as the three basic strategies. Source control is usually the first priority because it prevents pollution from being released indoors.

Ventilation is helpful when outdoor air is cleaner and can remove pollutants near their source through kitchen or bathroom exhaust. During a severe outdoor pollution episode, uncontrolled ventilation can instead bring more pollution inside. Particle filtration can supplement these measures but does not remove every pollutant or gas.

Using an indoor monitor without overinterpreting it

Place the monitor away from a wall, direct airflow, steam and a single source unless that source is the question.
Compare trends before and after cooking, ventilation or filtration rather than focusing on one isolated number.
Check what the device measures; PM, carbon dioxide, VOC estimates and carbon monoxide are not interchangeable.
Use a certified alarm—not a general air-quality sensor—for carbon monoxide and smoke safety.

Frequently asked questions

Does a good outdoor AQI guarantee good indoor air?

No. Indoor sources such as cooking, smoke, combustion, moisture or products can create poor conditions even when outdoor air is clean.

Should I open windows when AQI is high?

Usually avoid bringing in heavily polluted outdoor air unless ventilation is needed for an urgent indoor hazard. Conditions, building systems and indoor sources matter, so use current local guidance.

Will an air purifier remove all indoor pollution?

No. A suitable particle filter can reduce airborne particles, but performance depends on airflow, room size, placement and maintenance. Many particle cleaners do not remove gases.

Can I calculate indoor AQI from outdoor AQI?

Not reliably without indoor measurements and information about infiltration, sources, ventilation and filtration. Outdoor AQI is context, not a room reading.

Primary sources

This reference was prepared from the following primary public-health and regulatory sources: