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Air Quality Index Explained: How AQI Is Calculated

The EPA AQI is not a simple average. Here is how it works, what each pollutant is, and how to use the number day to day. Everything below comes from EPA's own AQI guidance.

Last updated October 2026

How AQI Is Calculated

EPA sets an AQI for five major pollutants. Particle pollution is split into two sizes, PM2.5 and PM10, so the breakpoint table has six columns. For each pollutant, EPA takes the highest reading in the area, finds which two breakpoints it falls between, and turns it into an index number. The AQI is the highest of those numbers.

So a day with high ozone but low PM2.5 can still show an AQI above 100, driven by ozone alone. The pollutant with the highest index is called the main pollutant.

The Formula (Plain English)

EPA's equation for each pollutant is:

Ip = (IHi - ILo) / (BPHi - BPLo) × (Cp - BPLo) + ILo

In plain English: the formula asks how far your reading sits between the two breakpoints, then moves the same distance along the matching AQI range.

EPA's own example: a PM2.5 reading of 35.9 falls between 35.5 and 55.4, which match AQI 101 and 150. (150 - 101) / (55.4 - 35.5) × (35.9 - 35.5) + 101 = 102.

PM2.5 Breakpoints and What EPA Says to Do

These are the PM2.5 breakpoints in EPA's May 2026 AQI guidance, for a 24-hour average in micrograms per cubic meter. Good stops at 9.0. Some older charts online use different numbers, so go by EPA's current table.

CategoryAQIPM2.5 (µg/m³)What EPA says to do
Good0 to 500.0 to 9.0It's a great day to be active outside.
Moderate51 to 1009.1 to 35.4Unusually sensitive people: consider making outdoor activities shorter and less intense.
Unhealthy for Sensitive Groups101 to 15035.5 to 55.4Sensitive groups: make outdoor activities shorter and less intense. Take more breaks.
Unhealthy151 to 20055.5 to 125.4Sensitive groups: consider moving all activities inside. Everyone else: keep outdoor activities shorter and less intense.
Very Unhealthy201 to 300125.5 to 225.4Sensitive groups: avoid all physical activity outdoors. Everyone else: limit outdoor physical activity.
Hazardous301 and higher225.5 and higherEveryone: avoid all physical activity outdoors.

For particle pollution, EPA's sensitive groups are people with heart or lung disease, older adults, and children, including teens. During smoke, EPA adds pregnant people. EPA also says people with asthma should follow their asthma action plan once the AQI is above 100.

The Pollutants in Detail

PM2.5: Fine Particles

Particles generally 2.5 micrometers and smaller. EPA says these pose the greatest risk to health. They can get deep into the lungs, and some may get into the blood. EPA links long exposure to heart and lung disease, lung cancer and early death.

PM10: Inhalable Particles

Particles generally 10 micrometers and smaller. EPA says particles under 10 micrometers can be breathed in. Sources include building sites, unpaved roads, fields and fires.

Ozone (Ground-Level)

Ground-level ozone is not given off directly. It forms when nitrogen oxides and VOCs from cars, power plants, factories and other sources react in sunlight. EPA says it is most likely to reach unhealthy levels on hot sunny days in cities. It can cause coughing and a sore throat and make asthma worse.

Carbon Monoxide (CO)

A colorless, odorless gas released when something is burned. Cars, trucks and other machines that burn fossil fuels are the biggest outdoor source. High levels cut the oxygen the blood can carry. EPA says very high levels are not likely outdoors.

Sulfur Dioxide (SO2)

Comes mostly from burning fossil fuels at power plants and other industrial sites. Short exposure can make breathing hard. EPA says people with asthma, especially children, are sensitive to it.

Nitrogen Dioxide (NO2)

Comes mostly from burning fuel in cars, trucks, buses, power plants and off-road equipment. It can irritate airways and make asthma worse. It also helps form ozone and fine particles.

How Often Is AQI Updated?

AQI Data Sources

Wildfire Smoke and AQI

EPA says smoke is a mix of gases and fine particles, and the biggest health threat is the fine particles. That is why PM2.5 usually drives the AQI during a fire. AirNow says smoke can travel a long distance from a fire.

EPA says portable air cleaners and high-efficiency HVAC filters can remove smoke particles. Activated carbon can help with some gases, but EPA says it works only when a large amount is used. See our picks for the best air purifier for smoke.

Limitations of AQI

This page explains EPA's published method and advice. It is not medical advice. If you have a health condition, ask your doctor what to do on bad air days.

Frequently Asked Questions

Why does a sensor map show a different AQI than AirNow?

Low-cost sensors and official monitors measure in different ways. EPA applies a correction equation to sensor data before it shows on the AirNow Fire and Smoke Map, so the two can be compared. EPA also says it does not advise applying AQI values to real-time sensor readings, especially 1-minute readings, because the AQI is based on health studies of longer exposures.

What AQI is OK for exercising outside?

EPA does not name a single safe number. For fine particles, its advice is: Good, it's a great day to be active outside. Moderate, unusually sensitive people should consider shorter, less intense activity. Unhealthy for Sensitive Groups, sensitive groups should make activities shorter and less intense. Unhealthy and above, everyone should cut back. For ozone, EPA adds that mornings are often better because ozone is lower.

What causes high AQI days?

EPA says ozone is most likely to reach unhealthy levels on hot sunny days in cities, and wind can carry it to rural areas. Particles come straight from sources like fires, roads, fields and smokestacks, and most also form in the air from sulfur dioxide and nitrogen oxides. Wildfire smoke carries fine particles too.

Does AQI include indoor air quality?

No. The AQI is for outdoor air. Indoor air can be better or worse, depending on ventilation, filters, air purifiers and indoor sources like cooking. An indoor air quality monitor shows what is in your own rooms.

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Sources and last review

Specifications on this page are taken from the documents below, on the dates shown. We do not physically test products; read our methodology. Page last reviewed .