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What Is AQI? Air Quality Index Explained (2026)

AQI stands for Air Quality Index. It is the US EPA's tool for telling people about outdoor air quality and health. The higher the number, the more pollution in the air and the bigger the health concern.

Last updated October 2026

What Is the AQI?

The Air Quality Index tells you how clean or polluted the outdoor air is where you live. You do not need to read raw pollution data. You just read one number and one color.

EPA splits the index into six categories. Each one has its own color and its own level of health concern. An AQI of 50 or below is good air. An AQI over 300 is hazardous air.

AQI Levels and Categories

The “what it means” column is EPA's wording from AirNow. The “what EPA says to do” column is EPA's advice for fine particles (PM2.5).

AQICategoryColorWhat it meansWhat EPA says to do
0 to 50GoodGreenAir quality is satisfactory, and air pollution poses little or no risk.It's a great day to be active outside.
51 to 100ModerateYellowAir quality is acceptable. There may be a risk for some people, mainly those who are unusually sensitive to air pollution.Unusually sensitive people: consider making outdoor activities shorter and less intense. Go inside if you have symptoms such as coughing or shortness of breath.
101 to 150Unhealthy for Sensitive GroupsOrangeMembers of sensitive groups may have health effects. The general public is less likely to be affected.Sensitive groups: make outdoor activities shorter and less intense. It's OK to be active outdoors, but take more breaks.
151 to 200UnhealthyRedSome members of the general public may have health effects. Sensitive groups may have more serious effects.Sensitive groups: consider rescheduling or moving all activities inside. Everyone else: keep outdoor activities shorter and less intense. Go inside if you have symptoms.
201 to 300Very UnhealthyPurpleHealth alert: the risk of health effects is increased for everyone.Sensitive groups: avoid all physical activity outdoors. Everyone else: limit outdoor physical activity.
301 and higherHazardousMaroonHealth warning of emergency conditions: everyone is more likely to be affected.Everyone: avoid all physical activity outdoors.

Who counts as a sensitive group? For particle pollution, EPA names people with heart or lung disease, older adults, and children, including teens. For ozone, it also names people who are active outdoors.

What Pollutants Does AQI Measure?

EPA sets an AQI for five major air pollutants. Particle pollution is tracked in two sizes, so you will often see six names. EPA works out an index for each one, and the AQI you see is the highest of them.

Why PM2.5 Matters Most

A human hair is about 30 times wider than the largest fine particle, EPA says. Particles this small can get deep into the lungs, and some may get into the blood. EPA links short exposure to heart attacks, irregular heartbeat and worse asthma. It links long exposure to heart and lung disease and lung cancer. Many home air monitors track PM2.5, so it is a good number to watch at home.

How to Check Your AQI

When to Take Action

Use the table above for what EPA says to do outside. Inside, these steps help when the outdoor air is bad:

Indoor vs Outdoor AQI

The AQI on AirNow.gov is for outdoor air. Your indoor air can be better or worse. It depends on:

An indoor air monitor shows what is in your own rooms. See our guide to the best air quality monitors.

How Air Purifiers Help

HEPA filters catch particles as air passes through. EPA says a HEPA filter can in theory remove at least 99.97% of airborne particles 0.3 microns in size. Larger and smaller particles are caught even better.

Activated carbon can catch some gases, but EPA says it works only when a large amount is used. The CADR rating covers particles only.

CADR (Clean Air Delivery Rate) tells you how fast a purifier cleans the air. A higher CADR cleans faster. EPA's chart, for an 8-foot ceiling, suggests a CADR of 65 for a 100 sq ft room and 130 for a 200 sq ft room. Compare picks in our best air purifier guide.

This page explains what EPA publishes about the AQI. It is not medical advice. If you have a health condition, ask your doctor what to do on bad air days.

Frequently Asked Questions

What is a good AQI number?

EPA calls 0 to 50 Good: air quality is satisfactory, and air pollution poses little or no risk. 51 to 100 is Moderate: air quality is acceptable, but there may be a risk for people who are unusually sensitive to air pollution. EPA says AQI values at or below 100 are generally thought of as satisfactory.

What does AQI measure?

EPA sets an AQI for five major air pollutants: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide and nitrogen dioxide. EPA works out an index for each one. The AQI you see is the highest of them, and that pollutant is called the main pollutant.

Is 100 AQI bad?

100 is the top of the Moderate range. EPA says an AQI of 100 generally matches the level of the short-term national air quality standard for that pollutant. Above 100, air is unhealthy: first for sensitive groups, then for everyone as the number climbs. At Moderate, EPA tells unusually sensitive people to consider making outdoor activities shorter and less intense.

How do I improve indoor air quality?

EPA says portable air cleaners and HVAC filters can reduce indoor air pollution, but they cannot remove all pollutants. Pick a portable air cleaner with a CADR big enough for the room. For a furnace filter, EPA suggests at least MERV 13 or as high as your system allows. Cutting indoor sources, like frying food or burning candles, also helps.

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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 .