What the Air Quality Index numbers and colors mean

Higher numbers mean more pollution

The US Air Quality Index (AQI) is the EPA's scale for reporting outdoor air. It runs from 0 to 500. The higher the number, the more pollution in the air and the greater the health concern.

The scale is tied to federal health standards. For each pollutant, an AQI of 100 generally matches the EPA's short-term national standard. The EPA says values at or below 100 are "generally thought of as satisfactory."

Each color is a category

Who counts as "sensitive" depends on the pollutant

The EPA lists different sensitive groups for each pollutant:

• Particle pollution (PM2.5 and PM10): people with heart or lung disease, older adults, and children, including teenagers. The EPA adds that some minority groups, people with lower incomes, and outdoor workers may get higher exposures that can worsen health conditions. During smoke events, the EPA also counts pregnant people as at risk.
• Ozone: people with lung disease such as asthma, children and teenagers, people who are active outdoors, and older adults.
• Carbon monoxide: people with heart disease.
• Nitrogen dioxide: people with lung disease such as asthma, children and teenagers, and older adults.
• Sulfur dioxide: people with asthma, children and teenagers, and older adults.

The worst pollutant sets the number

The EPA runs an AQI for 5 pollutants: ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide and nitrogen dioxide. Each gets its own index value. The AQI reported for your area is the highest one, and reports name it as the main pollutant.

Ozone and particles cause nearly all bad days. The EPA's AQI guide says these 2 are the main sources of unhealthy air "around 99% of the time." That's why most forecasts cover only those 2.

PM2.5 means particles 2.5 micrometers across or smaller. The EPA notes that an average human hair is about 70 micrometers wide, 30 times bigger than the largest of these particles. They can get deep into your lungs, and some may reach your bloodstream. Smoke, soot and exhaust are all sources. When wildfire smoke drives the AQI up, PM2.5 is the pollutant you're seeing.

Ozone is a gas. It forms near the ground when pollution from cars, power plants and industry reacts in sunlight. It's usually worst on hot, sunny days. Our ozone guide covers it.

The live reading and the daily AQI are measured differently

The official daily AQI uses a full day of data: a 24-hour average for particles and the highest 8-hour average for ozone. So the official daily number describes yesterday.

To show the air right now, AirNow uses a formula called the NowCast. For particles, the NowCast is a weighted average of the last 12 hours. When air quality is steady, the hours count about equally. When it's changing fast, as it does in smoke, the latest hours count more, close to a 3-hour average. Ozone has its own NowCast method.

Anywhere AirNow shows "current air quality," it's the NowCast, so expect it to move. The EPA points out that even on a day with a bad forecast, there may be hours when the air is fine for outdoor activity.

One caution about home sensors. The EPA does not advise applying AQI colors to 1-minute sensor readings, because the health studies behind the AQI looked at longer exposures.

The particle scale changed in 2024

On February 7, 2024, the EPA lowered the annual health standard for PM2.5 to 9.0 µg/m³ (micrograms per cubic meter of air). It also changed where the AQI categories for fine particles begin and end. The new scale took effect May 6, 2024, on AirNow and the Fire and Smoke Map.

The EPA expects many areas to see more yellow days, since Moderate now starts at 9 instead of 12. During wildfires, some days that once showed red may now show purple, and some purple days may now show maroon. The orange line didn't move, because the EPA kept the 24-hour standard of 35.

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