What the AQI numbers and colors mean

The Air Quality Index (AQI) is the number US agencies use to say how clean or polluted the air is. It runs from 0 to 500, and each range has a name and a color. The colors make it quick to read. The details behind them help you decide what to do, especially if you or someone you look after is more sensitive to pollution.

The AQI is run by the US Environmental Protection Agency (EPA) and published on AirNow together with state, local and tribal air agencies.

The six categories

The descriptions are paraphrased from AirNow. The colors are EPA's official ones, from its reporting guidance.

The anchor of the scale is 100. For each pollutant, an AQI of 100 roughly matches the national short-term health standard for that pollutant. At or below 100, air is generally considered satisfactory. Above 100 it's unhealthy, first for sensitive groups and then, as the number climbs, for everyone. The scale is usually described as running to 500, but EPA treats anything above 300 as Hazardous, including the rare readings above 500.

One number, five pollutants

The AQI covers five pollutants regulated under the Clean Air Act: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide and nitrogen dioxide. Each gets its own AQI value. The number you see reported for a place is the highest of them, and a good report names which pollutant that is.

In practice, two pollutants drive most bad air days. EPA says ozone and fine particles account for nearly all of them, which is why most local forecasts cover only those two. The AQI doesn't cover everything in the air. Air toxics and heat, for example, are outside it.

PM2.5 and ozone are different problems

They behave differently, so the advice for each is a little different.

Ozone near the ground isn't emitted directly. It forms when nitrogen oxides and volatile organic compounds, from vehicles, industry, power plants, paints and solvents, react in sunlight. That's why it tends to peak on hot, sunny summer afternoons and early evenings, and is often lower in the morning. Breathing it can cause coughing and breathing difficulty and can set off asthma attacks (AirNow ozone guide). On high-ozone days, moving a run or outdoor work to the morning is often enough.

PM2.5 means fine particles 2.5 micrometres across or smaller. They come from vehicle exhaust, power plants, industry, wood stoves and wildfire smoke. They can be high at any time of year and any time of day. In winter, a cold layer of air trapped under warmer air (an inversion) can hold wood smoke and exhaust near the ground in valleys. Fine particles reach deep into the lungs and are linked to asthma attacks, heart attacks and strokes (AirNow particle guide). Wildfire smoke is mostly PM2.5, so during smoke events the PM2.5 number is the one to watch.

The PM2.5 scale changed in 2024

In February 2024, EPA tightened the annual PM2.5 standard from 12.0 to 9.0 micrograms per cubic metre (EPA). Since 6 May 2024 the AQI breakpoints for PM2.5 have changed to match. Concentrations are 24-hour averages in µg/m³ (EPA fact sheet):

So the same air can now show as Moderate where it used to be Good, and heavy smoke can land a category higher than it would have before 2024. If you compare with older articles or screenshots, that's why.

Hourly numbers and the NowCast

The official AQI for particles and ozone is based on daily averages, but people want to know about the air now. AirNow's current readings use EPA's NowCast. For particles, it's a weighted average of the last 12 hours that leans heavily on the most recent hours when conditions are changing quickly, such as when smoke moves in. AirNow notes that its current data are preliminary and haven't yet gone through full validation (About the data).

Who counts as a sensitive group

The groups depend on the pollutant. EPA's current reporting guidance lists:

• For ozone: people with lung disease such as asthma, children and teenagers, older adults, and people who are active outdoors.
• For particle pollution: people with heart or lung disease, older adults, and children and teenagers. EPA also notes that outdoor workers, people with lower incomes and some minority groups can face higher exposure, and it recommends including pregnant people during smoke events.

Orange is the first color where these groups should take precautions. Typical steps are shorter or lighter outdoor activity, more breaks, and keeping rescue medicines at hand. At Red and above, the advice extends to everyone.

Air Quality Action Days and alerts

When the AQI is forecast to reach unhealthy levels, often Orange or worse, state and local air agencies may declare an Air Quality Action Day (AirNow). Some areas call them Ozone Action Days or Spare the Air days. They're voluntary. People are asked to cut pollution, for example by driving less or putting off jobs with gas-powered equipment, and to take care of their own health.

You may also see an Air Quality Alert from the National Weather Service. The alert is written by the state or local air agency, and the local NWS office passes it on (NWS Instruction 10-519). It's usually issued when the AQI is forecast at 101 or above.

Where to check

• AirNow and the free AirNow app show current readings and forecasts by ZIP code.
• During wildfires, the AirNow Fire and Smoke Map, from EPA and the US Forest Service, adds temporary smoke monitors and corrected readings from low-cost sensors. That helps where official monitors are far apart.
• EnviroFlash emails or texts your area's forecast and alerts.
• Your state or local air agency's website has local forecasts and Action Day announcements.

Herebeat is adding AirNow air quality for US places in version 1.0.1, shown like everything else in the app: the official number, its category and color, and when AirNow reported it.

A reading is a measurement at a monitor, so your own street can differ, especially near traffic or in smoke. When the number and what you see and smell disagree, it's reasonable to take the more cautious reading.