How to Read the Air Quality Index (AQI): What the Numbers and Colors Mean

When wildfire smoke drifts over a city or pollution settles in on a still day, one number tells you how worried to be: the Air Quality Index, or AQI. It compresses a tangle of invisible pollutants into a single value from 0 to 500, with a color to match. Once you know how to read it, you can decide in seconds whether to open the windows, move a run indoors, or keep the kids inside.

You can see live AQI readings for major cities around the world on the LEV live map by switching on the Air Quality (AQI) layer — each dot is colored by its current reading, with the number printed right inside it.

The index is built around your health, not the chemistry

The clever thing about the AQI is that it isn't a measure of one pollutant. It tracks several — fine particles, ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide — and reports back whichever one is worst at that moment, converted onto a common 0-to-500 scale. So an AQI of 120 means something in the air has reached a level that's a concern, even if you don't know which pollutant it is. The scale is anchored to human health: each band corresponds to a real change in how the air affects your body.

The six color bands

• 0–50 · Green · Good. Clean air. No precautions needed.
• 51–100 · Yellow · Moderate. Fine for almost everyone. A few unusually sensitive people might notice symptoms.
• 101–150 · Orange · Unhealthy for Sensitive Groups. Children, older adults, and people with asthma or heart conditions should ease off heavy outdoor exertion.
• 151–200 · Red · Unhealthy. Everyone may begin to feel effects; sensitive groups more so. Cut back on outdoor activity.
• 201–300 · Purple · Very Unhealthy. A health alert. Avoid prolonged outdoor exertion entirely.
• 301–500 · Maroon · Hazardous. Emergency conditions. Stay indoors with windows closed if you can.

A useful mental shortcut: green and yellow, carry on; orange, sensitive people take care; red and above, everyone should pay attention.

PM2.5 is usually the one that matters

Most of the time — and almost always during wildfire smoke — the pollutant driving a high AQI is PM2.5: fine particulate matter under 2.5 microns wide. These specks are so small they slip past your body's defenses, lodging deep in the lungs and passing into the bloodstream. That's why a smoky day can leave you with a scratchy throat, a headache, or a tight chest even if you only stepped out briefly. When you click an AQI dot on the live map, PM2.5 is the first pollutant listed for exactly this reason.

The other one worth knowing is ozone. Unlike smoke, ozone is invisible and tends to peak on hot, sunny, windless afternoons, when sunlight cooks car and industrial emissions into a low-level haze. This is why a perfectly clear summer day can still carry an unhealthy AQI.

How this connects to the smoke you can see

The AQI tells you the measured number on the ground. The satellite smoke and dust layer shows you the plume overhead. Together they're a powerful pair: turn on Air Quality and Smoke & Dust at the same time, and you can watch a wildfire's smoke spread across the satellite view while the city dots beneath it climb from green into orange and red. One is the cause drifting through the sky; the other is the effect arriving in the air you breathe.

To understand where that smoke comes from in the first place, see our guide on tracking wildfires from space.

The bottom line

The AQI exists so you don't have to be a chemist to protect your health. Check the number, match it to the color, and let it guide a simple decision about your time outdoors. On a clean day you'll never think about it — and on a smoky one, it's the most useful number on the map.