Air quality, decoded. What you are actually breathing.

Air quality is a measure of how clean — or dirty — the air is to breathe, and it gets a single number, the Air Quality Index, from 0 to 500. A low number means a clear-sky day for your lungs; a high number means the air is carrying something you would rather not inhale. This page is the short version of what that number means, what is actually floating in the air, where it comes from, and when the smart move is to take your run, your picnic, or your practice indoors.

ONE NUMBER, SIX COLORS.

The AQI runs 0 to 500. Green is go, maroon is stay in.

The Air Quality Index turns a pile of pollutant readings into one number from 0 to 500, split into six color-coded bands: 0 to 50 Good (green), 51 to 100 Moderate (yellow), 101 to 150 Unhealthy for Sensitive Groups (orange), 151 to 200 Unhealthy (red), 201 to 300 Very Unhealthy (purple), and 301 to 500 Hazardous (maroon). The number is set by whichever pollutant is worst at that hour, so a single bad actor — usually fine particles from smoke — can push the whole score up. The higher the number, the more the air can irritate eyes, throats, and lungs, and the shorter your outdoor plans should be.

WHAT IS ACTUALLY IN THE AIR.

PM2.5, ozone, and the usual suspects.

The main pollutants are fine particles (PM2.5, smaller than 2.5 micrometers — small enough to get deep into your lungs), coarse particles (PM10, dust and pollen-sized), ground-level ozone (sunlight cooking car exhaust into smog), nitrogen dioxide, sulfur dioxide, and carbon monoxide. Wildfire smoke is mostly PM2.5, which is why a fire hundreds of miles away can still turn your sky orange and your AQI red. Dust storms, traffic, factories, and pollen all add their own share — and hot, still, sunny days make ozone worse, which is why bad-air days so often feel like bad-heat days too.

WHAT TO DO ABOUT IT.

Over 100, sensitive lungs stay in. Over 150, everyone does.

If you are in a sensitive group — kids, older adults, or anyone with asthma or a heart or lung condition — keep outdoor exertion short once the AQI passes 100, and move it inside past 150. When the number climbs toward 200 and beyond, everyone should stay indoors, keep windows shut, and run an air purifier or the AC on recirculate. An Air Quality Alert means pollutants have reached unhealthy levels and the advice is to limit outdoor activity — and before you head out, check Brzzy’s forecast, live radar, and severe weather alerts so the sky does not make the decision for you.

What is air quality?

Air quality is a measure of how clean the air is to breathe. It is reported as the Air Quality Index, a single number from 0 to 500: low numbers mean clean air, high numbers mean the air is carrying pollutants that can irritate your eyes, throat, and lungs.

What does the AQI number mean?

The AQI runs 0 to 500 in six color-coded bands: 0 to 50 Good (green), 51 to 100 Moderate (yellow), 101 to 150 Unhealthy for Sensitive Groups (orange), 151 to 200 Unhealthy (red), 201 to 300 Very Unhealthy (purple), and 301 to 500 Hazardous (maroon). The number is set by whichever pollutant is worst at that hour.

What is PM2.5?

PM2.5 is fine particulate matter — particles smaller than 2.5 micrometers across, small enough to get deep into your lungs. It comes mostly from wildfire smoke, vehicle exhaust, and burning, and it is usually the pollutant that drives the AQI up on smoky days.

Who should worry about bad air quality?

Children, older adults, and people with asthma or heart or lung conditions are the sensitive groups — they should limit outdoor exertion once the AQI passes 100. Past 150, everyone should keep outdoor time short, and at very high numbers everyone should stay indoors with windows shut.

Does Brzzy tell me about air quality?

Brzzy shows your local forecast with hourly updates, live radar, severe weather alerts, GIF forecasts, and activity ratings that weigh the sky for your plans — including Air Quality Alerts when pollutants reach unhealthy levels. Brzzy is free to download on iPhone.

More from Brzzy: Full weather glossary, Haze, Sandstorm.

Download Brzzy on the App Store