How to Check Air Quality in Your City (AQI Explained)

Before a run, a school trip, or opening the windows, one number tells you whether the air is fine or worth avoiding: the Air Quality Index, or AQI. Checking it takes less than a minute once you know where to look. This guide shows you how to check air quality for any city, what the numbers and colours mean, and what to actually do on a bad-air day.

What the AQI number means

The AQI turns several pollutant measurements into one number from 0 to 500. Low is clean, high is polluted. The scale is colour-coded so you can read it at a glance: green (0–50) is good, yellow (51–100) is moderate, orange (101–150) is unhealthy for sensitive groups, red (151–200) is unhealthy for everyone, purple (201–300) is very unhealthy, and maroon (301–500) is hazardous. Two cities can both read 120 and still feel different, because the AQI reports the worst pollutant, not an average of all of them.

Check your city in three steps

Open the checker below and type your city. It shows the live AQI and the main pollutants for that place.
Read the colour first, the number second. The colour tells you the health category; the number tells you how far into it you are.
Check the main pollutant. PM2.5 (tiny particles) is the most common driver of bad readings, especially during wildfire smoke. Ozone is more common on hot, sunny afternoons.

Other free ways to check

Your phone’s weather app usually shows AQI for your current location — on iPhone it sits in the Weather app, on Android in Google’s weather card. Government sites such as AirNow in the United States publish station readings, and map apps offer an air-quality layer. The advantage of a dedicated checker is that it works for any city you type, not just where you are standing right now, which helps when you are planning travel.

What to do at each level

Green and yellow (0–100): Carry on as normal. Sensitive people may notice symptoms at the top of yellow on long outings.
Orange (101–150): Sensitive groups — children, older adults, and people with asthma or heart conditions — should shorten outdoor exertion. Everyone else can still go out but should take it easier.
Red (151–200): Keep outdoor exercise short or move it indoors. Close windows if smoke is the cause.
Purple and maroon (201+): Stay indoors if you can. If you must go out, a well-fitting mask helps against particles but does little against gases like ozone.

Common mistakes when reading AQI

Do not confuse AQI with pollen counts — they measure different things, and a low-pollen day can still have bad air. Do not trust a single neighbourhood sensor blindly: a sensor next to a busy road reads worse than one in a park a kilometre away. And remember that AQI is usually an average over recent hours, so a sudden smoke plume may take a while to show up. If you can smell smoke, act on that even if the number has not caught up yet.

Indoor air on bad days

On bad-air days, keep windows closed, run a purifier if you have one, and avoid adding to indoor pollution: skip candles, frying without ventilation, and vacuuming without a good filter. A simple box-fan filter can help in a pinch. When the outdoor AQI drops back to yellow, air the house out again — stale indoor air brings its own problems, from cooking fumes to dust.

Make it a habit

Check the AQI the way you check the weather: once in the morning if you exercise outdoors, and again before a long outing with children. Over a few weeks you will learn your city’s pattern — which seasons bring smoke, which afternoons bring ozone — and the number will stop being abstract. It becomes one more small decision that protects your lungs.