What Your Air Quality Index (AQI) Really Means
On some days the air looks hazy and you are told to "stay indoors" - and the reason is usually a high Air Quality Index (AQI). But what does an AQI of 60, or 180, actually mean for you? Here is a clear guide to reading it and acting on it.
What the AQI measures
The AQI condenses several air pollutants into a single, easy number from 0 upward. The main pollutants it tracks are:
- PM2.5 and PM10 - tiny particles that lodge deep in the lungs (the biggest concern in most cities).
- Ozone (O₃) - ground-level ozone, worse on hot sunny days.
- Nitrogen dioxide, sulphur dioxide, and carbon monoxide - largely from traffic and industry.
The index reports the pollutant that is currently worst, so a single high pollutant can drive the whole number up.
The AQI categories
- 0-50 (Good, green) - air is clean; enjoy the outdoors.
- 51-100 (Moderate, yellow) - acceptable, but unusually sensitive people should watch out.
- 101-150 (Unhealthy for sensitive groups, orange) - children, older adults, and people with asthma or heart conditions should limit prolonged exertion.
- 151-200 (Unhealthy, red) - everyone may feel effects; sensitive groups more seriously.
- 201-300 (Very unhealthy) and 300+ (Hazardous) - health warnings; avoid outdoor activity.
What to do on a high-AQI day
- Limit strenuous outdoor exercise - a run in unhealthy air does more harm than good.
- Keep windows closed and use an air purifier if you have one.
- Wear a well-fitting N95/FFP2 mask outdoors if levels are unhealthy or worse.
- Take extra care with children, the elderly, and anyone with asthma or heart or lung conditions.
Check before you head out
Air quality can change through the day and from one area to the next. Our free Air Quality tool shows the current AQI and category for any city, so you can plan outdoor activities - or decide to stay in - with the real numbers in front of you.
This article is general information and not medical advice.
Frequently Asked Questions
What is a safe AQI level?
An AQI of 0-50 is good and 51-100 is moderate. Above 100, sensitive groups should take care, and above 150 everyone may be affected.
Which pollutant usually drives a high AQI?
In most cities it is fine particulate matter (PM2.5), though ozone can dominate on hot, sunny days.
Should I exercise outdoors when the AQI is high?
No. Strenuous outdoor exercise makes you breathe in more polluted air, so it is best to move workouts indoors on unhealthy days.