What Is AQI?
The Air Quality Index is a standardised scale that communicates how polluted the air is and what health effects might occur. It measures key pollutants including ground-level ozone, particulate matter (PM2.5 and PM10), carbon monoxide, sulphur dioxide, and nitrogen dioxide.
AQI Scale
| AQI Range | Level | Health Guidance |
|---|---|---|
| 0-50 | Good | Air quality is satisfactory; no risk |
| 51-100 | Moderate | Acceptable; sensitive groups may notice effects |
| 101-150 | Unhealthy for Sensitive Groups | Active children, asthmatics, elderly should reduce outdoor exertion |
| 151-200 | Unhealthy | Everyone should reduce prolonged outdoor exertion |
| 201-300 | Very Unhealthy | Everyone should avoid outdoor exertion |
| 301-500 | Hazardous | Health emergency; stay indoors |
PM2.5 — The Key Number
PM2.5 refers to particulate matter smaller than 2.5 micrometres. These tiny particles penetrate deep into your lungs and even enter your bloodstream. PM2.5 is the primary driver of AQI readings in most cities.
What Worsens Air Quality?
- Traffic: Vehicle exhaust is the top urban polluter
- Wildfires: Can send AQI above 300 hundreds of kilometres away
- Industrial emissions: Factories and power plants
- Weather: Temperature inversions trap pollution near the ground
- Agriculture: Crop burning and ammonia from fertilisers
Practical Tips
- Check AQI before outdoor exercise, especially if you have asthma
- Use N95 or KF94 masks when AQI exceeds 150
- Keep windows closed on high-AQI days
- Run an air purifier with a HEPA filter indoors
- Exercise indoors when AQI exceeds 100
Plan outdoor activities with Weather Tomorrow.
Get tomorrow's forecast in your inbox
Free daily weather email. Choose your city, pick your time.
Subscribe — FreeRelated Articles
Running in Different Weather — A Complete Guide
Rain, heat, cold, wind — each weather condition changes your run. Here is how to adapt your pace, gear, and strategy.
Planning Outdoor Events Around Weather — A Practical Guide
Concerts, festivals, markets, and sports events all depend on weather. Here is how to plan smart and handle the unexpected.
Outdoor Wedding Weather Planning — The Complete Guide
Your outdoor wedding depends on weather. Here is how to plan for it, monitor it, and prepare backup options.