How AQI is calculated
The US EPA AQI converts each pollutant concentration to a 0–500 index using linear breakpoints. The final AQI is the highest sub-index across PM2.5, PM10, O₃, CO, SO₂, and NO₂.
AQI 0–50 is 'Good', 51–100 'Moderate', 101–150 'Unhealthy for sensitive groups', 151–200 'Unhealthy', 201–300 'Very Unhealthy', 301+ 'Hazardous'.
The PM2.5 problem
Fine particulates under 2.5 μm penetrate deep into lungs and enter the bloodstream. WHO's 2021 guideline lowered the annual PM2.5 limit to 5 μg/m³ — most cities exceed it.
What to do on bad-air days
AQI 101+: sensitive groups should reduce outdoor exertion. AQI 151+: everyone should. Run a HEPA purifier indoors (2 air changes per hour is typical). N95/KN95 masks reduce PM2.5 exposure by 90%+.
Frequently asked questions
- What AQI is dangerous?
- AQI above 150 is unhealthy for everyone. Above 300 is hazardous — avoid all outdoor activity.
- Is AQI the same everywhere?
- No. US EPA, EU CAQI, China AQI, and India NAQI use different breakpoints. A US AQI of 100 is not the same as India's NAQI 100.
AQI PM2.5 air pollution health