The Air Quality Index (AQI) is a number that shows how polluted the air is right now and how dangerous that is for your health. The higher the AQI value, the dirtier the air and the greater the health risk: an AQI of 30 means clean air, while an AQI of 180 means the air is unhealthy for everyone.
AQI is an index, not a concentration. Concentration is measured in micrograms per cubic metre (µg/m³), while AQI is a dimensionless value on a 0 to 500 scale that this concentration is converted into. So 35 µg/m³ of PM2.5 and an AQI of 100 describe the same air, expressed in two different ways.
There is no single global AQI scale: different countries use their own standards with different thresholds. This site uses US AQI, the scale of the U.S. Environmental Protection Agency (EPA), which is the most widely used one worldwide.
We calculate AQI from the three pollutants measured by the sensors in our network: particulate matter PM2.5 and PM10 and nitrogen dioxide (NO₂). The concentration of each is converted into its own AQI value using a table of ranges (see below), and the final AQI of a station equals the highest of them — in other words, it is set by whichever pollutant is currently the most harmful.
The AQI scale is divided into six categories — from 0 (clean air) to 300+ (hazardous pollution). Each category has its own colour and behavioural recommendations for different population groups.
| AQI | pm2.5, µg/m³ | pm10, µg/m³ | NO₂, ppb | Recommendations |
|---|---|---|---|---|
| 0–50 | 0.0-9.0 | 0-54 | 0-53 |
Clean AirNo restrictions, enjoy the fresh air! pm2.5: 0.0-9.0 µg/m³, pm10: 0-54 µg/m³, NO₂: 0-53 ppb |
| 51–100 | 9.1-35.4 | 55-154 | 54-100 |
Moderate PollutionThere may be a slight impact on people sensitive to pollution. pm2.5: 9.1-35.4 µg/m³, pm10: 55-154 µg/m³, NO₂: 54-100 ppb |
| 101–150 | 35.5-55.4 | 155-254 | 101-360 |
Unhealthy for Sensitive GroupsChildren, older adults, and people with chronic conditions should reduce outdoor activity. pm2.5: 35.5-55.4 µg/m³, pm10: 155-254 µg/m³, NO₂: 101-360 ppb |
| 151–200 | 55.5-125.4 | 255-354 | 361-649 |
Unhealthy AirAvoid physical activity outdoors. Vulnerable groups are advised to stay indoors. Use air purifiers if available. pm2.5: 55.5-125.4 µg/m³, pm10: 255-354 µg/m³, NO₂: 361-649 ppb |
| 201–300 | 125.5-225.4 | 355-424 | 650-1249 |
Very Unhealthy AirStay indoors and use air purifiers. Wear respirators with fine particle protection if going outside. pm2.5: 125.5-225.4 µg/m³, pm10: 355-424 µg/m³, NO₂: 650-1249 ppb |
| 301+ | 225.5+ | 425+ | 1250+ |
Hazardous PollutionAvoid going outside. Keep windows and doors tightly closed. Use respirators with fine particle protection and air purifiers indoors. pm2.5: 225.5+ µg/m³, pm10: 425+ µg/m³, NO₂: 1250+ ppb |
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