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The Air Quality Index (AQI) is a unified scale for assessing air pollution levels and their impact on health. The index was developed by the U.S. Environmental Protection Agency (EPA) and provides a quick way to understand how dangerous the air is at any given moment.
AQI is calculated based on the concentration of major air pollutants: particulate matter PM2.5 and PM10, nitrogen dioxide (NO₂), ozone (O₃), sulfur dioxide (SO₂), and carbon monoxide (CO). The concentration of each pollutant is converted to an AQI value using a specific table of ranges (see below), where each concentration range corresponds to a specific AQI range. The final AQI value is determined by the worst indicator among all pollutants.
The AQI scale is divided into six categories — from 0 (clean air) to 500+ (hazardous pollution). Each category corresponds to a specific color and behavioral 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 AirThe air is clean, and the risk is minimal. No 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 PollutionAir quality is acceptable, but there may be a slight impact on individuals 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 GroupsPeople with chronic illnesses may experience health effects. Children, the elderly, and those with respiratory issues should avoid prolonged outdoor exertion. 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 AirNegative health effects for everyone, especially high-risk groups. Avoid physical activity outdoors if possible. Vulnerable groups are advised to stay indoors. Use air purifiers indoors 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 AirSerious health effects for all residents. Stay 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 PollutionCritical air quality condition, posing a threat to everyone’s health. Avoid 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 |
