The Air Quality Index (AQI) is a standardized measurement that tells you how clean or polluted the air is. Understanding AQI helps you build features that protect user health, from outdoor activity recommendations to air purifier automation.
AQI Scale and Categories
AQI ranges from 0 to 500, divided into six categories: Good (0-50) poses little or no risk. Moderate (51-100) is acceptable but may concern unusually sensitive people. Unhealthy for Sensitive Groups (101-150) affects people with respiratory conditions. Unhealthy (151-200) affects everyone. Very Unhealthy (201-300) triggers health alerts. Hazardous (301-500) is an emergency condition.
Major Pollutants
AQI is calculated from multiple pollutants, with the highest value determining the overall AQI. PM2.5 (fine particulate matter under 2.5 micrometers) penetrates deep into lungs and is often the primary concern. PM10 (particles under 10 micrometers) includes dust and pollen. Ozone (O3) forms from vehicle/industrial emissions reacting with sunlight—worst on hot, sunny days. NO2 and SO2 come from combustion. CO (carbon monoxide) is typically only concerning near heavy traffic.
Health Recommendations by AQI Level
Good (0-50): No restrictions. Moderate (51-100): Unusually sensitive people should consider reducing prolonged outdoor exertion. USG (101-150): Sensitive groups should reduce prolonged outdoor exertion. Unhealthy (151-200): Everyone should reduce prolonged outdoor exertion; sensitive groups should avoid it. Very Unhealthy (201-300): Everyone should avoid prolonged outdoor exertion. Hazardous (301+): Everyone should avoid all outdoor activity.
Regional AQI Variations
Different countries use different AQI scales. The US EPA scale (0-500) is common in APIs, but China uses a 0-500 scale with different breakpoints, and Europe uses the Common Air Quality Index (CAQI). When building international apps, check which standard your API uses and consider normalizing for your users.