The Air Quality Index (AQI) is a valuable tool for understanding air pollution levels in a straightforward way. Typically, it’s calculated based on six key pollutants: particulate matter (PM2.5 and PM10), ozone (O₃), carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). However, not all of these pollutants are measured at every monitoring station. So, how is the AQI calculated when some of these pollutants are missing?
Each pollutant has a set of "breakpoints" — concentration ranges that correspond to different AQI values. These breakpoints allow the raw pollutant concentrations to be converted into AQI values that fall into one of six categories, from "Good" to "Hazardous." The image shown below provides breakpoints specifically for PM2.5, but similar tables exist for the other pollutants.
Once the AQI for each pollutant is calculated, the highest AQI value — called the "dominant" AQI — is reported. This means that even if multiple pollutants are measured at a location, the AQI shown to the public is based on the worst pollutant at that time, ensuring that the reported value reflects the most pressing health concern.
Note that different countries prescribe varying averaging periods for different pollutants when calculating AQI. For example, some AQI systems may use 24-hour averages for particulate matter, while others rely on shorter periods for certain pollutants like ozone.
To provide more timely and actionable air quality data, IQAir has developed the AQI⁺. The U.S. AQI⁺ and CN AQI⁺ are based on official guidelines but enhance the traditional AQI approach by using hourly concentration averages for all pollutants, rather than the longer averaging periods typically used in many national systems. This allows for more up-to-date and responsive reporting of air quality conditions, helping users make more informed decisions about their health and outdoor activities.
By focusing on the highest AQI value and applying hourly updates, IQAir’s AQI⁺ ensures that the public receives the most relevant and immediate information about air quality, making it a valuable tool for daily use.