Air Quality Explained
A practical guide to AQI, PM2.5, PM10, ozone and other pollutants — and why the same concentration can be interpreted differently under different air-quality systems.
What is AQI?
The Air Quality Index is a communication system that converts pollutant concentration information into an easier-to-understand index and category. Different authorities use different pollutants, averaging periods, breakpoints and health messages, so AQI values should always be read with their stated methodology.
What is PM2.5?
PM2.5 refers to fine particulate matter with an aerodynamic diameter of 2.5 micrometres or smaller. It is widely monitored because fine particles can penetrate deeply into the respiratory system and are an important air-pollution health indicator.
What is PM10?
PM10 is particulate matter with an aerodynamic diameter of 10 micrometres or smaller. It includes coarse particles as well as the fine-particle fraction represented by PM2.5.
What are ozone and nitrogen dioxide?
Ground-level ozone is a secondary pollutant formed through atmospheric reactions involving precursor pollutants. Nitrogen dioxide is commonly associated with combustion sources, including traffic and other fossil-fuel combustion. Both are included in major air-quality monitoring and index systems.
Why can AQI values differ between websites?
Differences can arise because services use different source networks, averaging periods, AQI methodologies, pollutant conversions, update times or atmospheric models. A model estimate is not the same thing as an observation from a monitoring station.
Monitoring data vs model data
Monitoring stations measure pollutant concentrations at specific locations. Atmospheric models estimate pollutant concentrations across a wider area using meteorology, emissions and atmospheric-chemistry information. AirQualityIndex.org labels model-based information so visitors can distinguish it from station observations.
What does an AQI category mean?
These labels are method-specific. India's CPCB AQI uses categories including Good, Satisfactory, Moderately Polluted, Poor, Very Poor and Severe. The European Air Quality Index uses a different five-band approach based on pollutant concentrations.