What is AQI?

An Air Quality Index turns pollutant concentrations into a simpler category or index for public communication. There is no single worldwide AQI formula: authorities choose pollutants, averaging periods, breakpoints, categories and health messages for their own systems. That is why the methodology should always accompany the number.

PM2.5: fine particle pollution

PM2.5 means particles with an aerodynamic diameter of 2.5 micrometres or smaller. Important contributors can include fuel and biomass combustion, traffic and industrial emissions, wildfire or open-burning smoke, and particles formed in the atmosphere from gaseous precursors. High PM2.5 alone cannot prove which source caused a particular episode.

PM10: coarse and fine particles

PM10 includes particles up to 10 micrometres, including the PM2.5 fraction. Elevated PM10 can be associated with road and construction dust, wind-blown soil or desert dust, quarrying and other mechanical processes as well as combustion. A high PM10-to-PM2.5 pattern can suggest a stronger coarse-particle contribution, but proper source attribution needs meteorology, composition and local emissions information.

Ozone, NO₂, SO₂ and CO

Ground-level ozone is mainly a secondary pollutant formed by atmospheric chemistry involving precursor emissions and sunlight. Nitrogen dioxide is strongly associated with combustion, including road traffic and other fuel burning. Sulfur dioxide is linked particularly to sulfur-containing fuel combustion and some industrial processes. Carbon monoxide is produced by incomplete combustion. Their meaning, averaging periods and AQI treatment differ between systems.

Why can pollution rise suddenly?

Emissions are only part of the story. Wind direction, atmospheric mixing, temperature inversions, humidity, wildfire smoke transport and dust events can change concentrations quickly. A monitor reports conditions at a particular place and time; it does not by itself identify the responsible source.

Monitoring data versus model data

Monitoring stations measure concentrations at specific locations. Atmospheric models estimate conditions over larger areas using emissions, meteorology and atmospheric chemistry. Both are useful, but they are not interchangeable. AirQualityIndex.org aims to identify the source, observation time and whether information is observed, calculated or modelled.

Why can two websites show different AQI values?

They may use different monitoring stations, update times, averaging periods, conversion methods or national AQI standards. One service may show a provider-supplied AQI while another calculates an index from pollutant concentrations. Before deciding that one value is wrong, compare the source, timestamp, pollutant, units and AQI method.

What should I do with an AQI reading?

First identify the AQI standard and dominant pollutant, then follow the health advice attached to that standard and your local public-health authority. AQI is a communication tool, not a diagnosis. People with heart or lung disease, children, older adults and other susceptible groups may receive more precautionary advice at elevated pollution levels.