Air Quality Index (AQI) Health Calculator

The AQI is a standardized index developed by the US EPA to communicate how clean or polluted outdoor air is and what associated health effects might be of concern. It runs from 0 to 500 — the higher the number, the greater the level of air pollution and health risk. Six color-coded categories range from 'Good' (0–50) to 'Hazardous' (301–500).

The US EPA AQI covers five major pollutants regulated by the Clean Air Act: ground-level ozone (O₃), particle pollution (PM2.5 and PM10), carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). Each pollutant has its own concentration-to-AQI breakpoints, and the overall AQI for an area is the highest single-pollutant AQI at that time.

The six AQI categories are: Good (0–50, minimal risk), Moderate (51–100, acceptable but some sensitive people may be affected), Unhealthy for Sensitive Groups (101–150, sensitive individuals should limit prolonged outdoor exertion), Unhealthy (151–200, everyone may begin to experience health effects), Very Unhealthy (201–300, health alert — everyone may experience serious effects), and Hazardous (301–500, emergency conditions affecting the entire population).

Sensitive groups vary by pollutant but generally include people with lung diseases such as asthma or COPD, people with heart or cardiovascular disease, children and teenagers, older adults, and people who are active outdoors. On high-AQI days, these groups are advised to reduce or avoid prolonged outdoor physical activity before the general public.

The EPA uses a piecewise linear formula: AQI = ((AQI_high - AQI_low) / (C_high - C_low)) × (C - C_low) + AQI_low, where C is the measured concentration and the high/low values come from the EPA breakpoint tables for each pollutant. This calculator applies those exact EPA breakpoints for PM2.5, PM10, O₃, CO, SO₂, and NO₂.

An Air Quality Action Day is declared by local air agencies when the AQI is forecast to exceed 100 — the 'Unhealthy for Sensitive Groups' threshold. On these days, sensitive individuals are urged to limit outdoor activity, schools may cancel outdoor sports, and the public is encouraged to reduce activities that contribute to pollution such as driving or using gas-powered equipment.

Yes, PM2.5 (fine particles smaller than 2.5 micrometers) is generally considered more hazardous than PM10 because the tiny particles can penetrate deep into the lungs and even enter the bloodstream, causing respiratory and cardiovascular problems. PM10 particles are larger and mostly filtered in the upper airways, though both are regulated and tracked in the AQI.

This calculator converts a known concentration value into an AQI score using official EPA breakpoints — it is accurate for that conversion. For real-time, location-based air quality data, check airnow.gov or a local air quality monitoring agency, which pull from live sensor networks and official monitoring stations.