Measuring Invisible Danger
Air quality is invisible but lethal — outdoor air pollution causes an estimated 4.2 million premature deaths annually worldwide. The Air Quality Index (AQI) translates complex pollutant concentrations into a single number that communicates health risk. This simulator lets you set concentrations of key pollutants and see how the AQI responds, with real-time visualization of dispersal patterns under different wind conditions.
The AQI Calculation
Each criteria pollutant (PM2.5, ozone, NO₂, SO₂, CO) is converted to an individual AQI using EPA breakpoint tables — piecewise linear functions that map concentration ranges to AQI bands (0–50 Good, 51–100 Moderate, 101–150 Sensitive, 151–200 Unhealthy, 201–300 Very Unhealthy, 301–500 Hazardous). The overall AQI is simply the maximum of all individual values, meaning a single elevated pollutant drives the entire index.
Pollutant Dispersal
Wind speed and atmospheric stability determine how quickly pollutants disperse from their sources. The Gaussian plume model — a foundational tool in air quality engineering — predicts concentration as a function of distance, wind speed, and atmospheric turbulence. Strong winds and unstable (convective) conditions favor rapid dispersal. Temperature inversions, common on clear nights and in valleys, trap pollutants near the ground, potentially for days.
Health Impacts and Thresholds
Fine particulate matter (PM2.5) is the deadliest common pollutant, causing cardiovascular and respiratory disease even at low concentrations. The WHO's 2021 guidelines recommend annual PM2.5 below 5 µg/m³ — a level exceeded by 99% of the world's population. Ground-level ozone inflames airways at concentrations above 70 ppb. Even 'moderate' AQI levels (51–100) pose risks for sensitive groups including children, the elderly, and people with asthma or heart conditions.