Air Quality Index Calculator

The Air Quality Index (AQI) is a standardized measurement used by governments worldwide to communicate how polluted the air is and what associated health effects might be of concern. The AQI scale runs from 0 to 500, with higher values indicating worse air quality and greater health risk.

The AQI is divided into six categories. Good (0 to 50) means air quality is satisfactory and poses little or no risk. Moderate (51 to 100) is acceptable, though some pollutants may be a concern for a very small number of people who are unusually sensitive. Unhealthy for Sensitive Groups (101 to 150) means members of sensitive groups (people with respiratory or heart conditions, children, and older adults) may experience health effects. Unhealthy (151 to 200) means everyone may begin to experience health effects, and sensitive groups may experience more serious effects. Very Unhealthy (201 to 300) triggers health alerts, meaning everyone may experience more serious health effects. Hazardous (301 to 500) is an emergency condition where the entire population is likely to be affected.

PM2.5 refers to fine particulate matter with a diameter of 2.5 micrometers or smaller. These particles are small enough to penetrate deep into the lungs and even enter the bloodstream. They come from vehicle exhaust, industrial emissions, wildfires, and cooking. PM2.5 is generally considered the most dangerous common air pollutant due to its ability to cause cardiovascular and respiratory problems.

PM10 refers to particles with a diameter of 10 micrometers or smaller. These include dust, pollen, and mold spores. While larger than PM2.5, they can still cause respiratory irritation and exacerbate asthma and other conditions.

Ground-level ozone (O3) is formed when pollutants from cars, power plants, and industrial facilities react with sunlight. It is the main ingredient of smog. High ozone levels are most common on hot, sunny days in urban areas.

The AQI calculation uses breakpoint tables established by the United States Environmental Protection Agency (EPA). For each pollutant, the measured concentration is compared against a table of concentration breakpoints. The AQI is calculated using linear interpolation between the breakpoints. When multiple pollutants are measured, the highest individual AQI value becomes the overall AQI, since the most dangerous pollutant determines the health risk.

The formula for calculating AQI from a concentration value is: AQI = ((AQI_high - AQI_low) / (C_high - C_low)) * (C - C_low) + AQI_low, where C is the measured concentration, C_low and C_high are the breakpoint concentrations below and above C, and AQI_low and AQI_high are the corresponding AQI values for those breakpoints.

Monitoring air quality is especially important for people who exercise outdoors, have respiratory conditions, or live in areas prone to wildfires or industrial pollution. Many cities now provide real-time AQI data through government websites and mobile applications.