The Air Quality Index: How to Interpret the Number That Protects Your Health

The Air Quality Index (AQI) is the universal language of outdoor air quality in the United States - a single number that translates complex atmospheric chemistry data into actionable health guidance. Understanding how to read and respond to the AQI can meaningfully reduce your risk of air pollution-related health effects.

What Is the AQI?

The Air Quality Index is a standardized, nationally uniform reporting system developed by the EPA to communicate daily air quality conditions to the public. The AQI converts measured concentrations of five major air pollutants - ground-level ozone, PM2.5, PM10, carbon monoxide (CO), and sulfur dioxide (SO2) - into a single numerical scale ranging from 0 to 500, with associated color categories and health messages. When multiple pollutants are measured, the AQI for a given location is determined by the pollutant with the highest index value on that day.

Understanding the AQI Scale

The six AQI categories, their color codes, and associated health implications are:

• 0-50 (Green - Good): Air quality is satisfactory. No health concern for any population.
• 51-100 (Yellow - Moderate): Air quality is acceptable. Some pollutants may pose a moderate health concern for a small number of unusually sensitive individuals.
• 101-150 (Orange - Unhealthy for Sensitive Groups): Members of sensitive groups - children, elderly, people with respiratory or cardiovascular conditions - may experience health effects. General public is not likely to be affected.
• 151-200 (Red - Unhealthy): Everyone may begin to experience health effects. Sensitive groups may experience more serious effects.
• 201-300 (Purple - Very Unhealthy): Health alert. Everyone may experience more serious health effects.
• 301-500 (Maroon - Hazardous): Health warning of emergency conditions. Everyone is more likely to be affected.

How AQI Data Is Collected

AQI calculations are based on real-time measurements from EPA-certified air quality monitoring stations operated by state and local agencies throughout the United States. These stations use sophisticated instruments including beta attenuation monitors and tapered element oscillating microbalances for PM2.5, ultraviolet photometers for ozone, and other reference-method analyzers for CO and SO2. Data is transmitted in near-real-time to EPA's AirNow platform and made publicly accessible through airnow.gov, mobile apps, and data feeds.

In recent years, lower-cost air quality sensors have proliferated (PurpleAir networks, for example), expanding coverage and enabling hyperlocal monitoring. While these sensors are not regulatory instruments and may have calibration limitations, they provide valuable supplementary information - particularly during wildfire smoke events where spatial variability in PM2.5 is high.

AQI and Climate Change

Climate change is making it more challenging to maintain healthy air quality. Higher temperatures accelerate ozone formation, more frequent and intense wildfires contribute PM2.5 over vast regions, and shifting wind patterns can increase transport of pollution from distant sources. Research projects that by mid-century, without continued emissions reductions, AQI exceedances will increase in frequency across much of the United States.

Practical Steps for Different AQI Levels

• Good to Moderate (0-100): Normal outdoor activities. If you have asthma or severe allergies, check pollutant-specific information.
• Unhealthy for Sensitive Groups (101-150): Sensitive individuals should limit prolonged outdoor exertion. Consider moving heavy outdoor activity to morning hours.
• Unhealthy (151-200): Everyone should reduce prolonged or heavy outdoor exertion. Sensitive individuals should avoid outdoor activity.
• Very Unhealthy to Hazardous (201+): Everyone should avoid outdoor exertion. Remain indoors with windows closed. Run air purifiers. This level is most commonly reached during wildfire smoke events.