Understanding Air Quality Index (AQI): Complete Guide

The Air Quality Index (AQI) is the EPA's standardized system for communicating daily air quality to the public. Understanding AQI is essential for protecting your health, especially if you have respiratory conditions, are elderly, or have young children. ## What Is the AQI? The AQI translates complex air monitoring data into a simple 0-to-500 scale. The higher the AQI value, the greater the level of air pollution and the greater the health concern. An AQI value of 100 generally corresponds to the national air quality standard for the pollutant, which is the level EPA has set to protect public health. The AQI is calculated for five major pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each pollutant has its own AQI calculation based on the concentration measured at monitoring stations. ## The Six AQI Categories **Good (0-50):** Air quality is satisfactory. No health risks. Everyone can enjoy normal outdoor activities without restriction. This is the target level for all US communities. **Moderate (51-100):** Air quality is acceptable for most people. Unusually sensitive individuals may experience mild respiratory symptoms. Most outdoor activities can proceed normally, but severely asthmatic individuals should monitor their condition. **Unhealthy for Sensitive Groups (101-150):** Members of sensitive groups — people with heart or lung disease, older adults, children, and pregnant women — may experience health effects. The general public is less likely to be affected. Sensitive individuals should reduce prolonged or heavy outdoor exertion. **Unhealthy (151-200):** Everyone may begin to experience health effects. Members of sensitive groups may experience more serious effects. All individuals should reduce prolonged outdoor exertion, and sensitive groups should avoid it entirely. **Very Unhealthy (201-300):** Health alert — significant risk for the entire population. Everyone should avoid prolonged outdoor exertion. Sensitive groups should remain indoors. Consider wearing N95 masks if outdoor exposure is unavoidable. **Hazardous (301-500):** Health emergency. The entire population is affected. All outdoor physical activity should be avoided. Stay indoors with air purification running. Follow local health authority instructions. ## How AQI Is Calculated The AQI is not a single measurement but a composite index. EPA monitoring stations measure pollutant concentrations continuously. The AQI for each pollutant is calculated by mapping the measured concentration to the AQI scale using EPA-defined breakpoints. For example, a PM2.5 24-hour average of 12.0 micrograms per cubic meter (the annual standard) maps to an AQI of approximately 50. A reading of 35.4 micrograms corresponds to AQI 100 (the 24-hour standard). The overall AQI reported for a location is the highest AQI value among all measured pollutants. If ozone AQI is 85 and PM2.5 AQI is 62, the reported AQI is 85, and ozone is identified as the primary pollutant. ## Where to Find AQI Data **AirNow.gov** is the EPA's official portal for real-time AQI data. It provides current conditions, forecasts, and fire and smoke maps for the entire United States. Many weather apps and websites also display AQI data sourced from EPA monitors. **PurpleAir** offers a network of low-cost air quality sensors that provide hyperlocal data. While less rigorously calibrated than EPA monitors, these sensors can reveal neighborhood-level air quality variations that official monitors may miss. **State and local agencies** often operate additional monitoring networks. Many cities publish real-time air quality dashboards with more granular data than the national AirNow system. ## Protecting Yourself Based on AQI When AQI rises above 100, take these steps: close windows and doors, run HEPA air purifiers on the highest setting, avoid activities that generate indoor pollutants (cooking with gas, burning candles), recirculate air conditioning rather than drawing in outside air, and reduce physical exertion. For outdoor workers, OSHA provides guidance on protecting employees during poor air quality events. Employers should monitor AQI forecasts and adjust work schedules when conditions deteriorate. Long-term exposure to elevated AQI levels is associated with increased rates of heart disease, stroke, lung cancer, asthma, and chronic obstructive pulmonary disease (COPD). Choosing where to live based on historical air quality data can have significant long-term health implications. ## AQI Limitations The AQI has limitations you should understand. It represents conditions at the monitoring station, which may not reflect your exact location. It does not account for indoor air quality. The reported AQI may be a forecast rather than a current measurement. And it does not capture all air pollutants — only those regulated under the Clean Air Act. Despite these limitations, the AQI remains the most accessible and useful tool for making daily decisions about outdoor activity and exposure management. Checking the AQI should be as routine as checking the weather forecast.