The Air Quality Index (AQI) is an index for reporting daily air quality—it tells you how clean or polluted your air is, and what associated health effects might be a concern for you. The AQI is calculated for four major air pollutants regulated by the Clean Air Act: ground level ozone, particle pollution, carbon monoxide, and sulfur dioxide. For each of these pollutants, EPA has established national air quality standards to protect public health. Of these, ground-level ozone and airborne particles are the two pollutants that pose the greatest threat to human health.

The AQI translates air quality data into numbers and colors that help people understand when to take action to protect their health, focusing on health effects that may be experienced within a few hours or days after breathing polluted air. 

AirNow reports current and next-day forecasts for AQI conditions for over 400 cities, as well as nationwide and regional real-time ozone and particle pollution air quality maps, updated hourly, covering 46 states and part of Canada.

Data Collection

Map and forecast data are collected using federal reference or equivalent monitoring techniques or techniques approved by the state, local, or tribal monitoring agencies. To maintain "real-time" maps, the data are displayed after the end of each hour. Although preliminary data quality assessments are performed, the data in AirNow are not fully verified and validated through the quality assurance procedures monitoring organizations used to officially submit and certify data on the EPA Air Quality System (AQS).

This data sharing and centralization creates a one-stop source for real-time and forecast air quality data. The benefits include quality control, national reporting consistency, access to automated mapping methods, and data distribution to the public and other data systems. AirNow data are used only to report the AQI, not to formulate or support regulation, guidance or any other EPA decision or position.