The United States Environmental Protection Agency (EPA) has developed an Air Quality Index (AQI) that is used to report air quality. This AQI is divided into six categories indicating increasing levels of health concern: Good, Moderate, Unhealthy for Sensitive Groups (USG), Unhealthy, Very Unhealthy, and Hazardous.

The EPA has established National Ambient Air Quality Standards (NAAQS) in order to protect public health. An AQI value of 100 generally corresponds to the level of the NAAQS for the pollutant. The Clean Air Act (USA) (1990) requires the EPA to review its National Ambient Air Quality Standards every five years to reflect evolving health effects information. The Air Quality Index is adjusted periodically to reflect these changes.

Real-time monitoring data are typically available as 1-hour averages. However, computation of the AQI for some pollutants requires averaging over multiple hours of data. (For example, calculation of the PM 2.5 AQI requires a 24-hour average.) To accurately reflect the current air quality, the multi-hour average used for the AQI computation should be centered on the current time, but as concentrations of future hours are unknown and are difficult to estimate accurately, the EPA uses surrogate concentrations to estimate these multi-hour averages. This surrogate concentration is called the NowCast. Nowcast is a particular type of exponentially weighted moving average that provides more weight to the most recent air quality data when air pollution levels are changing. The Air Quality Index itself is a piecewise linear function of the input pollutant concentration.

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