According to the Environmental Protection Agency, the AQI is calculated using data from 24 hours. That's because the EPA's science about air pollution exposure and health is based on 24 hours; therefore, the EPA's air quality standards are based on 24 hours. It is not valid to use shorter-term (e.g., hourly) data to calculate an AQI value. The NowCast is the EPA's method for relating hourly data to the AQI.

The NowCast is designed to be responsive to rapidly changing air quality conditions, such as during a wildfire. The calculation uses longer averages during periods of stable air quality and shorter averages when air quality is changing rapidly. The NowCast allows AirNow's current conditions to align more closely with what people are actually seeing or experiencing. This gives people information they can use to protect their health when air quality is poor and helps them get outdoors and exercise when air quality is good.