For ozone, the standard is an eight-hour average, and for particle pollution it is a 24-hour standard. Why don’t we provide a one-hour, stand-alone value? Because there is not a one hour health-based standard. More recent hourly air pollution readings (e.g., last three hours) are given greater weight in determining the NowCast index value when air quality is changing more rapidly, as may be the case when wildfire smoke affects air quality. The NowCast calculation is designed to be responsive to rapidly changing air quality conditions. To address this, EPA developed the NowCast to report air quality that is near real-time (referred to as “Current Air Quality” reported at the top of this webpage.) A rolling 24-hour average report also isn’t very useful when conditions rapidly change, such as during a dust storm or wildfire. This means the official daily AQI is not known until the following day. ![]() For particulate matter, it is a 24-hour average, midnight-to-midnight. ![]() Each pollutant has its own health-based standard.
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