What’s the difference between the Current AQI and the Daily (24-hour) AQI ?
First – some quick basics about the AQI
The U.S. Air Quality Index, or AQI, was developed by EPA as a tool for communicating daily air quality. Pollutant concentrations are converted to the AQI scale. The higher the AQI value, the greater the level of air pollution and the greater the health concern.
By definition, the AQI is calculated using data from a 24-hour period. That’s because the science we have about air pollution exposure and health is based on 24 hours and therefore EPA’s air quality standards are based on 24 hours.
It is not valid to use shorter-term (e.g. one hour) data to calculate an AQI value. However, we want to report shorter-term data to caution people in time for them to reduce their 24-hour exposure so EPA developed a NowCast calculation for the AQI to report near real-time conditions.
Current AQI (aka the NowCast)
Spokane Clean Air reports current conditions as the “Current Air Quality Index,” using the NowCast calculation. It is updated each hour with the most current data. The NowCast calculation uses longer averages during periods of stable air quality and shorter averages when air quality is changing rapidly such as during a wildfire.
Think of the “current air quality” as near real-time snapshot of the air quality you are currently experiencing. This provides timely information so you can make choices about your outdoor activities.
AQI (daily)
As previously stated, the AQI is a 24-hour average based on National Ambient Air Quality Standards. Each day’s AQI is reported the following day since it is the average from midnight-to-midnight. Think of the “daily air quality” as a report on how the air quality was the previous day.
AQI summary for Spokane County, 2016-2019, pdf.
For more details about the NowCast calculation, click here.
