AirNow averages the previous 3 to 12 hours of data and a weight factor to determine the "NowCast AQI."
IQAir provides real-time air quality data by publishing an average reading from the previous hour.
Because of this weight factor and difference in time frame used, AirNow and IQAir air quality readings are different.
IQAir values real time data, so in case of fires or other pollution releases that impact the air quality in your area, you know within the hour, rather than knowing hours laters or the next day.

You can learn more about the data provided through the link below:

What is the air quality index (AQI)? | IQAir

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