1. Calculation: AirVisual and Airnow calculate PM 2.5 AQI in different methods. AirVisual calculates 1-hour average PM 2.5 concentration and then converts to AQI, because this is the most representative of the current air quality, and it’s a helpful guide to people’s breathing behaviors. AirNow calculates 24-hour average (see details about AirNow calculation here: doucument).
2. Measured Pollutants: The Air Quality Index (AQI) is calculated by individually weighting 6 key pollutants in a formula - so that each pollutant has both a concentration value and AQI value. Whichever individual pollutant is at the ‘riskiest’ concentration (highest AQI number), will dictate overall AQI. Since the main pollutant is most often PM2.5 (particularly from wildfires or local industry), stations that do not report this pollutant often understate true AQI. For example, some stations that only report Ozone may consistently show a 'green' AQI, when stations that measure PM2.5 and other pollutants will register as 'red'.
(You can check monitored pollutants on AirVisual app by scrolling down the station page)
3. Data source: Sometimes readings from within a city may come from different sources. For instance, AirVisual displays both official government data and crowdsourced data from AirVisual Pro air monitors. Meanwhile, we additionally offer a "City aggregate" option - which is the average of all stations within a city. It may be worth checking that you are comparing identical stations across the two platforms.
(Data source is shown on AirVisual app)
4. Update frequency: It is possible that different sources update at different times. This can cause different locations to show different data - some current, some outdated. Update time is listed under the air quality reading on the AirVisual app - and can also be found on the US EPA (AirNow) website.
5. Air pollution index: AirVisual supports China and US AQI standards, and this can be switched in the settings of the app or AirVisual Pro. AirNow website uses US AQI. So please ensure you are comparing the data with the same AQI standard.