All data from the AirVisual app is sourced from ground stations - these are either government monitoring stations or community driven AirVisual Pro air monitors.
Still, there are quite a number of places all over the world that lack air quality monitoring data granularity, so people have little idea exactly how much pollution they are breathing in.
In response, many people around the world have decided to take matters into their own hands and launch community driven air quality monitors in places that lack this information. You can learn more about this process here: https://www.iqair.com/us/air-quality-community
Meanwhile, we look forward to continue rapidly growing the global network with the help of engaged community members, organizations and initiatives in the coming future. If you happen to know of an open data source that shows PM2.5 data for locations we missed, please share a link with us, we would be very interested to add it to our map.