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For indoor monitoring, the coverage of an AirVisual Pro is a room (or open space) of up to 100sqm.
For outdoor monitoring, the coverage of an AirVisual sensor is usually for a 5km radius. The network of sensors become more accurate as the density of the sensors increases.

Source: 
https://www.iqair.com/newsroom/what-area-coverage-are-the-airvisual-pro-s-measurements-valid-for