Dataset from a large-scale air quality citizen science campaign is presented (xAire, 725 measurements). A broad partnership with 1,650 citizens from communities around 18 primary schools across Barcelona (Spain) provided the capacity to obtain unprecedented high-resolution NO2 levels which had in turn the capacity to provide an updated asthma Health Impact Assessment. Nitrogen dioxide levels being obtained in a 4-week period during February and March 2018 with Palmes’ diffusion tubes are herein provided. Dataset includes NO2 levels from outdoor locations n=671, playgrounds n=31, and inside school buildings (mostly classrooms) n=23. Data was calibrated and annualized with concentration levels from automatic reference stations. It is shown that NO2 levels vary considerably with at some cases very high levels. Strong differences might also however be explained by the fact that ambient air pollution is reduced exponentially with distance from an emission source like traffic meaning that two samplers located about 100m away can measure a tenfold difference concentration level. Columns: Schoolname (xAire, an additional community); Latitude; Longitude; Address; Type (traffic, background, playground, classroom); NO_2 raw data; NO_2 adjusted unbiased data - monitoring stations; NO_2 annualized taken with 2017 annual data).