The current datasets comprise of air quality data collected or created within the hackAIR project (https://platform.hackair.eu/) all around Europe from February 2018 until November 2018. i) "measurements_arduino.xlsx": PM10 and PM2.5 measurements collected by hackAIR users with stationary hackAIR sensors (https://www.hackair.eu/hackair-home-v2/). These sensing devices are based either on an Arduino or a Wemos board. The first column is the unique identifier for the measurement in the hackAIR database. The date/time is in UTC timezone, while the unit of the pollutant value is μg/m3. ii) "measurements_bleair.xlsx": PM10 and PM2.5 measurements collected by hackAIR users with mobile hackAIR sensors (https://www.hackair.eu/hackair-mobile/). The first column is the unique identifier for the measurement in the  database. The date/time is in UTC timezone, while the unit of the pollutant value is μg/m3. iii) "measurements_sky_photos.xlsx": Air pollution estimations from photos depicting sky. The hackAIR platform estimates the particulate matter content in the air from Flickr photos, photos from webcams and sky photos that users upload on the hackAIR mobile application, based on the colour of the sky. This is expressed as Aerosol Optical Depth (AOD). In the current dataset, the timezone is UTC, while AOD is unitless. The pollutant index is based on a scale created for the purposes of the hackAIR project.