The OK Lab Stuttgart has a project to build fine particulate matter sensors by yourself. The data can then be sent to the database of https://luftdaten.info. This app shows you data from this network. It is not necessary to operate an own sensor to use this app.
Currently supported sensor types are: SDS011, SDS021, HPM, DHT11, DHT22, HTU21D, BME280 (temperature & humidity only)
Besides the main application, this app also provides three widgets:

A small widget that takes minimum space and displa