Thanks to italian site centralinedalbasso.org I got to know about this project I love : https://sensor.community/en/. Around 13000 sensor in 70 countries are currently measuring pm25, pm10, temperature, humidity, noise (beta) and making this data available to everyone. But more, you can build your own sensor and contribute to the network. I’ll be building one and will post the results.
Relations between covid-19 transmission and pm25 levels are being studied. If you’re interested: