OpenAQ has collected 231,965,688 air quality measurements from 8,469 locations in 65 countries. Data are aggregated from 105 government level and research-grade sources.

https://medium.com/@openaq/where-does-openaq-data-come-from-a5cf9f3a5c85

Disclaimers:

• Some records contain encoding issues on specific characters; those issues are present in the raw API data and were not corrected.
• Some dates are set in the future: those issues also come from the original data and were not corrected.