# Which Big Cities Cleaned Up Their Air? (Data Settles It, Episode 4)

Decade trends in annual PM2.5 for 441 cities of 300,000+ people, 2013 to 2024, from the WHO Ambient Air Quality Database (v8.0, June 2026). 304 cities in 37 countries are ranked; US and UK cities are listed but not ranked (instrument changes in their national networks).

Files:
- `ranking_441_cities.csv`: one row per city (see `data_dictionary.csv`)
- `data_dictionary.csv`: column definitions
- `METHOD_AND_SOURCES.md`: rules, caveats, and a source for every statement in the video

Licence: CC BY-NC-SA 4.0. Data Settles It Lab, November 2026.