# Which Big Cities Cleaned Up Their Air? Method and sources

Data Settles It, Episode 4. Data Settles It Lab, November 2026.

## Question
Which big cities' air got cleaner over the last decade, and which got worse, measured by fine particles (PM2.5)?

## Data
- **Air quality:** WHO Ambient Air Quality Database, version 8.0 (June 2026 update). Annual mean PM2.5 by city and year. https://www.who.int/publications/m/item/who-ambient-air-quality-database-(update-jun-2026)
- **Population (filter only):** the population recorded in the WHO database where present; otherwise the GeoNames `cities15000` place (CC BY 4.0) with a matching name within 20 km. The larger of the two is used.

## Rules
1. Years 2013 to 2024 only.
2. A city-year counts only if the WHO database reports PM2.5 temporal coverage of at least 75% of the year.
3. A city needs at least 6 qualifying years, spanning at least 7 years, with its first year between 2013 and 2016 and its last between 2021 and 2024.
4. Cities of 300,000 people or more.
5. WHO entries within 20 km of a larger WHO entry in the same country are merged into it (for example Istanbul districts, Mexico City boroughs). The merged city's trend is the median of its entries' trends.
6. Trend: least-squares slope of log(annual PM2.5) on year, converted to % per year and compounded over ten years ("change per decade"). Every qualifying year counts, not only the first and last.
7. For balance, no country has more than 4 cities in either the "most improved" or "got worse" list shown in the video.
8. WHO-listed district councils that are not cities (Waterberg, South Africa) are excluded.

Result: 441 cities pass the data rules; 304 cities in 37 countries are ranked. 293 improved, 11 got worse. Median change per decade: -42%. Four ranked cities have a latest three-year average at or below the WHO guideline of 5 µg/m³: Tallinn, Adelaide, Stockholm and Halifax.

## Why the US and UK are not ranked
- **US:** many PM2.5 sites switched to Teledyne T640/T640X continuous monitors around 2016 to 2020. EPA reports these read high compared with reference filter samplers (on average about 20%) and published a data correction in May 2024. EPA's "Update of PM2.5 Data from T640/T640X PM Mass Monitors", https://www.epa.gov/system/files/documents/2024-05/2_supplemental-info_t640-data-update_final-05-13-2024.pdf. Example: at Melbourne, Florida (site 12-009-0007) in 2019, the filter sampler read 5.3 µg/m³ and the T640X 7.1 (EPA AirData annual_conc_by_monitor_2019, parameter 88101).
- **UK:** Defra replaced all FDMS PM analysers in the national network by the end of 2021 (Defra, Ongoing Equivalence Report 2021), and Scottish Government guidance notes the instrument change can cause a step change in reported concentrations.
- US and UK cities remain in the dataset with their figures, flagged "excluded".

## Caveats
- The ranking is limited to places with long official records. Most of Africa, South Asia and the Middle East are missing.
- Some trends depend on unusual end years (Canadian wildfire smoke in 2021 and 2023; Saharan dust in the Canary Islands 2020 to 2022).
- Johannesburg: the City of Johannesburg's own later readings (about 14.5 µg/m³, 2024) are far lower than the WHO database's 2021 value (38), which suggests station or method differences. Vitória: the state agency IEMA's own series (12 to 5.1 µg/m³, 2015 to 2024) is lower than the WHO figures but shows the same steep fall.
- Al Ahmadi: no published study attributes the fall to a cause; Kuwait's PM2.5 is dominated by desert dust that varies year to year.

## Sources for statements in the video
- PM2.5 about 30 times thinner than a human hair: US EPA, "Particulate Matter (PM) Basics".
- WHO guideline 5 µg/m³ (2021): https://www.who.int/news/item/22-09-2021-new-who-global-air-quality-guidelines-aim-to-save-millions-of-lives-from-air-pollution
- Ottawa, 7 June 2023: CTV News, "Wildfire smoke makes Ottawa's air among the worst in the world Wednesday" (IQAir ranking). https://ottawa.ctvnews.ca/wildfire-smoke-makes-ottawa-s-air-among-the-worst-in-the-world-wednesday-1.6430659
- Mexicali 122 days vs Calexico 11 (2016 to 2018): California Air Resources Board, Mexicali data analysis. https://ww2.arb.ca.gov/sites/default/files/2023-01/MexicaliDataAnalysis_Spanish.pdf
- Winnipeg, July 2021 smokiest July on record (Environment Canada): CTV News Winnipeg. https://winnipeg.ctvnews.ca/manitoba-wildfires-lead-to-smokiest-july-in-history-for-winnipeg-1.5537352
- Calgary, 499 smoke hours to early September 2023 (record 450, 2018): The Weather Network / CTV News Calgary.
- Celaya, 168 days over the standard in 2019: Milenio, 25 Dec 2019, quoting Guanajuato's environment ministry. https://www.milenio.com/politica/comunidad/es-celaya-el-municipio-con-mayor-contaminacion-del-aire
- Edmonton, 291 smoke hours by 18 Sept 2023 (record 229, 2018): Global News. https://globalnews.ca/news/9968621/edmonton-record-smoke-hours-alberta-wildfires/
- Timișoara, electric street lighting November 1884: Transelectrica. https://transelectrica.ro/en/web/tel/istoric-1884 ; air plan: Primăria Timișoara, Integrated Air Quality Plan (2022).
- Canary Islands Saharan dust 2020 to 2022: CSIC press release and Atmospheric Chemistry and Physics 24, 12031 (2024). https://doi.org/10.5194/acp-24-12031-2024
- Johannesburg sources (generators during load-shedding, waste burning, mine dumps): City of Johannesburg, "Clean Air Jozi", Sept 2024.
- Plovdiv, about 18,000 households heating with wood and coal: Energy Agency of Plovdiv via EU BUILD UP.
- Sofia, EU Court of Justice ruling C-488/15 (5 April 2017); over 15,000 heaters replaced: Balkan Green Energy News / IEEP Sofia case study (2026).
- Kraków, solid-fuel heating ban from September 2019: Małopolska Regional Assembly resolution (2016); IEEP case study (2026). Wawel Dragon runs on gas and "does not contribute to smog": krakow.pl.
- Vilnius: 83% renewable district heating since 2024 and European Green Capital 2025: Eurocities.
- Vitória, black dust limit 10 g/m² in 30 days: Espírito Santo state decree 6.076-R/2025, via MPF.
- Baoding named most polluted city in China for 2014: Ministry of Environmental Protection report, Feb 2015 (UPI, Eco-Business).
- Beijing: UNEP (2019) "Beijing air improvements provide model for other cities"; last large coal-fired power plant (Huaneng) suspended March 2017: China Daily.
- Lhasa measures: Xinhua / China Tibet News (2015), "according to Chinese state media".
- Yiwu and Christmas decorations (reported share around 60%): Al Jazeera (2016).
- Al Ahmadi, last 1991 oil well fire extinguished in the Burgan field (737 wells): Arab Times.
- Sub-Saharan Africa monitoring (33% of 48 countries): OpenAQ Data Landscape 2024.
- India PM2.5 stations, 78 (2017) to 346 (2024): Health Effects Institute / CPCB.

Full per-city research notes: `research/city_facts.json`.

## Licence
Dataset: CC BY-NC-SA 4.0. Derived from the WHO Ambient Air Quality Database (WHO data are published under CC BY-NC-SA 3.0 IGO, which allows adaptations under later versions of the same licence) and GeoNames (CC BY 4.0).