Global Air Quality Outlook According to the 2023 Air Quality report, only 10 out of 135 reporting countries met the WHO PM 2.5 standard in 2023. Europe is home to some of the cities with the cleanest air in the world. In 2023, 135 cities in the region achieved the WHO annual PM2.5 standard of 5 g/m3. The United Kingdom led the region with the highest number of cities meeting the air quality standards. Canadian wildfires led to an increased particulate matter concentration in the country