Air quality is measured on the fine particulate matter (PM2.5) scale. PM2.5 are tiny particles in the air that are harmful to health and reduce visibility and cause the air to appear hazy when levels are elevated.
The data below shows PM2.5 levels by country. A high PM2.5 level indicates very poor air quality. A very low PM2.5 level denotes clean air.
Data source: World Health Organization, Geneva