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Air pollution is a big problem - it's responsible for an estimated 3.3 million deaths every year.
A tiny pollutant called PM2.5, or particulate matter smaller than 2.5 microns across, is considered the most harmful pollutant. It's found in soot, smoke, and dust. When it lodges in the lungs, it can cause long-term health problems like asthma and chronic lung disease.
While we typically tend to focus on the places that are the least healthy, here's a list of the most healthy.
Countries are ranked according to their number of clean cities, using data from the WHO.