Air pollution is deadly – in fact, a recent study found that on average, it cuts human lifespan by three years, which is higher than the global effect on lifespan caused by smoking. No level of pollution is safe, but the World Health Organization (WHO) has set a target of 10 micrograms per cubic metre (µg/m³) of fine particulate matter (known as PM2.5 particles), a level that it says minimises the risk of adverse health effects. However, many countries have much higher levels of PM2.5 particles, which can lead to "low birth weight, increased acute respiratory infections, and stroke".

While the coronavirus pandemic and subsequent lockdowns enforced by governments across the world have seen 84% of monitored countries report air quality improvements, including across most major cities, in some nations pollution levels are already starting to surpass what they were before the pandemic even started. As we celebrate world Earth Day (22 April), click or scroll through the 15 most and 15 least polluted countries in the world according to the 2020 World Air Quality Report produced by air pollution tracking organisation IQAir.