We've covered the world's most polluted cities before. Delhi is in a league of its own, as this reporter can attest to; half an hour of sightseeing is enough to leave you with a sore throat. According to the World Health Organisation (WHO), the average concentration of PM 2.5 particles in the Indian city is 150 micrograms per cubic metre of air. PM 2.5 refers to fine particles (2.5 micrometres or smaller in diameter) produced by combustion, including motor vehicles, power plants, forest fires, and some industrial processes. By comparison, London's average is 16.

But what of the most polluted countries? Pinpointing them is a little more problematic. The WHO tracks air quality at 1,622 locations in 92 countries - but all are urban areas. So while Pakistan, Egypt and Mongolia are among the most polluted countries according to the map below, this only refers to pollution in its cities. Air quality in the Karakoram mountain range or the Gobi Desert will, of course, be pristine. Similarly, Russia appears to be among the worst performing countries - but its ranking is based only on air quality in Moscow.

The website aqicn.org crunched 2014 WHO data to produce the map. It shows countries according to the average concentration of PM 2.5 particles in its cities, with figures weighted according to the population of each city. So, if a country's largest city has good air quality, but a handful of small cities have bad air quality, it will perform better overall. Tap on or hover over a country to reveal further statistics, including levels of coarse dust particles (PM10), the result of crushing or grinding operations and dust stirred up by vehicles.

Pakistan's urban areas are, on average, the world's most polluted, followed by Qatar and Afghanistan. Europe's most polluted cities are found in Turkey, Bulgaria and Serbia.

The UK came 21st out of 92 countries for air quality

The 20 countries with the most polluted urban areas

Pakistan - average PM 2.5 concentration: 115.7 Qatar - 92.4 Afghanistan - 86 Bangladesh - 83.3 Egypt - 73 UAE - 64 Mongolia - 61.8 India - 60.6 Bahrain - 56.1 Nepal - 50 Ghana - 49 Jordan - 48 China - 41.4 Senegal - 40 Turkey - 39.1 Bulgaria - 38.6 Mauritius - 38.1 Peru - 38 Serbia - 35.8 Iran - 34.2

Of the 92 countries to feature, Australia has the least polluted urban areas, followed by Brunei and New Zealand. Estonia is Europe's top performing nation, followed by Finland and Iceland. The UK just misses out on the top 20, coming 21st.

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The 20 countries with the least polluted urban areas