We say "probably" here for two reasons. First, not all cities collect good enough data to be measured (a problem in itself). And secondly, it depends on how you measure it. When studying air quality, scientists look at the size of particles found in the air. Generally, a lot of attention is placed upon particles of a PM2.5 size, which is around one-30th the size of a human hair. These particles are particularly worrying because they have the ability to enter lungs and blood tissue and cause serious health problems.
However, you can also measure larger particles in the air. At PM10, the particles are coarser, though they can still be inhaled and cause health problems.
If you look at WHO's PM2.5 data, the Iranian city of Zabol comes out on top. This city, which has a population of more than 100,000, sits on the border with Afghanistan in Iran's east. The city is known for the "120 day wind," a lengthy dust storm through the hot and dry summer. Zabol is followed by Gwalior and Allahabad, two Indian cities, while Riyadh, Saudi Arabia's capital and largest city, comes in fourth.
Delhi comes 11th in this ranking, while Beijing is far down the list at 57. For reference, WHO suggests that 10 micrograms per cubic meter of PM2.5 should be a guideline level for safe air.
WHO's PM10 data paints a somewhat different picture. Nigeria's Onitsha, a city of half-a-million people on the Niger River, is atop this list. The city has been known for its rapid growth and gridlock, though some locals were apparently still surprised that it appeared so high on the list. “We know pollution is very bad here. But this city must be much better than Lagos,” Solomon Okechukwa, a local official, told the Guardian.
Zabol also appears in the PM10 top 10, surrounded by cities from Pakistan and Nigeria as well as those from Saudi Arabia and India. In this ranking, Delhi comes in 25th and Beijing 153rd.
The relatively low ranking of Delhi in both scales has led to some muted celebration in the Indian press. In a WHO ranking released just two years ago, Delhi had had a PM2.5 rating of 153, among the worst in the world. Reuters notes that the city has gone to some lengths to tackle this problem, including ordering cars off the road at certain periods. However, there are large numbers of other Indian cities on the list, showing just how much of a problem pollution is for the country.
More broadly, WHO's latest report paints a worrying global picture of pollution. As The Washington Post's Chris Mooney and Brady Dennis report, the bigger picture in WHO's report is that air pollution is getting worse in urban cities around the globe, and the poorest cities are hit hardest. Around 80 percent of the cities and towns where air control is monitored have more pollution than WHO recommends, the organization found, and on average pollution levels worsened by 8 percent between 2008 and 2013.