What’s the most polluted city in the world? Some might point to Beijing, the Chinese capital, and its now legendary smog problem. Others may point toward India, where Delhi’s own air pollution problems have become similarly infamous. However, a new report from the World Health Organization suggests that these megacities are actually only the tip of the iceberg — and the actual city with the world’s worst pollution is probably in Iran.
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.