The title for the world’s most polluted city may not be held by the first sprawling megapolis that springs to mind. In fact, it could be found in Iran.

Your first thoughts might spring to the Chinese capital of Beijing, where smog levels regularly grab the headlines. Or perhaps you’ve heard about the pollution in Delhi, India. However you might be surprised to hear that the city with the worst pollution may be in Iran, writes Adam Taylor for The Washington Post.

Iran May Be Home To The World's Most Polluted City

WHO data reveals air pollution levels

That is according to a new report from the World Health Organization, which says that the megacities famed for smog problems are only the beginning. However there is an important caveat to Iran’s questionable honor.

The thing is that the data on air pollution is variable. First of all, not all cities collect sufficient data. Second you can measure air pollution in different ways.

Many scientists concentrate their efforts on particles known as PM2.5. These are around 1/30th the size of a human hair and cause particular concern as they can enter lungs and blood tissue, which can lead to serious health issues.

There are also larger particles known as PM10 which are sometimes measured. They are coarser but can be inhaled and lead to health issues.

Zabol, Iran, takes title of world’s most polluted city

The WHO data on PM2.5 particles shows that the Iranian city of Zabol is the most polluted city in the world. It may have a population of just over 100,000, but it is home to the “120 day wind.” This length dust storm buffets the city, which sits on the border with Afghanistan, throughout its summer months.

Next on the list are the Indian cities of Gwalior and Allahabad, with the Saudi Arabian capital of Riyadh in fourth place. According to PM2.5 data Delhi is only the 11th most polluted city in the world, while Beijing is 57th.

The WHO says that 10 micrograms per cubic meter of PM2.5 is a safe level. Zabol has 217 micrograms per cubic meter.

Indian cities feature heavily despite improvements in Delhi

According to PM10 data the ranking is slightly different. The Nigerian city of Onitsha tops this list, with its 500,000 residents suffering PM10 levels of 594 micrograms per cubic meter.

Onitsha sits on the banks of the Niger River and has become famous for gridlocked streets caused by rapid growth. Locals expressed surprise that their city topped 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 is third on the PM10 list, surrounded by cities from Pakistan and Nigeria. There are various cities from Saudi Arabia and India near the top of the list, while Delhi comes in 25th and Beijing 153rd.

The Indian press has celebrated the relatively low ranking of Delhi by both measures. Just two years ago WHO data revealed that Delhi had a PM2.5 rating of 153, which was among the worst in the world.

Authorities have implemented a raft of schemes to combat air pollution, including restricting the movement of cars at certain times. However the presence of multiple Indian cities on the list of the world’s most polluted shows just how much work still needs to be done.