THOUGH it is often cloaked in a fetid smog, China does not have the most toxic air in the world. Nor does India, despite its congested roads and belching power-stations. The most polluted air hangs over the United Arab Emirates. So says the World Bank, at least. It claims that the UAE’s air is a bit worse than China’s and more than twice as bad as India’s, if one measures particulates of 2.5 microns or smaller, known as PM2.5 (see chart). These tiny particles can penetrate deep into the lungs. The UAE protested. Granted, it is the world’s eighth largest emitter of carbon dioxide per capita. Cement manufacturing, power generation, desalination and cars all add to its pollution. But one of the biggest contributors of PM2.5 in the region is dust made of sand, kicked up by construction or windstorms. This skews the data, say UAE officials.

Fahed Hareb, the director of air quality at the ministry of environment and water, says carbon-based particles and dust should be viewed differently. Otherwise it is unfair to countries with deserts. The UAE has 46 monitoring stations and reckons that even with the dust, its average PM2.5 levels are “not even half of what [the World Bank] stated,” he says.

Dust is probably not as dangerous as man-made pollutants, which are often carcinogenic. But most scientists think PM2.5 is toxic regardless of its source or composition. “Whether it contains dust or not, there is a severe health effect,” says Johann Engelbrecht of the Desert Research Institute in America, adding that the bank’s measurements are “in the ballpark” of those previously taken in the region. Studies have found that exposure to wind-generated dust is associated with higher hospital-admission rates for respiratory illnesses and other problems.

A stronger criticism is that the bank’s data need updating. It relied on the Institute for Health Metrics and Evaluation (IHME), which combined satellite imagery, ground-level monitoring and atmospheric modelling to produce the PM2.5 numbers. Michael Brauer, who led the effort, says the data were then weighted to come up with a national average. But many of the measurements are a decade old and the bank admits the methodology “has its limitations”.

An update is already in the works. The new numbers will benefit from more monitoring stations, including in the UAE, which has taken steps to curb emissions. They will also reflect lower-than-expected dust levels in the Gulf. That’s good news for the UAE, but its air is still among the worst, says Mr Brauer.