DUBAI - Schools across Tehran will be closed on Sunday and residents have been warned to stay at home as a thick smog of pollution hangs in the air, officials said on Saturday. ‘All educational establishments in the city of Tehran, and some towns in the province, are suspended on Sunday due to air pollution,’ the city’s top education official Esfandiar Chaharband was quoted as saying by the Mehr news agency. Outdated vehicles and inconsistent enforcement of emissions regulations are the main drivers of pollution in Iranian cities, which officials have blamed for thousands of deaths from respiratory illnesses. The warning came as officials in China issued a ‘red alert’ to prepare for smog and a week after almost 200 countries reached a landmark accord to combat climate change.

‘The most important sources of air pollutants in Tehran are motorcycles and cars whose catalytic converter has been removed,’ health official Khosro Sadegh-Niat was quoted as saying by state news agency IRNA. Tehran, a dense city of at least 12 million inhabitants, suffers the most as the snow-capped Alborz mountains form a basin that shields its air from cleansing winds. Pollution is more severe in winter as cold air prevents the smog from rising.

Three years ago, officials blamed smog for more than 4,000 deaths across the country. Since then, President Hassan Rouhani’s government has pledged to tackle pollution and introduced measures such as converting vehicles to cleaner fuel. Last month, Supreme Leader Ayatollah Ali Khamenei issued a set of environmental policy directives and called for the government to support investments in clean energy.