Iranian authorities urged young schoolchildren and many civil servants in the capital Tehran to stay at home Wednesday due to a spike in air pollution levels.
"One third of the employees of all executive bodies will work remotely on Wednesday due to the accumulation of pollutants," the official IRNA news agency reported, citing a government committee tasked with monitoring pollution.
It also said that pre-school and elementary school classes in Tehran "will be held virtually on Wednesday due to worsening air pollution levels".
The committee warned people across Tehran against engaging in any "outdoor activities."
Tehran, a city of around nine million people, suffers from chronic air pollution and regularly ranks among the world's most polluted cities.
Air pollution has affected several Iranian cities in recent days, including Mashhad, Isfahan and Ahvaz.
On Tuesday, schools were closed in three towns in the southwestern province of Khuzestan, IRNA said.
Around 40,000 deaths a year in Iran are attributed to air pollution, according to Iranian media.
Tehran lies in the southern foothills of the Alborz mountains which tower over the city trapping polluted air.
The phenomenon, known as thermal inversion, peaks during winter, when cold air and a lack of wind traps hazardous smog over the capital for days on end.
In December 2019, authorities ordered schools around Tehran shut for an entire week due to severe air pollution.