Tehran city was cloaked by thick toxic smog for a third successive day on Monday that blanketed out landmarks like Milad Tower and the snow-capped Alborz mountains.
"All of Tehran province's schools... except Firuzkuh and Damavand counties are closed for the week due to increasing air pollution," governor Mohsen Bandpey told reporters.
He added that forecasts indicated stable weather and "increasing pollutant density" across urban areas.
Average airborne concentration of the finest and most hazardous particles (PM2.5) was at 147 microgrammes per cubic metre in the 24 hours to midday Monday, according to government website air.tehran.ir.
That is close to six times the World Health Organization's recommended maximum of 25 microgrammes per square metre.
Pollution worsens in Tehran during winter, when cold air and a lack of wind traps hazardous smog over the capital for days on end, a phenomenon known as thermal inversion.
Schools in northern Alborz province were also closed for the week, state news agency IRNA quoted the deputy governor as saying.
The measure aims to stop pollution-belching buses from traversing the city while also keeping vulnerable youngsters indoors.
Iranian media and officials have however slammed the school closures as a limited measure to address the problem.
Shutting schools on Saturday and Sunday had evidently "not helped to improve Tehran's air quality", Donya-e Eqtesad daily wrote, calling it the "minimum necessary measure" the authorities could take.
Iran's judiciary chief Ebrahim Raisi said the "roots" of the problem must be addressed.
Tehran's mayor Pirouz Hanachi called for industries using the fuel oil mazout and gasoline to be targeted.
The sprawling capital is one of the world's most polluted cities.
According to a World Bank report last year, most of the pollution in the city of eight million is caused by heavy duty vehicles, motorbikes, refineries and power plants.