Iran closed many government offices and state firms and cancelled some flights on Tuesday because of wind-blown dust caused by sandstorms in neighbouring Iraq, official media reported.
State industries will be shut for two days in the central Tehran province and medical authorities advised people with heart or respiratory problems to stay indoors.
"Dust pollution closes down Tehran," state Press TV said on its website.
With dust clouds reducing visibility, some domestic flights were grounded or delayed for a third straight day, especially in western areas close to Iraq, which has suffered one of its worst sandstorms in living memory.
The streets of Tehran, a city of 12 million people, were unusually calm as government employees and others stayed home.
Many of those who ventured to work wore facemasks and ambulances were deployed in squares.
The Tehran Times newspaper said pollution had risen to 21 times the normal level in the western province of Kermanshah, which borders Iraq, and that dust was also affecting some central regions.
"The dust has brought life to a standstill in some western cities," Tehran Times said. "Reports received from across the country indicate that the situation has gone from bad to worse."
Twenty-two people have been taken to hospital in the neighbouring province of Ilam over the last few days, the official IRNA news agency said.
Iranian authorities expect the situation to improve towards the end of the week.
Iraq has long suffered periodic, blinding sandstorms, but several years of drought have aggravated the situation this year.