More than 1,600 people have been taken to hospitals in Tehran as pollution in the Iranian capital reaches critical levels, health officials have said.
Hospitals have reported increased cases of heart attacks and breathing problems, while many residents are complaining of fatigue and headaches.
Public offices and schools have been closed in an attempt to reduce traffic, and clear the city's blanket of smog.
Authorities have warned of thousands of casualties if pollution levels persist.
There is no wind or rain and the dirty air is trapped on top of the city by the mountains surrounding it, reports the BBC's France Harrison in Tehran.
Sheydar Malik Afzeli, the head of the anti pollution unit of the health ministry, speaking to Ettemad newspaper, said the extent of deaths and casualties from pollution were "not less" than those in the plane crash in the capital on Tuesday which killed more than 100 people.
However there is no official confirmation of any smog-related deaths.
From Monday, cars will only be allowed into the city centre on alternate days, depending on whether their number plates start with odd or even numbers.
Authorities have blamed the severe smog on emissions from the capital's three million cars, many of which lack modern exhaust filters.
It is estimated that up to 5,000 people die every year from air pollution in the city.