Iran shut primary schools in the capital and other parts of the country on Sunday due to choking levels of air pollution.
Local authorities late Saturday announced the closure of all primary schools in the province of Tehran, which is home to 14 million residents, except in two towns.
A blanket of smog has covered neighbourhoods in the capital in the past few days.
Airborne concentration of fine particles (PM2.5) hit 185 microgrammes per cubic metre in the south of Tehran and 174 in its centre on Sunday morning, local authorities said.
That is far above the World Health Organization recommended maximum of 25 microgrammes per m3 over a 24-hour period.
Authorities also ordered mines and cement factories in Tehran province to close and reinforced regular traffic restrictions in the capital's centre.
They called on the elderly, children, pregnant women and people with heart problems to stay indoors.
In the northwestern cities of Tabriz and Urmia, schools remained closed for the second day straight on Sunday, official news agency IRNA said.
Every year, Tehran suffers some of the worst pollution in the world when cool temperatures cause an effect known as "temperature inversion".
The phenomenon creates a layer of warm air above the city that traps pollution from more than eight million cars and motorbikes.
In 2014, almost 400 people were hospitalised with heart and respiratory problems caused by pollution in Tehran. Nearly 1,500 others required treatment.
The health ministry estimated that pollution in 2012 contributed to the premature deaths of 4,500 people in Tehran and about 80,000 across the country.