All schools in Tehran will remain closed on Tuesday because of dangerously high levels of air pollution blanketing the Iranian capital, authorities said.
State television said the closure of primary schools on Monday was being extended to also cover high schools and colleges in the whole of Tehran province apart from four areas where lower levels were recorded.
Average concentrations of the finest and most hazardous airborne particles hit 173 microgrammes per cubic metre in Tehran on Monday and peaked at 231 in an eastern sector, municipal authorities said.
That is far above the World Health Organisation's recommended maximum of 25 microgrammes per cubic metre on average over a 24-hour period.
The microscopic particles lodge deep in the lungs and are harmful to human health.
A grey cloud hung over Tehran on Monday, obstructing the view of the mountains which overlook the city to the north.
Authorities have imposed traffic restrictions in Tehran, which is home to 8.5 million inhabitants, and ordered a suspension of polluting industrial activities and work on construction sites.
Iranian media reports say 80% of the pollution in Tehran is caused by fumes from vehicle exhaust pipes.
Similar measures were taken in December when primary schools in Tehran and most of its province were forced to close for several days because of pollution.
Working mothers are being given a day off on Tuesday to look after their children, said Qumars Kalantari, head of the environmental protection service for Tehran province.
With pollution causing 20,000 deaths a year in Iran, President Hassan Rouhani said on Tuesday that "the Iranian government attaches great importance to the question of the environment".
But ordinary Iranians voiced scepticism.
"If there was a strong will, things would be changing," said Mohammed Soultani, a private sector employee.