Governor Anoushiravan Mohseni Bandpey said kindergartens, preschools, and primary schools would be shut in the city and the counties of Gharchak, Pishva, and Varamin.

“The air quality index for the city of Tehran still has not passed the unhealthy status for sensitive groups,” he said.

Average concentrations of hazardous airborne particles hit 133 micrograms per cubic meter in the city and were as high as 150 for 10 districts, he said.

That is far above the World Health Organization’s recommended maximum of 25 micrograms per cubic meter on average over a 24-hour period.

Warnings were issued for children, pregnant women, the elderly and people suffering from cardiovascular or respiratory diseases to stay indoors.

The decision to suspend schools in Tehran is the first to come at this time of the year when the cold and dry weather in Tehran causes pollution to accompany rising warm air, leading to the creation of a smog blanket over the city.

Air pollution has been worsened by a lack of wind and rain in recent days.

Many people were seen wearing face masks to avoid fumes as they waited for buses on the sides of traffic-choked streets of southern Tehran during morning rush-hour.

A layer of thick smog, caused by heavy traffic and factory pollution, covered Tehran on Tuesday, but it appeared to dissipate in northern areas on Wednesday morning with fewer school buses on the roads.

Air pollution was the cause of nearly 30,000 deaths per year in Iranian cities, IRNA reported earlier this year, citing a health ministry official.

Each winter, Iran’s sprawling capital suffers some of the worst pollutions in the world through thermal inversion – a phenomenon that traps hazardous air over the city.

Tehran is normally spared of severe levels of pollution seen in other populated cities in Asia. The air quality index is normally at levels between 150 and 200 at their harshest, much lower than figures recorded in cities like New Delhi in India and Beijing in China.

However, to help reduce the smog, authorities normally impose restrictions on travel inside Tehran while ordering factories around the city to close down for several days.

Government officials said earlier this year that the quality of the gasoline produced inside Iran and pumped into the cars had drastically improved, helping to reduce the number of days each year affected by pollution in Tehran and other large cities.

According to a World Bank report last year, most of the pollution in the city of eight million inhabitants is caused by heavy-duty vehicles, motorbikes, refineries and power plants.

AFP and Press TV contributed to this story.