Iranian authorities have decided to shut down schools in the capital Tehran and several other cities as seasonal heavy pollution continues to pose risks to the lives of many people in populated cities.

Elementary schools were declared closed for Sunday in all but two towns in Tehran province, which includes the capital and is home to 14 million residents.

In the northwestern cities of Tabriz and Urumiyah, schools remained closed for a second day.

Authorities normally announce such closures in this time of year, when pollution created by millions of cars is trapped under a layer of warm air that blankets cities as a result of "temperature inversion".

Interim traffic restrictions were introduced for Sunday and sensitive groups of population, including the elderly, children, pregnant women and people with heart problems were recommended to stay indoors. Mines and cement factories in Tehran province were also ordered to stay closed.

Authorities said they would introduce “harsher” measures if air pollution persisted in its current choking levels. They said airborne concentration of fine particles (PM2.5) per cubic meter was 185 micrograms in southern Tehran and 174 in the city’s central neighborhoods as of Sunday morning. A maximum recommended by World Health Organization is 25 micrograms per m3 over a 24-hour period.

A general view shows the Milad telecommunications tower behind a blanket of smog in Tehran on December 17, 2017. (Photo by AFP)

The government estimates that air pollution costs Iran billions of dollars from gross domestic product. It said in 2012 that 4,500 people in Tehran and about 80,000 across Iran died prematurely as a result of the problem.

Many blame government policies in car making and public transportation for the worsening of pollution in Tehran over the past years. They say heavy traffic jams in Tehran have even affected the city’s climate, helping pollution stay above the city for a longer period of time.