After a winter weather front passed from central Iran on Friday, the Iranian meteorological agency announced a “yellow” pollution alert for five major cities.

Citizens have been warned not the engage in heavy physical activities outdoors in Tehran, Esfahan Mashhad, Karaj and Arak. Vulnerable people have been told to stay indoors.

It is not yet clear if burning of dirty, heavy fuels for electricity generation has led to the heavy pollution, but Iranian officials had warned months ago that with shortages of natural gas, they would resort to using diesel fuels in winter months.

Consumption of gas and electricity increases during cold winter and hot summer days.

Last winter, Iran burned mazut, a type of dirty diesel that polluted many large cities, which led to protests. Tehran and Esfahan were listed in category red during last December-January.

The same energy crisis happened again in June when air-conditioners kicked in raising electricity consumption. Iran has one of the cheapest electricity rates in the world.

Iranian officials said earlier this year that up to 40,000 people die prematurely every year because of air pollution.