Air pollution in Iran’s major cities has reached dangerous levels this week as power plants have switched from scarce natural gas to dirty fuels in colder weather.

Javad Rahmati, deputy governor of Eastern Azarbaijan province said the provincial capital Tabriz is enveloped in smug because from late summer power plants were ordered to switch from natural gas to mazut, a heavy diesel fuel banned in most of the world.

This week, Tehran, Esfahan, Mashhad and Arak reported heavy pollution and people at higher risk were advised to stay indoors.

Iran has the second largest gas reserves in the world but because of lack of investment and advanced technologies it is unable to produce more than what it extracts now, which is around 700-750 million cubic meters a day. Domestic usage has climbed to 530 million cubic meters a day officials announced this week, leaving little for industries and export.

Iran has the world’s cheapest energy prices in par with Venezuela, which is the main factor in high consumption, while various sanctions have made it difficult to upgrade its oil and gas industries or to adopt more alternative fuels.