In Iran, schools in Tehran province have been closed until Friday due to severe air pollution, authorities announced, extending their closure to a full week.

State news agency IRNA reported schools in northern Alborz province were also closed for the week.

Tehran city was cloaked by thick toxic smog for a third successive day today that blanketed out landmarks like Milad Tower and the snow-capped Alborz mountains.

Pollution worsens in Tehran during winter, when cold air and a lack of wind traps hazardous smog over the capital for days on end, a phenomenon known as thermal inversion.
The sprawling capital is one of the world’s most polluted cities.

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