<span style="color: #222222;">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.<br />''<br />''State news agency IRNA reported schools in northern Alborz province were also closed for the week.<br />''<br />''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.<br />''<br />''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.<br />''The sprawling capital is one of the world's most polluted cities.<br />''<br />''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.</span><br />