Clouds of filth envelope Asian cities: 'you can't escape'

This year has seen some of Asia's worst urban smog episodes in nearly 20 years, as India's air pollution soars above levels recorded in China

The winter air in Tehran is often foul but for six days last week it was hardly breathable. A dense and poisonous chemical smog made up of traffic and factory fumes, mixed with construction dust, burning vegetation and waste has shrouded buildings, choked pedestrians, forced schools and universities to close, and filled the hospitals.

Anyone who could flee the Iranian mega-city of 15 million people has done so, but, say the authorities, in the past two weeks more than 400 people have died as a direct result of the pollution, known as the Asian "brown cloud".