BEIJING: Heavy smog descended upon the Chinese capital this week as Beijing issued its first three day pollution 'red alert'.

The unprecedented move followed scathing public criticism aimed at the city's weak response to last week's thick haze, which saw pollution skyrocket to record levels. The key cause of smog is the burning of coal (for electricity and heating), which also contributes the the country's greenhouse gas emissions.

Beijing put its air pollution emergency plan into action on Monday, pulling half of all private vehicles off the streets, ordering many factories to close and recommending that some schools allow students to remain at home. The use of conventional petrol-powered and hybrid cars was limited to alternate days, an unprecedented boon for China's nascent electric car market.

The recurrent bad air drove residents to hospitals in growing numbers, and pollution mask sales soared by as much as 400 per cent. Meanwhile, Chinese e-commerce giant Alibaba was overrun for searches for air purifiers and masks.

The chronic haze blanketing northeastern China earlier this month was so thick that, unlike the Great Wall, it could be seen from outer space, according to satellite photographs from NASA.

One traveller on a high-speed train from the capital to the central province of Hubei posted said, “Every city in north China that I passed was covered by smog and looked like a dead town.”