A Real Chinese Peasouper - Pollution in Harbin

11 million people live in the Chinese city of Harbin. But, you might find it hard to see many of them if you decided to venture out for a stroll. In recent days, Harbin, one of the most-polluted cities in China, has been in the grip of a smog which has paralysed the city.

In parts of Harbin, the PM2.5 measure of air pollution reaches 1,000 – 40 times the WHO's recommended daily exposure level. As the smog formed, visibility fell to under 10 metres in places.

According to the Guardian, officials blamed the first day of winter heating in the city - leading to increased coal burning - low winds and the burning of crop stubble as well as vehicle emissions. Others said a sudden temperature change and humidity might have contributed.

Why does this matter so much? Whilst the disruption to public services and everyday life is fairlyu inconvenient, the most significant threat from air pollution comes to the health of the population.

PM2.5 is particularly dangerous because the matter is small enough to penetrate deep into the lungs and enter the bloodstream. Outdoor air pollution has been officially classified as carcinogenic by the cancer arm of the World Health Organisation.