The pollution monitor on the roof of the US Embassy in Beijing was catapulted into the international spotlight as air pollution deteriorated in the Chinese capital in mid-January.

The instrument broadcasts a Twitter bulletin each hour providing a record of the airborne particle pollution. This peaked at 25 times the World Health Organisation (WHO) guidelines on the 13th implying an increased daily death rate of around 50%. This was a classic example of wintertime smog with cold air settling over the city trapping pollution. Managing Beijing's air pollution is not easy, typical pollution composition in the Chinese capital reveals fumes from coal burning as well as traffic and heating in the city itself. Beijing is not alone among developing mega-cities; Tehran reported air pollution problems in 2012, with estimates of 4,400 deaths annually.

In the UK cold, settled, weather conditions also led to brief periods of wintertime smog in many UK cities in January with pollution reaching two to three times the WHO Guidelines. High particulate pollution was measured in Derry on the 10th and 13th January, in London on the 16th and 17th and in the east Midlands in Chesterfield on the 17th and in Nottingham on the 24th. Almost all towns and cities across England and south Wales experienced particulate pollution on the 24th and 25th, ahead of January's last bout of snow, but a change to wet and windy weather brought low air pollution to end the month.