Smog engulfs the city of Beijing this weekend, as quiet weather conditions encourages the pollution to settle across the city.



The quiet weather is due to an area of high pressure which is dominating the region. High pressure acts as a lid on the atmosphere, effectively trapping the polluting particles near the earth, and preventing them from dispersing.



The forecast of quiet weather prompted a yellow alert to be issued on Thursday, ahead of the weekend.



Yellow is the second highest on the new scale of four levels which the Environmental Protection Bureau issued in October last year. This is the first time that a yellow warning has been issued.



People were warned to reduce outdoor activities, and many commuters opted to wear masks.



On Friday and Saturday, the city reported an air quality index of over 400. This is extremely high; levels above 300 are classed as hazardous by the World Health Organisation.



The air pollution index measures the number of unhealthy particles in the air. It refers to the number of particles known as PM2.5, those which are 2.5 microns in diameter. This includes ammonia, carbon, nitrates and sulphates have been linked to diseases such as emphysema and cancer.



Pollution is an increasing concern in China, which adopted a growth-at-all-costs economic model that has tainted much of the country's air, water and soil.



This is a particular problem in Beijing, which is surrounded by the Hebei province. Hebei the country’s biggest steel producer, and home to many of the most polluted cities in the country.



A wind from the south or east will therefore draw pollution into Beijing from the surrounding cities. This pollution then gets trapped within the city, its path blocked by the mountains to north and west.



A strong wind or heavy rain will clear the pollution, but at this time of year there are often long spells of calm and quiet weather.



The authorities in China are well aware of the problem, and last year over 8,000 heavily polluting companies were shut down in the Hebei province.



This year, more action is being taken. At the end of January, Beijing's mayor pledged to cut coal use by 2.6 million tonnes and set aside $2.4 BN (15 billion yuan) to improve air quality.



