by Xian Wan and Biodun Iginla, BBC News, Beijing

A visitor is silhouetted at an observatory tower during a polluted day in Beijing

A yellow pollution alert, the third highest in the four-tier system, has been in place since Saturday

Pollution has soared to hazardous levels in Beijing, reaching 20 times the limit recommended by the World Health Organisation.

Since Saturday visibility has been low and many people were seen wearing masks. Experts say car emissions and construction in cities are to blame.
Parts of China experience heavy smog periodically.
After an intense bout in Beijing last year in January, the government pledged to improve air quality.

Visitors take a walk during a polluted day at Tiananmen Square in Beijing January 15, 2015. Beijing issued its first smog alert of 2015 on Tuesday.

Last year Beijing experienced a smog crisis which continued for days

A man wears a mask as he makes his way during a polluted day at Tiananmen Square in Beijing

PM2.5 particles have a diameter small enough to get into your lungs and are considered very dangerous

A man stands on a platform as he poses for a picture at an observatory tower during a polluted day in Beijing

China has previously limited car usage and restricted industrial activity to try and control air quality

On Wednesday, the government said it would ban the construction of new oil-refining, steel, cement and thermal power plants starting from March, state-run newspaper China Daily said.
On Thursday, levels of PM2.5 particulates, which are the most hazardous to health, rose to 568 micrograms per cubic metre. according to the US embassy.
The WHO recommends a maximum of 25 micrograms per cubic metre.
'Smoking bacon' A yellow pollution alert, the third highest in the four-tier system, has been in place since Saturday, according to the Beijing Meteorological Bureau.
Less severe pollution is also expected in Tianjin, Hebei province and parts of Shanxi and Sichuan provinces.
A government official in the south-western city of Dazhou in Sichuan Province said earlier in January that the lingering smog was caused by smoking bacon, State-run Xinhua news agency said.
Rao Bing, deputy head of Dazhou's Environment Protection Bureau, was referring to the traditional method of preserving pork and sausages common in Sichuan.
On Sina Weibo, China's micro-blogging site, many mocked the argument saying "smoking bacon has a long history, but smog does not", Xinhua reported.
During previous bouts of heavy smog Chinese government officials have given other controversial explanations for smog including blaming farmers burning straw, another traditional practice.
China has also previously restricted industrial production and limited car usage in big cities.