China's top observatory issued a yellow alert for smog in central and eastern regions on Thursday, the first day of chunyun, or the Spring Festival transportation period.

Heavy fog and smog shrouded 13 provincial regions on Thursday, with visibility reduced to less than 500 meters in parts of Hebei, Tianjin and Jiangxi, and was expected to last until Friday, the Central Meteorological Center (CMC) said Thursday.

Chinese passengers will make 3.2 billion road trips during the 40-day chunyun, up 5.8 percent from 2013, according to the Ministry of Transport.

"Smog may become the biggest obstacle for those travelling home," the CMC said. On Thursday, four highways in Beijing and 15 in Hebei Province were forced to close.

The smog in Beijing started Wednesday afternoon and pollution levels continued to rise, which led to a blue alert on Wednesday night, the capital's first air pollution warning of the year.

Figures from the Beijing municipal environmental monitoring center showed that Beijing's average PM2.5 index stood at 400 to 500 micrograms per cubic meter in most parts of the city at 6 am on Thursday, far surpassing 300, the threshold for the worst level of a six-scale table.

In October 2013, outdoor air pollution was linked for the first time with cancer deaths by the World Health Organization's International Agency for Research on Cancer. It said PM2.5 contributed to 223,000 deaths from lung cancer worldwide in 2010.

Besides health concerns, the capital is hazy on polluted days with skyscrapers seeming to have vanished. Some experts believe severe air pollution to be a factor in the city's declining tourist numbers.

Beijing's inbound tourism decreased by 12.9 percent from 2012 figures during the first three quarters of 2013 , according to the city's tourism commission.

Data showed that severe air pollution appeared on more than 15 percent of days in 2013 in Beijing, which averaged at one day a week. Beijing's average density of PM2.5 is more than double the national standard.

Wang Anshun, Beijing's mayor, said on Thursday that the city will cut coal use by 2.6 million tons and restructure 300 polluting companies in 2014, in an "all out effort" to tackle air pollution.

He added that research into pollutant reduction technology and the sources of PM2.5 will be carried out.

The Beijing Youth Daily reported Thursday that an experiment in artificial haze dissipation is being researched in Beijing.

Wang Gengchen, a research fellow with the Institute of Atmospheric Physics at the Chinese Academy of Sciences, questioned the theoretical basis for this method.

"This is different from artificial fog dissipation since the pollutant particles are small and thick, and smog always spreads widely," said Wang. "Even if the experiment works, it will be too costly for practical use."