The heavy smog that affected north China in January contained a heavy concentration of deadly organic compounds, according to the Chinese Academy of Sciences.

A woman takes precautions against pollution in downtown Liaocheng in eastern Shandong Province. Smog again shrouded Beijing and other areas yesterday, disrupting traffic and flights. Extreme pollution was recorded in Shijiazhuang, capital of north China's Hebei Province. [Shanghai Daily]

Wang Yuesi, an academy researcher, said the smog that shrouded Beijing and other cities in the region contained chemicals that were present in London's Great Smog of 1952, in which about 12,000 people died, and the photochemical smog in Los Angeles from 1940 to 1950 which killed about 800 people.

The academy's study found that the source of the air pollution was vehicle and industrial emissions.

Chen Liangfu, another academy researcher, said the problem was made worse as small airborne particles increased in size by two to three times when combined with water in the air.

In Beijing, vehicle emissions were the biggest problem, making up about a quarter of the air pollution, the study found.

This was followed by the burning of coal and dust and sand blown in from outside the city which each made up about a fifth of the pollution.

The academy's experts said coal burning should be controlled and attention paid to emissions by diesel vehicles and the quality of the fuel they used.

Yesterday, smog again disrupted traffic and flights in Beijing, with moderate or serious air pollution reported and visibility down to 500 meters during the early morning, the Beijing Environmental Protection Monitoring Center said. At one point, visibility at the Beijing Capital International Airport fell to 5 to 10 meters, causing flights to be delayed, Xinhua news agency reported.

Meanwhile, fireworks caused PM2.5 readings to skyrocket in many cities during the Spring Festival holiday, the Ministry of Environmental Protection said yesterday.

Figures released by the ministry showed that 42.7 percent of 74 cities reported higher-than-normal readings of the dangerous particles measuring 2.5 microns or less in diameter, Xinhua said.

The highest average reading in a single day was 426 micrograms per cubic meter, or 5.7 times the country's standard.

North China's Tianjin hit 577 on lunar New Year's Eve, the highest of all cities. Some cities also saw excessive readings for PM10 and sulfur dioxide, Xinhua said.