BEIJING - Experts are worried that particulate matter in the air, which is in higher concentrations in North China because of the heavy fog since the weekend, may lead to various respiratory diseases including lung infections and cancer.

Shi Yuankai, vice-president of the Chinese Academy of Medical Sciences Cancer Hospital, said longtime exposure to particulate matter especially the particulate matter smaller than 2.5 micrometers (PM2.5) which can go directly to the alveoli of the lungs, is a major health hazard.

"Even if we can manage to keep the country's smoking rate flat, the lung cancer rate is expected to keep rising for 20 or 30 years and worsening air pollution could be the major culprit," he said.

Air quality in the capital has deteriorated because of the heavy fog. Beijing's air was slightly polluted on Monday, according to the China National Environmental Monitoring Center.

However, the United States embassy, which conducts its own measurements based on the concentration of PM2.5 in the air, rated Monday's air in Beijing as "hazardous".

Zhong Nanshan, a respiratory expert and academician of the Chinese Academy of Engineering, echoed Shi's concern.

He said that without intervention, PM2.5, a major cause of choking smog, would replace smoking tobacco as the top risk factor for lung cancer.

In Beijing, the lung cancer rate has increased by 60 percent during the past decade, even though the smoking rate during the period has not seen an apparent increase, said Mao Yu, deputy director of the Beijing Health Bureau.

"Increasing air pollution might be largely blamed for that," said Zhi Xiuyi, director of the Lung Cancer Treatment Center at Capital Medical University.

Lung cancer, the leading killer among cancers, claims more than 600,000 lives on the mainland annually, according to the Ministry of Health.

In the past 30 years, the mortality rate among lung cancer patients increased by 465 percent on the Chinese mainland.

"Long-term exposure to that (particulate matter) also substantially increases the risk of developing cardiovascular diseases," said Ma Changsheng, a cardiologist at Anzhen Hospital in Beijing.

"The smaller the particle, the more hazardous it is for public health," Shi Yuankai said. "Worse, protective measures like wearing face masks barely help because the particles are too small."

The disastrous effects of smog are nothing new.

In the winter of 1952, dense smog in London caused mainly by heavy coal combustion killed about 12,000 people in the city.

That incident prompted the British government to move to clean up the air, and the effort paid off, for today, PM10 concentrations read at the significantly lower level of around 30 micrograms per cubic meter in the city.

According to the World Health Organization, the main components of particulate matter are sulfate, nitrates, ammonia, sodium chloride, carbon, mineral dust and water.

Even in the EU, average life expectancy is 8.6 months shorter due to exposure to PM2.5, though the lung cancer rate has slightly decreased there in recent years, studies found.

"It is possible to derive a quantitative relationship between pollution levels and specific health outcomes," according to a 2005 World Health Organization report. Yet, in China, "we have few such studies", said Shi Yuankai.

The bad air has also disrupted traffic in the country.

Starting at the weekend, heavy fog shrouded northern and eastern parts of China. On Monday, about 219 flights were canceled and 118 delayed as of 3 pm at the Beijing Capital International Airport, according to the airport's website.

Apart from Beijing and Tianjin, nine other provinces, including Hebei, Shandong, Anhui, Zhejiang, Fujian and Jiangxi, were shrouded in heavy fog, snarling traffic.

Cars along the highway from Beijing to Harbin were backed up more than 50 kilometers in Liaoning province on Monday morning, China National Radio reported.

The heavy fog in the capital is likely to clear before Wednesday with a cold front passing and scattered snowstorms are forecast on Tuesday, according to the Beijing Meteorological Bureau.

Zheng Xin contributed to this story.