China is expected to have more than 800,000 lung cancer patients diagnosed annually by 2020, with nearly 700,000 people dying from the disease each year, according to the latest forecast by medical experts.

Lung cancer has been a growing problem in China since 2000 due to risk factors such as an aging population, air pollution and smoking. "The rapid increase of the disease will last for at least 20 years," warned Zhi Xiuyi, head of the Lung Cancer Diagnosis and Treatment Center of Capital Medical University, Beijing.

And Zhong Nanshan, a respiratory expert and academic at the Chinese Academy of Engineering, expects that PM2.5, a major cause of smog, will replace smoking as the top risk factor for lung cancer.

Currently, lung cancer is the top killer among all cancer types, causing about 600,000 deaths per year in China. In 2015, roughly 700,000 Chinese developed lung cancer in China.

China already has the highest lung cancer prevalence and death rate in the world, according to the World Health Organization. However, the disease and its death rate has been in decline in most Western countries, said Zhi.

In response, the Chinese government has launched health initiatives, including efforts to cut down smoking. But the emerging challenge from the smog "should be addressed as well", Zhi said.

Shi Yuankai, vice-president of the Chinese Academy of Medical Sciences Cancer Hospital, said continuous exposure to particulate matter, particularly PM2.5, could be a major health hazard.

The matter goes directly to the alveoli of the lungs and cannot be coughed out, Shi explained. But how exactly the smog, or the PM2.5, harms the lungs remains unclear.

Shi urged related government departments, such as health and environment, to carry out more research to help with targeted interventions.

According to the National Health and Family Planning Commission, a nationwide network monitoring the impact of air pollution on human health is underway.

More than 40 monitoring sites will be set up in 16 provinces and municipalities that are frequently engulfed by smog to support research on air pollutants in different regions.