Chinese cities are speeding up construction of subway lines as the increasing number of vehicles in urban areas has been largely blamed for smog.

Construction of six new subway lines in Beijing is expected to start by the end of this year with a total length of more than 90 km, including downtown lines and lines linking suburban areas with the downtown, according to the railway construction company.

Currently, there are 17 subway lines running in Beijing with a total length of 456 km. The city's underground network carries approximately 10 million passengers daily on workdays.

Apart from big names like Beijing and Shanghai, less populated large cities, such as Nanchang, Changsha, Zhengzhou, Hefei, Nanning, Chongqing, Chengdu and Kunming, are also constructing or operating urban railway lines.

There are 35 Chinese cities engaged in such construction. Some 70 subway lines are currently being built, with a total investment of 800 billion yuan (131.28 billion U.S. dollars). Adding approved subway line construction plans, the spending reaches 1.5 trillion yuan.

As winter is approaching, air pollution has been clearly visible in many regions of China. In an extreme case, dense smog shrouded major cities in northeast China last week, closing schools, highways and airports, with a visibility of less than 10 meters in certain areas.

Experts believe developing urban rail traffic is one of the solutions to help ease the situation, by getting people out of pollution-emitting cars.

According to research by the Chinese Academy of Sciences, vehicle exhaust fumes contribute 22.2 percent of PM 2.5 particles in Beijing, exceeding the figure for industrial emissions.

PM 2.5 are airborne particles measuring less than 2.5 microns in diameter, which can pose health risks.

Lei Xiaoyan, a professor with East China Jiaotong University, said cities which are equivalent to mid-sized cities abroad should develop rail traffic.

In Nanchang, capital of east China's Jiangxi Province, five subway lines are being constructed, with a total length of 168 km, according to Hu Mengda, deputy chief engineer with the city's rail construction company.

Provincial capitals with large populations, such as Nanchang, serve as cores in less developed central and western regions, and rail traffic is necessary there, but smaller cities have to consider whether their population will be large enough to support the operation of the subway lines in the future, said Hu. "For them, a subway is a luxury."