BEIJING, Jan. 11 (Xinhua) -- Experts called for an outright ban on smoking throughout China's railway network, after air quality inspections found dangerous levels of harmful pollutants on slow trains, most of which currently allow tobacco use in designated areas, China Daily reported Friday.

Researchers led by the Chinese Association of Tobacco Control examined four slow trains in October. Three of the trains had set up designated smoking areas, and one had banned tobacco use due to an earlier lawsuit filed by a passenger.

Results released on Thursday showed the concentration of PM2.5 in carriages of the three trains where passengers can smoke exceeded 500 micrograms per cubic meter, meaning the air could be considered hazardous to human health.

China's high-speed rail networks all prohibit smoking, but passengers on slow trains are allowed to smoke in designated areas, often the connecting areas between carriages.

Cui Xiaobo, deputy director of the Beijing Tobacco Control Association, said secondhand smoke is likely to cause life-threatening, acute illnesses.

"A lot of emergency medical requests occurring on trains are linked to vulnerable groups, including the elderly, children, pregnant women and those with chronic diseases who breathe in smoke," he said.

The researchers also interviewed 94 passengers on the four trains, and found that 77 of them would applaud a complete ban on smoking on slow trains.

China saw 3.37 billion railway trips in 2018.