Commuting regularly by underground metro trains may pose hazardous health risks as the air inside the compartments have higher concentrations of cancer-causing agents, claims a study.

The study, published in the journal Aerosol and Air Quality Research, showed that people are exposed to significant amounts of airborne particulate matter (PM) during their daily commutes, which often exceed 30-60 minutes each way. The two major compounds found in airborne particulate are polycyclic aromatic hydrocarbons (PAHs) and hexavalent chromium, both which include carcinogens, as well as chronic non-cancer health risks, such as cardiovascular and respiratory diseases. Owing to the high levels of exposure to carcinogenic transition metals hexavalent chromium, for a person travelling in the underground trains, the maximum "excess lifetime cancer risk" was found to be 10-times higher than the acceptable threshold of one-in-a-million, set by the World Health Organisation (WHO) and the US Environmental Protection Agency (EPA), the researchers said. 

Despite these electric-powered trains operating with mandatory closed windows and a mechanical ventilation system, airborne hexavalent chromium gets built up due to dust from friction on the steel tracks, as well as the lack of ventilation. Based on these data, the best option for commuters is to use above-ground light-rail transportation, which allows for reduced exposure to both traffic-generated PAHs and railway-related metals, said Constantinos Sioutas from the University of Southern California-Los Angeles. 

The researchers suspect that individuals who spend more time in the subway, particularly those who work there, would also have a significantly higher health risk. For the study, the team collected air samples using battery-operated devices with particle sensors on both train platforms and inside train cars in Los Angeles. Cancer and non-cancerous health risks were calculated based on a lifetime of exposure commuting one hour a day, five days a week, for 50 weeks a year, and assuming 30 years of employment.