Irrespective of whether you are traveling by tube in London or the subway in Los Angeles and New York, you are more likely to suffer from pollution than regular car travelers. A recent study in the US has stated that public transport might not be as healthy as you think.

The effect of toxic air is more prevalent in cities that have scarcely ventilated underground rail systems. In Los Angeles, for instance, where it is mandatory to keep railcar windows closed all the time, the level of toxins in the underground air is 10 times higher than the safe limit approved by global health organizations.

Toxic dust is produced when trail wheels grind against the rails. This dust keeps getting accumulated in the subway track and train cars due to lack of proper ventilation. Regular exposure to this level of toxicity may cause liver and kidney damage, says the study.

The pollution levels were measured in LA, both inside rail cars and on train platforms. However, LA is much less crowded than other subway lines like that of New York and London. This suggests that billions of daily commuters in these cities, who spend at least 30 to 60 minutes on an average in the subway daily, are in constant danger. The findings, published in Aerosol and Air Quality Research, suggest that subway cars need more ventilation than is currently available to dissipate the toxic dust.

According to last summer's statistics, subway ridership has declined by 20 percent in Los Angeles. However, still 1.65 billion people take the metro in New York and 2.18 billion use the London tube, reports Daily Mail.

All of these people are exposed to the PAHs (polycyclic aromatic hydrocarbons) produced through the train wheel chaffing, which are capable of causing gastrointestinal, lung, skin and bladder cancer. A PAH called hexavalent chromium, caused by steel rails, cannot be prevented from entering our lungs and is extremely carcinogenic.

Such a revelation is bound to cause worry in millions of people who have chosen this public transport after learning about the harmful effects of automobile pollution. In this age, air pollution causes a large number of deaths globally. And as we know, car exhaust fumes create air pollution, increase the amount of greenhouse gases and also contribute greatly to global warming.

In such a scenario, the study suggests us to opt for above-ground trains whenever possible, as it has very less PAH emission and causes minimal pollution.