People may think that they are doing the healthy thing by taking the subway daily, walking to and from the train, but a new study suggests that it may pose a large health risk.

There has been considerable research on the effects on workplace air quality where metal dust is prevalent in the air, but little has been studied of the environment that people in every city in the world are exposed to in the subway system.

A new study published in Environmental Science and Technology looked at small particulate matter that is in underground transit systems is different from dust in the air at street level.

"We studied the ultrafine dust (or particulate matter) found in an underground station in Europe. Typically, ultrafine dust is composed of inert matter that does not pose much of a risk in terms of its chemical composition. However, in the underground station we studied, the ultrafine dust was at least as rich in metals as the larger dust particles and therefore, taken together with their increased surface area to volume ratio, it is of potential significance in understanding the risks of working and travelling in the underground. These tiny dust particles have the potential to penetrate the lungs and the body more easily, posing a risk to someone's health," explained Matt Loxham, PhD student at the University of Southampton.

Ultrafine dust created from the trains' steel on steel movement contains particles that are so small they can go deeper in the lungs than larger dust particles found elsewhere in the environment. Larger particles can be trapped and sequestered in nasal passages.

These super small dust particles can also infiltrate the top layer of lung tissue and enter the bloodstream where they can travel throughout the body and enter the brain, liver and kidneys.

"Underground rail travel is used by great numbers of people in large cities all over the world, for example, almost 1.2 billion journeys are made per year on the London Underground. The high level of mechanical activity in underground railways, along with very high temperatures is key in the generation of this metal-rich dust, and the number of people likely to be exposed means that more studies into the effects of particulate matter in the underground railway environment are needed, as well as examining how the levels of dust and duration of exposure might translate to effects on health," Mr Loxham concluded.

The report published in the journal Environmental Science and Technology can be found here.