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Travelling on the Tube could be bad for your health because of tiny metal dust particles in the atmosphere, new research has claimed.
Experts found that the small dust particles in the air, or particulate matter, in an underground railway were quite different to the dust breathed in every day and that could have health implications because they could penetrate the lungs and body easily including the liver, brain and kidneys.
The research team called for more work to be done to discover what the risks were as the finding could mean that working or travelling on an underground railway for a sustained period is bad for health.
Matt Loxham, PhD student at the University of Southampton, said:
"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."
Previously published work suggested that working in environments such as steel mills or welding plants, which are rich in airborne metals, like iron, copper and nickel, could have damaging effects on health.
But little research has been done on the effects of working in an underground railway environment - a similarly metal-rich environment - and results of studies that have been conducted are often inconclusive.
While coarse dust is generally deposited in the conducting airways of the body, for example nasal passages and bronchi, and the fine dust generally can reach the bronchioles or smaller airways, it is almost exclusively the ultrafine dust which is able to reach the deepest areas of the lungs, into the alveoli, where oxygen enters the blood and waste gases leave, to be exhaled.
There is evidence that this ultrafine dust may be able to evade the protective barrier lining the airways (the epithelium), and enter underlying tissue and the circulation, meaning that the toxicity of ultrafine particles may not be limited to the airways but may involve the cardiovascular system, liver, brain, and kidneys.
Mr Loxham added:
"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."
Researchers initially collected airborne dust from a mainline underground station underneath an airport in Europe.
The metal content of the dust was analysed and a detailed elemental profile was established for each dust sample.
These profiles were then compared to profiles from other dusts analysed at the same time, for example dust from wood-burning stoves and a heavily-trafficked road tunnel, showing that underground particles were very rich in metals, especially iron and copper.
The team then showed that the dust was capable of generating reactive molecules which are fundamental to their toxic effects, and that this was dependent on the metal content of the particles and, importantly, occurred to a greater extent as the size of the individual particles decreased.
Further work is now being undertaken to examine the effects of underground dust on airway cells in more detail and the potential mechanisms by which cells may be able to protect themselves.
The study was funded through the Integrative Toxicology Training Partnership studentship provided by the Medical Research Council UK and published in Environmental Science and Technology.
London Underground chief operating officer Howard Collins said:
"We have carried out monitoring of dust levels on the Tube for many years to reassure our passengers and staff. That research has consistently shown that mineral levels within the dust are perfectly safe and that dust levels are less than a third of the limit set by the Health and Safety Executive for general dust.
"That standard remains the level required by the Health and Safety Executive and would be amended if they felt there was any danger to our passengers or staff.
"This is supported by an independent report from the Institute of Occupational Medicine, which concluded tunnel dust on London Underground is highly unlikely to be dangerous.
"It is clearly wrong to attempt to draw any conclusions about London Underground from this new research that studies one station elsewhere in Europe."