Matt Loxham, a doctoral student at the University of Southampton, and colleagues, report the results of their investigation of a European underground station in Environmental Science and Technology.
They studied the ultrafine dust or particulate matter in the air of the underground station and found it was quite different to that which most people breathe in every day: it was more like the that found in steel mills and welding plants, where there is a much higher proportion of airborne metals.
Loxham tells the press in a recent statement:
"Typically, ultrafine dust is composed of inert matter that does not pose much of a risk in terms of its chemical composition."
But, in the underground station they studied, he says, "the ultrafine dust was at least as rich in metals as the larger dust particles".
And because these particles are much smaller than the larger dust particles, when you add them all up, for the same amount of metal contained, they present a much larger surface area for contact compared to their volume than the larger particles, one reason that makes them potentially more damaging to health. Another reason is, because they are smaller, they can penetrate further into the lungs and body.
Coarse, Fine and Ultrafine Particulate MatterThe dust in the air comprises differently sized particles or particulate matter. For studies these tend to be graded as coarse, fine, and ultrafine.
Coarse particulate matter contains the biggest particles with a size between 2.5 and 10 micrometers, or about 25 to 100 times thinner than a human hair. This grade of dust is called PM10 (short for Particulate Matter up to 10 micrometers in size).
Fine particulate matter comprises particles smaller than 2.5 micrometers (PM2.5), and ultrafine particles are smaller than 0.1 micrometers (PM0.1).
The authors write in their background information that:
"Compared to coarse (PM10) and fine (PM2.5) particulate fractions of underground railway airborne PM, little is known about the chemistry of the ultrafine (PM0.1) fraction that may contribute significantly to particulate number and surface area concentrations."
We know that coarse particles (PM10) generally don't get further into the body that the nasal passages and the bronchi, while fine dust (PM2.5) reaches the smaller airways (the bronchioles).
Ultrafine Dust Reaches Deep Into BodyBut ultrafine dust (PM0.1) reaches the deepest areas of the lungs, and get right into the alveoli, the little sacs where oxygen transfers to the bloodstream, and waste gases pass the other way for breathing out.
There is also a suggestion that ultrafine dust can penetrate the protective epithelial barrier that lines the airways and get into underlying tissue and the bloodstream. If this is the case, then the harm that any toxins in the ultrafine dust pose is not limited just to airways but may also cause damage to the cardiovascular system, liver, brain and kidneys.
Previous studies have already shown that working in heavy industries like steel mills and welding plants potentially exposes unprotected workers to airborne metals like iron, copper and nickel, which can harm health.
The large amount of mechanical activity in an underground railway combined with a high ambient temperature creates a lot of metal-rich airborne dust.
But there is little understanding of what harmful agents might be in the air of an underground railway, never mind the health risks it might pose. The few studies that have been published tend to be inconclusive.
Dust in the Air of a Mainline Underground Station In Europe Found to Be Rich in MetalsFor the study, geochemists, toxicologists and environmental scientists collected and analyzed airborne dust from a mainline underground station located underneath an airport in Europe.
They compiled detailed profiles of the metal content of the dust they collected. They then compared these profiles to other profiles collected at the same time from different environments, such as samples from wood-burning stoves and a road tunnel containing heavy traffic.
The comparisons revealed that the underground particles were very rich in metals, especially iron and copper.
The researchers also looked at the shapes of the particles, to discover clues about how they are generated.
Plus, they found that the particles were capable of producing reactive molecules that play a key role in their toxicity. This depended on the type and amount of metal they contained, but toxicity tended to increase as particle size got smaller.
They are now doing further tests to look at the effect of the dust on airway cells, and what mechanisms cells might use to guard against any potential damage.
"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," says Loxham, who calls for further studies to find out more about the airborne particulate matter in underground railways and how varying degrees of exposure to it affects health.
Funds from the Integrative Toxicology Training Partnership studentship provided by the Medical Research Council UK financed the study.
In another recent study, researchers found that PM2.5 air pollution is linked to faster thickening of the carotid artery lining, a reliable marker for atherosclerosis or hardening of the arteries, a major cause of heart disease.
Written by Catharine Paddock PhD