Matt Loxham, PhD student at the University of Southampton, explains: "We studied the ultrafine dust (or particulate matter) found in an underground station in Europe...It was at least as rich in metals as the larger dust particles...These tiny dust particles have the potential to penetrate the lungs and the body more easily, posing a risk to someone's health."
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 (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 adds: "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."