These were then compared to profiles from other dusts analysed at the same time. These included dust from wood-burning stoves and a heavily-trafficked road tunnel, and showed 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, importantly, they showed the dangerous effects were greater as the size of the individual particles decreased.
While coarse dust generally sticks to the nasal passages and windpipe, fine dust 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.
‘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 [dust] in the underground railway environment are needed.'World Steel Tube Report
Follow us: @MailOnline on Twitter | DailyMail on FacebookMr 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.
But Howard Collins, London Underground Chief Operating Officer, said: 'We have carried out monitoring of dust levels on the Tube for many years to re-assure our passengers and staff. World Steel Tube Report
'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.World Steel Tube Report
“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.'World Steel Tube Report