Data has shown that people using the London Underground can inhale more than 12 million toxic particles a minute, according to Transport for London (TfL) data.
According to The Sunday Times, the “nanodusts” are mostly iron oxides, from the grinding of wheels and rails in braking, or blown from the tracks by the air movements of trains. They are chemically different from road pollutants — but just as toxic.
The extent of the pollution was discovered by monitors placed around the network by TfL.
Originally TfL refused to release the data until a freedom of information request was submitted by The Sunday Times.
A monitor on the Central line showed particle levels surging with train movements, with peaks exceeding 2 million particles per litre of air. Adults breathe 6-10 litres of air a minute and so would inhale 12-20 million particles.
More than half the particles were so small they could pass through the lungs or nasal membranes to enter people’s organs and brains.
On other Tube lines, TfL measured the mass of particles per cubic metre of air. On the Northern line it recorded daytime peaks ranging from 250 micrograms to 1,000 micrograms, with similar levels on the Piccadilly and Victoria lines.
This compares with World Health Organisation (WHO) guidelines for air pollution exposure — the basis for UK rules — which say humans should not be exposed to more than 50 micrograms of particles per cubic metre, averaged over 24 hours.
TfL said that, as passengers and staff did not spend all day in the Tube, their daily exposure would not breach health rules.
It also said that city dwellers were exposed to metal particles and pollutants from other sources, such as diesel vehicles.