Pollution levels on London's underground rail system are so high that an hour's travel is the same as spending a whole day in traffic, new research has found.
In some Tube stations the air can be up to 50 times dirtier than on the street, with pollution particularly bad on lines that run a long way under the city, found the study, commissioned by Transport for London (TfL).
"Mass concentrations of PM at the platforms on London Underground lines are typically much higher than in ambient air," said the report, referring to the concentration of tiny poisonous particulate matter in the air.
Nearly 9,500 London residents die prematurely every year as a result of long-term exposure to air pollution, a 2015 study by researchers at King's College London showed.
London's subway is the world's oldest and some of its 11 lines and 270 stations date back to 1863 - a likely cause of high pollution as "deep, poorly ventilated tunnels" make up part of the system, the report said.
Air drawn into the tunnel network becomes contaminated by the wear and tear of railway components, such as train wheels and brake blocks, it said.
At the deepest station, Hampstead, the concentration of PM 2.5 - the smallest particulates that do the most damage because they penetrate into the bloodstream - averaged 492 over a 10-day period in 2018.
That compares with an average of just three in rural Scotland, and 16 on a busy London road. World Health Organization (WHO) guidelines advise levels should not exceed a daily mean of 25.
Globally air pollution kills about 7 million people every year, according to the WHO. Long-term exposure can affect human respiratory and inflammatory systems and lead to heart disease and cancer.
The report, published on Wednesday, said there was not enough information to assess the effects of exposure to underground pollution on commuters but health risks could not be ruled out. "Given that there is strong evidence that both long and short term exposure to particle pollutants in ambient air are harmful to health, it is likely that there is some health risk associated with exposure to underground PM," it said.
However, the authors said they did not believe travelling posed a serious risk.
Peter McNaught, TfL's director of asset operations, said the company was committed to maintaining the cleanest air possible.
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"We closely monitor dust levels on the Tube and, through a wide range of measures, ensure that particle levels are well within Health & Safety Executive guidelines," he said in a statement.
"We have already enhanced our sampling regime by including tests for additional metals and we will continue to investigate ways we can keep dust and particles to an absolute minimum."