“On any given weekday, the London Underground sees up to five million passengers hopping on and off its network. Its 11 lines serve 272 stations, and at peak times there can be over 500 trains hurtling around beneath the streets of London… But how fresh is the air that you’re breathing on the tube? ‘The air, even before it gets to the Underground, isn’t perfectly clean.’ explained Dr David Green, senior research fellow at MRC Centre for Environment and Health, Imperial College London and a member of the UK’s Committee on the Medical Effects of Air Pollution (COMEAP). Green is also part of a group commissioned by TfL to regularly assess the COVID-19 risk on the Underground. ‘The urban background air already has a low level of particulate matter, but on top of that you have all these extra emissions (coming from the tube).’ Dr Green talks to BBC Science Focus.