We all know that exhaust fumes from traffic can cause high levels of air pollution at street level but what about the air we breathe underground?

More than a billion journeys are made on the tube every year. To ensure the air is safe to breathe its monitored everyday either on the platforms or on monitors worn by staff.

These monitors collect the very fine particles which are likely to go deep into the lungs. Samples are weighed and measured against health and safety standards to ensure they're not at dangerous levels.

Samples are measures against health and safety standards. Credit: TfL

What you're breathing in is iron and the iron is formed from the friction action of the brakes on the wheels and the wheels on the rails of our tube trains. The other thing which is in the dust is as a result of the number of people who use the tube so we get a lot of rubbing off of fibres of clothes of skin scale and of hair. Dr OLIVIA CARLTON, TRANSPORT FOR LONDON

Overnight platforms and tunnels are deep cleaned to keep dust levels down but they never get fully clean.

Tube platforms are cleaned overnight. Credit: TfL

Transport for London say there is nothing to worry about breathing in the dust.

The dust on the underground is nuisance but it is not harmful to health and we've actually done a lot of research and we've invited researchers with a lot international reputations and they have investigated and reassured us that it's highly unlikely to be harmful to health. Dr OLIVIA CARLTON, TRANSPORT FOR LONDON

Credit: TfL

Whilst it may not be pleasant, the best science has said the tube's dust is probably safe.