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.
Overnight platforms and tunnels are deep cleaned to keep dust levels down but they never get fully clean.
Transport for London say there is nothing to worry about breathing in the dust.
Whilst it may not be pleasant, the best science has said the tube's dust is probably safe.