London Underground insists passengers and staff are not at risk from dust-filled air on the Tube.

The company's assurance comes on Friday after the Liberal Democrats questioned air quality on the Underground following research carried out last year.

Scientists from University College London warned last October that air quality in carriages and at stations was up to 73 times worse than at street level.

They said a 20-minute journey on the Northern line through central London had the same effect as smoking a cigarette.

Exposure levels

London Undergroung (LU) said on Friday: "London Underground constantly monitors tunnel dust and its possible health risks.

"Past extensive investigation of employee health in relation to exposure to tunnel dust showed no measurable health effects.

"Exposure levels to tunnel dust are much lower than the standards set by the Health and Safety Executive."

LU said it was working to understand better all the evidence available by commissioning an independent review of all the data on tunnel dust.

The review is being carried out by the Robens Centre for Occupational Health and Safety.