A major inquiry is to be launched into the health risks to commuters from dust-filled air on the Tube.

The move comes after scientific research showed London Underground passengers were breathing potentially dangerous levels of dust.

Researchers discovered air quality in carriages and at stations was up to 73 times worse than at street level and a 20-minute journey on the Northern line through central London had the same effect as smoking a cigarette.

Scientists from University College London warned commuters last October that they were breathing in dangerous levels of dust. Using pollution monitoring equipment, a team found readings for toxic particulates or dust particles in the air on the system were up to 30 times the safe levels recommended by the World Health Organisation.

The team took measurements at Warren Street, a deep level station on the Northern line, and nearby Tottenham Court Road and found 996 parts of dust per million - 70 times that found in an average office.

Environmental scientist Dr Ben Croxford, who led the UCL team, said: "The Northern line had the worst readings, probably because it is the deepest line. Passengers and staff are inevitably going to be breathing in a lot of dust."

The findings prompted the Liberal Democrats to call for immediate government action to tackle the problem, which sees passengers breathe in dust particles which include ground-up steel, concrete and quartz.

Malcolm Bruce, the party's environment spokesman, called the situation "a disgrace" adding: "Not only do Tube passengers suffer massive overcrowding, regular delays, smelly and poorly maintained carriages, graffiti and large annual fare rises, but travelling on LU is bad for their health as well."

He said it was 23 years since the extent of air quality problems on the Underground had first been understood but no government had acted on the issue. "You don't have to be a genius to realise breathing dust and dirt into your lungs is bad for your health," he said. "Given that 150,000 passengers enter the Tube system every hour, enough to fill Wembley Stadium twice over, it is staggering that the Government has no idea about the impact of poor air quality on passengers."

Today London Underground said it had ordered its own research into the issue, which will investigate air quality throughout the system. It is the first time the potential impact on passengers has been investigated.

A spokesman said the system "constantly monitors tunnel dust and its possible health risks" and added that previous investigations into the health of Tube workers had found "no measurable health effects".

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

A report from the Council of Europe's Committee on Science and Technology showed microscopic particles in particular were a "major" health risk. "They penetrate directly into lungs and cause allergies and cardiovascular and respiratory diseases," the study found.

London Underground said dust levels had fallen in tunnels since new braking technology was introduced. A spokesman said one of the main causes of dust was particles produced when brake pads rub against wheels and new technology had eliminated harmful material in the brake pads.