Potentially dangerous dust is building up on London Underground (LU) because its only vacuum train is broken.
The dust could damage organs
The Tube's network of tunnels have not been cleaned for six months and dust is building up in its deepest sections.
Dust made up of oxides of iron and silicon as well as particles from hair, skin and the street is leaving a residue on ventilation systems on the Northern Line.
And one driver who took a mask to work was told to remove it, or face disciplinary action.
He told BBC London: "Managers are worried drivers wearing masks will send the wrong signal to customers - but it is a truthful signal and it is that truth we are trying to get out."
Iron has been linked to heart and liver problems and silica to lung disease.
The important thing is we have got to come up with the best method of collecting the dust
But although LU admits dust levels are rising, it says exposure levels will not reach dangerous levels.
LU's health adviser Dr Olivia Carlton does not believe dust in the Tube to be dangerous, but has asked the Institute of Occupational Medicine to investigate further.
One solution could be a fleet of mobile vacuum cleaners, prototypes of which are currently being developed.
Martin Brown of Tubelines, one of the private companies who have taken over responsibility for Tube maintenance, said: "The important thing is we have got to come up with the best method of collecting the dust.
"We think this may be a much better method than the tunnel-cleaning train because it allows us to take out a much lower level of dust particle. That is a big improvement on what we can currently do."
'Like smoking a cigarette'
A working model of the Tube vacuum cleaners is expected to be ready in a few months.
In October 2001, scientists from University College London warned 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.