Like the best Tube myths, it's got an element of believability about it (the one about semen & rats droppings being found on Tube seats meant the London Underground had to make a statement debunking it). However, a blog post from Nature network has now refuted the smoking myth (thanks to Ian for spotting this)
"The myth originated from work done by Dr Ben Croxford, a researcher at University College London who at the request of a journalist carried out an estimate of the weight of particulate matter inhaled when smoking a cigarette. Croxford subsequently calculated that a person would have to spend 20 minutes in the most polluted part of the London Underground to breathe in the same amount of matter.
However, the calculation only made a comparison of the weight of matter breathed in and Croxford’s research never intended to suggest that breathing in Underground dust was as harmful as smoking cigarettes. The subsequent interpretations of this data by the media is a classic example of how lying can be easily achieved with statistics and how scientists must be careful that their data does not mean the lay-reader draws an incorrect conclusion."
This made me think about Tube snot myth though, as a number of people have said that if you spend a length of time in the Tube (let's say 25 minutes) & blow your nose on leaving, you get much darker deposits on your hanky. All we need now is another scientist with large box of Kleenex and some willing commuters to put the theory to the test in the bowels of the system. Any volunteers?!