A study (published in ther BMJ I think) about 18 months ago showed that approximately 10% of passengers on long haul flights end up with some form of clot in their lower limbs.
I haven't read the BMJ in years, but was that the 10% have clots, 10% of those extend above the knee, 10% of those break off and 10% of those cause nasty problems?
A cardiologist friend of mine was in talks with a large airline to do some more definitive studies on this. This would involve nurses at check in selecting & consenting passengers, and taking blood. At the other end of the long haul flight, nurses would question the passenger, take more blood and relevant investigations performed.
The airline was very interested in this, but really wanted it to prove that the problem existed in all classes of service, and also wanted it to prove that the same problem existed in car/rail/bus travel. Last I heard the airline was plucking up the courage to agree to the trial, but petrified that the study would show that those nasty economy seats might be blamed.
I think that hurdle was overcome and more definitive studies are being done.