According to The Times in their article 'All forms of travel can lead to blood clots':
'BLOOD clots are linked to many different modes of transport, not just flying, according to a study.
Travelling for more than four hours by air, car, bus or train can all increase the risk of blood clots, with air travel no worse than the others, researchers said yesterday.
A team in the Netherlands studied 2,000 people who had suffered a venous thrombosis — a blood clot in a vein — for the first time. They found that 233 of them had travelled for more than four hours in the eight weeks preceding the event.
Travelling doubled the chances of having a venous thrombosis. The hazard was greatest in the first week after travelling, and the overall risk of flying was no worse than that of going by car, bus or train.'
Mobility it seems does come with a price - another aspect of the 'burden of mobility'? Is it worth the blood-price??