Various people have varying degrees of predisposition to DVT. Things that particularly increase the risk are any previous DVT, pregnancy and cancer. A lot of people wander round life with a mildly increased risk from various genetic causes and a longhaul may be enough to trigger it off. The "Pill" also mildly increases the risk.
They occur because gravity, in flight swelling, no movement and the kink in the veins with the knee bent on the seat dilate the veins and slow the flow in the vessels. The thombus starts in an area of blood stasis. It may then grow, and it's the bits that break off and end up impacting in the lung that are potentially fatal (called pulmonary embolism:PE). By no means is this problem confined to the air or steerage class.
I've been wondering for a while now when someone would bring up the keep your seatbelt on/ move around trade off. Effectively what happened is we trade off M&M from clear air turbulence with DVT/PE. Not something I would be arguing that one could run a suit on, but I guess that the class action lawyers will find a more cooperative "expert opinion".
Email me if you've specific Qs or want some indepth reading: i'm finishinng off some research into a rare cause of the things
