A new study in The Lancet finds 1% of air travelers developing injurious blood clots. Following 878 air travelers for three months after their flights, Rodney Hughes (et al) found four developed potentially fatal pulmonary embolism and five developed deep vein thrombosis (DVT) which, more often than not, leads to disabling chronic swelling and pain.

A surprise in this study was that four of the nine were wearing compression stockings. Previous studies have found compression stockings to be effective in preventing blood clots during air travel. Frequent flexing of the legs, even while seated, is recommended to reduce chances of clotting.

Airhealth.org has developed a registry of nearly two hundred cases of blood clot injuries and deaths caused by air travel. Endurance-type athletics turns out to be a major risk factor for ECS, due to the slower resting blood flow and tendency to have bruising and similar conditions that can trigger clotting. Another unexpected finding from the case registry is that short flights of three to five hours cause blood clots.

A new study by Gianni Belcaro (not yet published) finds 4.3% of travelers on short flights developing blood clots, about the same percentage as on longer flights. Belcaro's LONFLIT series of studies uses ultrasound scans before and after air travel, looking for blood clots of .5 cm or larger, large enough to be injurious. Most of these dissolve naturally, accounting for the difference between Hughes' finding of 1% and Belcaro's finding of 4.3%.