Low cabin air pressure and poor oxygenation enhance the risk of
deep-vein thrombosis (DVT), a study says, also identifying a minority of
people who are the most prone to developing these dangerous blood clots.
DVT occurs when a clot forms in leg veins during periods of sedentary
activity. The clot can then migrate to the heart, lung or brain,
sometimes days or weeks later, and inflict a heart attack or stroke.
Even though the phenomenon has been known since World War II, it has
in recent years become a major issue for the airline industry, through
lawsuits in which DVT victims placed the blame on cramped economy
seating in long flights.
The airlines retort that DVT can occur long after a flight, which
thus makes it impossible to establish a link, and point out that
clotting can occur in other forms of sedentary activity, even from
sitting and reading a book.
The latest research, published in next Saturday's issue of The
Lancet, puts the ball back into the airlines' court, pointing the finger
at cabin air quality as a potential risk factor.
A team led by Frits Rosendaal, a professor of clinical epidemiology
at Leiden University Hospital in the Netherlands, monitored levels of a
key clotting protein called thrombin-antithrombin (TAT) complex among 71
healthy men and women aged 20-39.
Forty percent of the guinea pigs were selected as they took the birth
control pill and/or had a gene varation called factor V Leiden, both of
which are known to greatly increase blood clotting.
The volunteers were taken onboard a specially-chartered Boeing 757
for a non-stop eight-hour flight, which cruised at at around 11,000
feet) with a cabin pressure that corresponded to an altitude of
1,800-2,100 metres (2,600-6,825 feet).
Blood samples were taken before, during and after the flight.
A couple of weeks later, the same group underwent the same tests, but
this time at a movie marathon, watching comedy and action films for
They were not allowed to drink alcohol, take aspirin or wear elastic
stockings during the flight or the movies, and were asked to remain
seated as much as possible.
The group was monitored for the final time during eight hours of
regular daily activity.
After the flight, the concentration of TAT complex rose by 30.1
percent on average across the group. It fell by 2.1
percent after the
cinema, and by7.9 percent after the "daily life" experiment.
The post-flight rise was overwhelmingly concentrated in just 11 of
the 71 individuals, especially those who took oral contraceptives and
also had factor V Leiden.
The findings suggest that "flight-associated factors" are the cause
behind increased clot formation after a long trip, say the team.
They put the ring of suspicion around hypobaric hypoxia -- a
combination of low cabin pressure and a low oxygen level compared with
terrestrial air conditions.
The data also provide a useful pointer for medical help to those most
There already exists an array of simple techniques or
over-the-counter drugs for combatting DVT.
These include cutting out alcohol on a flight, stretching one's legs,
taking an aspirin and wearing compressive stockings that improve blood
There is also a powerful anti-clotting prescription drug called
This medication has side-effects, which means it should be targeted
at those most at risk, such as women on the Pill and with factor V
Leiden, cancer patients and recent surgery patients, said Hans Stricker,
a doctor at the Charity Hospital in Locarno, Switzerland, in a
Previous studies have found a two-to-fourfold risk of DVT after air
travel, while a study in 2003 estimated that DVT may occur among one in
every 100 frequent long-haul air travellers, in business class as well