Individuals involved in the aircraft industry, either as workers or passengers, appear to be particularly prone to certain illnesses. Passengers, flight attendants and pilots appear to be prone to deep vein thrombosis (DVT), passengers to ‘air rage’, female flight attendants to temporal lobe (brain) damage, and pilots and aircraft maintenance crew to bloodclotting.

Research (Scurr et al. 2001) showed that, in 5% of people with no known risk factors (age, pregnancy, previous history of clotting, etc.) even single long-haul return flights (average length 24 hours) caused small or 'silent' blood clots. A short-haul trip (3-4 hours) caused clots in1-2%. All those found to have these potentially fatal clots were treated with heparin (an anticoagulant) and referred to their doctor.

Infrasound
Largely ignored by current research into the causes of DVT, exposure to the infrasound made by aircraft engines may be the underlying factor. It is known to cause damage at cell level and has been linked to changes in serotonin production in the brain, a tendency for blood coagulation, and neurological disorders, including spontaneous rage. The temporal lobe damage noted above may be due to this neurological damage.

Infrasound is sound below twenty pulses a second and is inaudible to the human ear. It may also be characterised as low/high air pressure change pulsing below twenty pulses a second. It is produced by jet engines (much effort has been made to reduce this by lining the engines with lead) and can cause the air cabin, which acts like a gigantic organ pipe, to resonate. In fact, a 30 metre cabin attached to any strong source of sound between 5 and 250 hertz would tend to pulse at around 6 hertz. Longer cabins would resonate at lower speeds.

Research on aircraft passenger compartments have not found evidence of resonance, but measurements were not continuous and thus may have missed intermittent resonance. Intermittent resonance may explain the dips in blood oxygen levels noted in a separate study (see below). The absence of resonance does not mean that passengers and aircrew are not exposed to infrasound.

Research from the Eastern bloc identified “intense infrasounds from arc furnaces”. Other research (Scholy, 2002) found that “exhaust systems are one of the most important sources in modern turbine power plants (where) acoustic resonance can occur, producing very high sound-pressure levels, usually at low frequencies”. Denmark, Norway, Poland and Russia have specific workplace infrasound guidelines or regulations.

Immobility and age
Immobility for a long period is favoured as the main cause of DVT, especially for older people, but this may not be the major factor. Flight attendants are relatively mobile but suffer from DVT, and some young athletes have also succumbed during or after a long flight. Deep sea divers appear particularly prone. Millions and millions of non-flyers (e.g. office workers, people in care, in old people's homes or in a coma) are relatively immobile day after day for many years but do not develop DVT.

Reduced air pressure
Reduced air pressure leading to low blood oxygen levels resulting in an increased tendency for clotting appeared to be a strong candidate as a cause for DVT until research showed that people living at high altitudes did not suffer particularly either from low blood oxygen or DVT. Attention turned to the effect of sudden changes in air pressure. Research which exposed individuals to precisely what they would experience in an aircraft cabin found a 5% reduction in blood oxygen levels across two hours (their blood was tested every 30 minutes).

Another change in blood chemistry (called tissue factor pathway activation), however, was marked during the thirty minutes after take-off. A separate study noted an average 3.1 dips of up to 10% in blood oxygen levels during long-haul flights and 3% dips in shorter flights, suggesting a cumulative effect. Pilots' risk of DVT also appears to be cumulative in line with flight hours logged (but not to age).

Other transport
Trains and coaches are also large acoustic chambers. With the windows open they can generate significant levels of infrasound, as can even cars. In one instance, even a car at cruising speed with the windows closed generated 90 decibells of infrasound. This, rather than immobility, may explain certain categories of car drivers' susceptibility to DVT, particularly on journeys exceeding four hours, as noted by the RAC.

The size of the problem

• A round a million (5%) of the people carried on European airlines each year develop DVT. [1]
• The organisation AirHealth has collated 21 medical reports on people using US airlines. There were a million diagnoses of DVT each year, resulting in 100,000 deaths. The chances are that DVT deaths from all causes worldwide exceed three million. [2]

Ed.- Late in December 2002, DVT victims lost their legal battle for compensation from the air companies on the basis that DVT was not "an accident", the only event for which aircraft operators are liable under the outmoded but still extant 1929 Warsaw Convention. British Airways stated that they had shown videos to passengers on long-haul flights encouraging exercise since 1993. Virgin Airways restated that there was no proven link between flying and DVT (which is true).

See also BSE, infrasound and deep vein thrombosis - Is there a link? and Windfarm infrasound brings migraine and depression