In the first half of this year, nearly as many people died in flight on British airlines as in the whole of 1990. The numbers are small (24 deaths in 1991), but the upward trend is being blamed on the growing armies of elderly people who fly around the world. Also at risk are those business travellers who collect air miles in much the same way as the rest of us collect petrol tokens.
'People ought to think very carefully before they put Granny on a long flight,' Dr John Mills, an aviation expert, warns. 'It's a hostile environment. Firstly, there is less oxygen in the air and secondly there are pressure changes.'
Few people realise that when they sit in a pressurised aircraft they are still living at altitude. The amount of oxygen in the air you breathe in flight is the same as that on a 6,000-8,000ft mountain. Most people won't start getting tired and breathless until they try walking about at 10,000-12,000ft. But those with any kind of heart disease, or a lung condition such as bronchitis or emphysema, will begin to notice the difference much sooner - at the oxygen levels found in aircraft.
Remarkably little research has been done on the effects of reduced oxygen levels on long-haul flights. Dr Mills believes that even healthy individuals may have difficulty concentrating after hours of breathing slightly less oxygen than they are used to, and people with some mental impairment are visibly affected.
'You frequently see elderly people getting off a flight from Australia in a thoroughly confused state. They were probably a bit mixed-up when they left home - but by the time they get here, they don't know where they are. It's reversible, of course, and they'll feel better after breathing normal oxygen levels for a while.'
'Dizzy spells' among passengers are the commonest problem reported by British Airways crews, followed by diarrhoea and hysteria. In 1990/91 crews had to deal with 25 passengers having heart attacks. Farrol Kahn, author of a new book Why Flying Endangers Your Health (Aurora Press, pounds 9.95), believes that the death and illness figures reported by airlines are an underestimate, because they can't take account of people who die in the weeks after their journey. 'Many people fly who, knowingly or unknowingly, have heart or lung disease, cancer or other serious conditions,' he says. 'I believe that flying can push them into a critical phase of their disease which may prove fatal.'
Anyone who has had a heart attack is advised not to fly for 10 days to six weeks after their attack, depending on its severity. If you have had a major haemorrhage, such as a bleeding stomach ulcer, you should not fly for three weeks; if you have had an operation, you should stay on the ground for at least 10 days afterwards (although experts admit that all these figures are quite arbitrary). Sitting on a plane for many hours - especially in economy class where the leg space is so cramped - makes the blood pool in the legs. Blood clots may form in the veins and later lodge in vital organs like the heart, lungs or brain.
Pregnant women should not take a flight lasting more than four hours after their 35th week - or a shorter flight after 36 weeks. Flying too soon after an operation could be very painful. Air is inevitably trapped inside the body during surgery and takes time to find its way out. Pressure changes in an aircraft make any air in the body expand, possibly even bursting the scar.
Much more commonly, pressure changes in an aircraft cause burst eardrums among people who fly when they have a cold. Dr Jim Dunlop of British Airways explains that, as an aircraft goes up, the pressure inside the cabin falls below that behind the eardrum - so air must escape down the tube between the ear and the nose to equalise the pressure on either side of the eardrum. This is what happens when your ears 'pop'.
During descent the reverse needs to occur, so that air goes back up the tube from the nose into the ear. If you have a cold the air cannot get back into the ear, the eardrum is sucked inwards and your ears won't 'pop'. The pain is only relieved if the eardrum actually ruptures. Fortunately, it will heal. But people with bad colds should think twice before flying - or use a nasal decongestant before descent.
They should also, of course, consider the bugs they are passing on to their fellow passengers. It is illegal to fly with a contagious disease, and that includes measles and chickenpox. But it is hard to keep people with colds and flu off planes. The result in many cases is 'sick plane syndrome', with passengers experiencing many of the flu-like symptoms suffered by people who work in so-called sick buildings.
Most of the air you breathe on a plane is recycled and, to save fuel, many airlines take in less fresh air than they used to. The result, especially in the smoking sections of aircraft, is an unpleasant fug. Airlines actually have to scrape out the tar in the air-ducts which builds up from cigarette smoke.
If all this isn't enough to put you off flying, there are also the better- known dangers of enjoying yourself on a long-haul flight: drink yourself into a stupor to pass the time and you'll wake up with the worst hangover of your life. Everyone gets dehydrated in the air and alcohol will make this much worse. The effects of overeating are also worse than they are on the ground.
There is very little you can do to prevent jet lag. If you decide to take a sleeping pill, don't drink alcohol or you could feel confused when you wake up. The best advice is to switch to your new time zone as soon as you arrive and try to resist the desire to sleep if it's still daylight.
Dr Mills believes that we expect far too much of our bodies, and that frequent business travellers in particular are pushing themselves too hard. At the very least he recommends an annual medical check for high blood pressure, abnormal heart rhythms and breathing problems.
Clearly, the fat, flabby and under-exercised are at greatest risk. The stress of flight delays and packed airports could be enough to give them a heart attack before they even get on a plane. Bon voyage]-