Afraid of flying? The good news, according to David Newman, aviation medicine specialist and head of aviation research at Swinburne University in Melbourne, is that almost everybody who travels by air will tolerate the flight reasonably well.
The main exceptions were people who had a pre-existing illness that deteriorated and would probably do so whether they were in the air or on the ground. About 5 per cent of the total number of people who fly each year have a chronic illness.
"The basic rule of thumb is that if anyone is suffering from an unstable medical condition, it's not going to improve by going flying," Dr Newman said.
"With the addition of hypoxia and maybe a little bit of disorientation due to time zone shifts, they may actually get worse."
Any significant lung disease, heart disease or mental illness, that was untreated and unstable, was potentially going to be a problem at altitude.
If patients are stable and under medical care, they will probably be OK, according to Dr Newman, but it is still worth their while consulting their doctor, the airline medical staff or an aviation medicine specialist to check.
"You really don't want to get worse at altitude a long way from care," he said.
"As a general rule, if someone can carry their own luggage and navigate their way in and throughout the airport to the gate without significant shortness of breath, that gives a clue that they are probably going to tolerate the flight OK."
Then there are people who don't heed medical advice and hop on a plane when they shouldn't.
These include people who have had certain types of surgery or any penetrating injury and fly before the requisite waiting period of 7-10 days. In some operations, such as abdominal surgery or an appendectomy, air may enter the cavity and will take 7-10 days to be absorbed by the body.
"If after two days (post-surgery) you feel fine and decide to fly off on a holiday . . . air that is still in your abdomen can expand due to the low atmospheric pressure," Dr Newman said.
"At 8000 feet you get a 30 per cent increase in the volume of gas, so that can cause mild to moderate or severe pain and it can do damage to the surgery recently done. In people with an eye injury, if there is any air in the eye, the air can expand due to the same mechanism and cause problems."
The average passenger on a plane, which may be cruising at a height of 30,000-40,000 feet (9000-12,000m) is exposed to an internal altitude on the plane of 5000 to 8000 feet. "That is clearly higher than most people live," Dr Newman said.
"What we know is that once you get above 10,000 feet, the lack of oxygen tends to impair performance significantly and the higher you go, the greater that effect is.
So if you have people at 39,000 feet in an unpressurised aeroplane, their time of useful consciousness would be about a minute, probably less." The cause is hypoxia, or lack of oxygen.
Because a plane performs best aerodynamically at an altitude of about 40,000 feet, but people don't, some of the engine power is used to pressurise the craft to an altitude at which the passengers will survive.
In the event of an explosive decompression of the plane, such as occurred in 1988 during a Boeing 737 flight in Hawaii when the roof of the first-class section ripped off, the plane must descend immediately to an altitude of 10,000 feet or less.
During the descent, the passengers breathe using the emergency oxygen masks that are automatically deployed. In the case of the Hawaii emergency, the only fatality among the 95 passengers and crew was a flight attendant who was blown out of the plane.
Dr Newman said there was some evidence that prolonged exposure to a pressurised environment such as a long-haul flight was fatiguing, mainly because of the low level of hypoxia that occurred at 8000 feet.
It was also dehydrating because the air in the cabin was dry and time zone changes could be disruptive, all adding up to "jet lag".
His research into data from the Australian Transport Safety Bureau on in-flight injuries and medical problems among passengers in the 30 years to March 2006 shows there were only 284 reported medical events, or fewer than an average of 10 a year.
"Given that two billion people fly on the world's airlines every year, it is a very, very safe environment," he said.
With regard to deaths on board a plane, there is about one a week globally. The most common cause is heart attack, followed by stroke, but other causes include a pre-existing illness, such as terminal cancer, which the passenger finally succumbs to during the flight.
"There is some stress in air travel these days, mainly to get to your seat on the aeroplane so it's hard to say whether that contributes to it or not," Dr Newman said.
"Getting from home to the airport, making sure you are not late, checking in, fighting your way through security, especially depending on what country you are in, a busy airport, finding the gate, lugging your luggage and getting it in is all quite mentally and physically taxing, particularly if someone has already got, maybe, heart disease. It can trigger a terminal episode."
Deep-vein thrombosis is also a potential problem. "If you have prolonged immobility, dehydration and cramped conditions, wherever that occurs, potentially you are at risk of a clot," Dr Newman said.
Other illnesses that flare up on board include air sickness and gastro-intestinal disease while physical trauma from objects falling from overhead lockers can occur. There have also been cases of unbuckled passengers being flung out of their seat during turbulence and sustaining head injuries or broken limbs.
Most airlines had the capacity for tele-medicine, allowing the cabin crew to contact doctors on the ground for specialist advice to deal with mid-air crises.