As the car trundles slowly towards the summit, the thoughts of the foolhardy thrillseeker ascending the Oblivion rollercoaster at Alton Towers in Staffordshire tend to dwell on the more fundamental questions of life: its transience, its fragility - and how, under the wrong circumstances, a vertical drop at high speed from a very great height can bring it rather swiftly to an end. Once the car has reached the precipice, though, there's no time to think: screaming participants are plunged face-first into a black hole at 70mph, their bodies withstanding forces reaching four-and-a-half times that of gravity. It takes several moments, at the end, before paralysing terror is replaced with a surge of relief, exhilaration and an overpowering smugness towards those who opted to wait on the ground instead.
In recent months, though, a hand ful of riders on some of the world's most stomach-churning rollercoasters in the US and Japan have been reporting some more disturbing sensations: persistent headaches, symptoms of blood clots in the brain which, if left untreated, can cause brain damage, seizures, and even death. This month, America's consumer product safety commission put at 16 the number of reported cases of brain damage believed to be related to the extreme g-forces experienced on white-knuckle rides. In one of the most serious incidents, Japanese researchers reported that a 24- year-old woman had been diagnosed as suffering from subdural haematomas - ruptured veins on the brain's surface, better known as a key symptom of shaken baby syndrome - after taking a 70m plunge on the giant Fujiyama rollercoaster.
The thought that one's safety might be in immediate and serious danger is, of course, as crucial to the thrill of a good rollercoaster ride as is the underlying faith that death won't actually result from the experience. And, by and large, the industry has an excellent safety record: 500m rides are purchased in Britain each year, but the nightmare mechanical failures - the loose screw, the insecure harness, the heart-stopping mid-loop halt - are so rare as to prompt just 400 accident reports to the health and safety executive in total. Theme parks enjoy reminding critics that the chances of having an accident while driving to an attraction are seven times greater than when riding on it; equivalent US figures show that fishing is 17 times more dangerous.
But the new generation of giant rollercoasters are raising a different kind of safety fear, as parks race to outdo each other in speed, altitude and nerve-shattering terror. This is not a field in which records remain standing for very long, but the current leaders - according to the Rollercoaster Club of Great Britain - give a sense of the territory: Millennium Force at Cedar Point, Ohio, is the tallest conventional rollercoaster at 95m; and Superman - The Escape, at the Magic Mountain park in California, the fastest, firing riders down its 377m of track at 100mph, exposing them to forces approaching those previously only experienced by astronauts and jet-fighter pilots.
"We suspect that many cases have been overlooked," warned Toshio Fukutake, a neurologist at Chiba University in Japan, who wrote the Fujiyama study. He acknowledged that the problem was rare, but added: "Giant rollercoasters that are higher and faster than typical roller coasters may be more dangerous_ the woman's subdural haematomas and resulting headaches may have been caused by the up-and down, back-and-forth motions of the rollercoaster, or the acceleration force may have been strong enough to rupture veins on the surface of her brain."
Haematomas - usually encountered in older men suffering from alcoholism, hypertension or diabetes - are extremely rare in younger women, but eminently treatable: eight weeks after presenting her symptoms, the woman made a full recovery. But her case adds weight to earlier studies of "rollercoaster headache", including a paper published in the New England Journal of Medicine in 1995 in which the effects were likened to whiplash injuries."The swooping ride induces marked rotatory and other positional changes in a deformable brain that is moving within a relatively rigid skull, thus causing tensile and shearing stresses," the authors observed.
The forces experienced while hurtling down hundreds of metres of steel tracks demand years of professional training in other contexts: astronauts reach a g-force of 3 during the launch of the Space Shuttle; Blackpool Pleasure Beach's £12m Pepsi Max Big One - so tall it requires aircraft warning beacons from the Civil Aviation Authority - subjects riders to 3.5g; Alton Towers' Oblivion reaches 4.5. Only jet-fighter pilots experience a greater hammering - and they wear anti-gravity suits to pump air around their lower bodies, compressing blood vessels to prevent the rapid drain of blood from the head as g-forces spiral towards 9 and 10.
The British Association of Leisure Parks, Piers and Attractions is adamant that no British rides have ever been linked to these risks. "We are aware that there are a number of rides in this country that exert moderate g-forces, and there is no evidence that this is known to affect people's health, providing that passengers follow the operators' safety warnings and advice," says a spokesman for the group.
In any case, much as theme parks crow about them - "Some TV Gladiators were actually sick after sampling the world's ultimate white-knuckle ride," says Alton Towers of its 4g Nemesis attraction - high forces are not all they seem. "You can't compare absolute figures," says Volker Damman, head of medical operations at the European Space Agency and an expert on the effects of space travel on the human body. "It depends on how long you're exposed to high forces for. On a rollercoaster you're talking about milliseconds and you can experience that turning a sharp bend in a car.
"It also matters how your body is positioned: the worst is vertical force, pushing blood away from the head. Russian astronauts on the Soyuz withstood 5g for much longer, but they were lying on their backs. It was much, much harder for them to breathe, but the blood was drawn from their chest to their back, which is less of a problem."
Ultimately, all this may be of some concern to those who spend endless hours on some of the world's biggest rollercoasters: men and women of impenetrable motives such as Richard Rodriguez, the American who seized the world record for the world's single longest rollercoaster ride by spending 1,013 hours onboard in 1998. For the rest of us, sheer, jelly-legged terror may prove a surprisingly effective form of preventative medicine.