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There is still more speculation than information surrounding actress Natasha Richardson's fateful ski accident. Part of the confusion is the very nature of the accident — an improbable injury, little more than a head bump on a bunny slope, that has felled an otherwise healthy 45-year-old woman. It has also left onlookers wondering not just what happened to Richardson, but whether a helmet could have prevented it.

The details of Richardson's accident are sketchy, but what is known sounded benign — at first. She was taking a lesson on a beginner slope at the Mont Tremblant ski resort north of Montreal, with an instructor but without a helmet. She fell at the end of the lesson and struck her head, but was alert and conversational afterward and did not complain of any ill effects. An hour later, in her hotel room, she developed a severe headache. The next day, she was flown to Lenox Hill Hospital in New York City in critical condition, where she died on Wednesday. (See pictures of Natasha Richardson's life.)

Richardson's family and doctors had released no information regarding her condition, prior to her death. But it appears that Richardson was the victim of an unfortunate collision of biology and physics — a collision that is becoming scarily common in the worlds of athletics and organized sports. The human body is a sturdy one, but only up to a point, able to withstand collisions of about 15 m.p.h., which is about as fast as an average person can run. The skull is designed to be especially rugged — the permanent home and helmet for the brain — but even it can't take a much more serious hit. The problem is that over the centuries, we've developed all manner of ways to exceed a mere 15 m.p.h. creep. (Read a TIME cover story on the brain.)

One of the most common collision-related head injuries is a concussion, which occurs when the head moves at high speed and stops suddenly as it strikes a hard object. The brain, which is snug but not completely stationary inside the head, may continue moving, colliding with the inside of the skull. This leads to swelling or bruising or — much worse — bleeding. A brain-bleed is immediately life-threatening, but swelling is less so and may not even be evident for a little while, which is what appears to have happened in Richardson's case.

"In organized sports," says Jasper Shealy, a professor emeritus at the Rochester Institute of Technology and a sports-injury expert who has published more than 100 papers on the subject, "the practice had been for players with a head injury to sit on the bench for a little while and, once they felt better, to go back in."

That's a mistake — and not only because it takes more than a few minutes to know how serious the injury is. An initial concussion, neurologists are now learning, can make a second concussion more likely, and the second injury, in turn, increases the risk of subsequent ones over the years. That's precisely the reason some NFL players become known as more concussion-prone than others. Worse, the danger is cumulative: later concussions may become not just more likely, but also more serious. "A sequence of mild events — even just two or more — can equal one big hit," Shealy says. That may have been what happened in Richardson's case, though no one has said publicly if she had any other head injuries in her past.

The fact that Richardson was not wearing a helmet may or may not have made a difference in the gravity of her injury. If skiers are moving slowly — say 10 m.p.h. or slower — and they fall on soft snow, they're probably not going to be hurt severely, whether they're wearing a helmet or not. If they're moving faster than 15 or 20 m.p.h. and strike ice, hard-packed snow or another solid object with the head, they're likely to suffer severe injury, and again the presence of a helmet may not make much difference. It's in the middle area — at speeds that are neither very slow nor very fast — that a helmet can play the biggest role. The trick, of course, is that you never know when you're going to be in that gray zone, since even slow beginner skiers can lose control and speed up, and high-speed skiers have to slow down eventually. (See pictures of skiing.)

Physics makes things trickier still, causing different parts of the body to move at different speeds. Your skis or snowboard may be sliding along at a slow 10 m.p.h., but if you catch a tip or edge on something stationary, the rest of you plunges forward and accelerates. "The body acts as an inverted pendulum, so the upper body moves much faster than the lower body," says Shealy.

Once you do fall and hit, the brain can do much more than just bump the inside of the skull. "You can have stretching of cortical connections or stretching of blood vessels, and that can lead to bleeding," Shealy says. "You can also have linear or rotational acceleration [of the brain]. There's a lot that can go wrong in there."

Even some experts acknowledge that helmets are no panacea, and not only because they become less effective at higher speeds. Skiers argue that they reduce peripheral vision and also provide a false sense of security that encourages speeding. Those claims may well have some truth, but seat belts too may create a false sense of security, yet few people argue the wisdom of wearing them. Helmets may not provide the same level of protection as a seat belt, but in some cases, even inconsistent protection may make all the difference.

See pictures of Natasha Richardson's life.

Read "The Year in Medicine 2008: From A to Z."

• Jeffrey Kluger
• Two days after a seemingly innocent fall on a beginner ski slope, the actress died in a New York City hospital. What happened?