Saturday, 20 November 2004

Using, not losing, your head

Cycle helmets are a good thing, aren't they? It's obvious. They protect your head. They must be a good thing: it's common sense. Why then is the cycling community, in the face of proposed mandatory helmet legislation, fighting internecine helmet wars?

Don't panic

Before going into the details of this argument, let's start by putting this into perspective. Cycling is actually a very safe activity. Nothing, of course, is absolutely safe. Last year, in Britain, 114 cyclists were killed. Of those, 95 (83%) died as a result of collisions with motor vehicles. But that's out of millions of cyclists, covering billions of miles. In fact, according to the National Statistics Office, there is on average one fatal accident for every twenty one and a half million miles cycled. Twenty one and a half million. If you were to cycle ten miles every single day, it would be nearly six thousand years before you had a fatal accident.

At the same time as those 114 cyclists died, over three thousand people died from accidents and mishaps in their own homes. Do you think your home is a dangerous place to be?

Of course, in the modern world, there are dangers other than accidents. We live highly stressed lives in which opportunities for exercise get fewer and fewer, and opportunities to eat and drink become more and more available. We get fat. We get unfit. And our health suffers in consequence, with the incidence of illnesses such as obesity, heart disease, osteoporosis and diabetes increasing rapidly. Cycling is a good general exercise both for the cardiovascular system and for the limbs. Unlike walking, jogging or running, the movement is smooth and so does not cause impact damage to the ankles, knees and hips. Yes, there is a finite risk of accident when cycling but it is nevertheless undoubted that if you cycle regularly not only are you likely to live longer but you're more likely to enjoy a fit, active and healthy old age.

Got that? Good. Now let's talk about helmets.

Use no hooks: or, A box for a computer

In the more tragic and more bloody wars of the Democratic Republic Congo, many warriors wear or carry lucky charms which they believe will protect them against bullets. We sophisticated westerners read stories of this and we think 'how quaint, and sad, and ignorant, are these uneducated child soldiers going into battle, believing superstitiously in the protection of lucky charms'. And then we cycle off into the traffic, wearing our cycle helmets.

This note was written as a web page. If you're reading it on a web page, you're reading it on a computer. I'd like you to stop for a moment and think about that computer. When it arrived from its maker - possibly when you bought it - it was packed in a strong cardboard box. Inside the strong cardboard box was almost certainly some polystyrene foam packaging material. Probably at least 40mm of it, surrounding and protecting your computer from the inevitable bumps it would incur in transit - bumps like being dropped from someone's hands onto the warehouse floor, or thumped up against another, similarly packaged computer.

By and large, for these sort of bumps, the packaging works, and your computer probably arrived home safe and sound.

Now think about your bicycle helmet. Like the packaging your computer came in, it is worn to protect a very valuable object - your brain. Like the packaging your computer came in, it is made of polystyrene foam - and typically it's a good bit less than 40mm thick.

Putting the boot in

I would like you to stop again, and think about the box your computer came in. I'd like you, as a thought experiment, to imagine taking your computer, putting it back it in its original box, and placing the box in the middle of the street. Now I want you to imagine getting into a car and driving into the box at just thirty miles an hour. You've imagined that? Good. Now do you think you would be able to use the computer afterwards?

Polystyrene foam is just polystyrene foam. Polystyrene foam is a light, weak, compressible solid which rapidly becomes brittle with age and is easily damaged by solvents. It doesn't become magically stronger just because it's formed into a cycle helmet. The same foam that didn't protect the computer in the thought experiment is equally not going to protect your head in similar circumstances.

Ticking the box

Nor do the manufacturers, nor the standards writers, believe it should. The European test for cycle helmets involves dropping the helmet, containing a dummy head weighing not more than 6Kg, onto a flat surface from a height of 1.5 metres. I don't know about you, but I'm 1.88 metres tall and I weigh 82Kg. If I just fall over from standing upright, I already exceed the impact which cycle helmets sold in Europe are tested to protect against - and exceed it by a very substantial margin. And that's before I've even got on my bicycle and started moving.

In practice, cycle helmets are expected to be helpful in accidents up to about 15mph (24Km/h). You might (common sense) expect a 30mph impact to be only twice as bad as a 15mph impact, and you might think that something which offered reasonable protection at 15mph would offer some degree of protection at 30mph. Unfortunately, it doesn't work like that. Firstly, the force of the impact scales with the square of the speed, so your 30mph impact is four times, not twice, as severe as your 15mph one. But secondly, and even more scarily, it is widely accepted that the probability of injury scales with the fourth power of the speed. So your 30mph impact is sixteen times as likely to cause injury than your 15mph impact.

And that's before you consider what happens to polystyrene foam when its design load is exceeded. It snaps. It suffers 'brittle failure'. You can do this experiment quite easily with the foam packing your computer came in. Take a piece of the foam about as long as your helmet, and about as thick as your helmet. Try to crush it between your finger and thumb. It's surprisingly strong, isn't it? You can squeeze it very hard and it doesn't deform a lot. Polystyrene foam is quite strong in compression, that's why it is used. Now take your piece of foam and snap it between your two hands. That's amazingly easy, isn't it? It takes far less force than crushing it does... which means it has absorbed far less force. When a helmet breaks, it offers no further protection. The more an impact exceeds the helmet's design parameters, the more likely it is to break, and the less likely it is to offer any protection.

You saw the whole of the moon

But let's step back a bit. Let's suppose, for the moment, a helmet provides 100% protection for the part of the body it covers. Because, let's face it, the part of the body a cycle helmet protects is the scalp. What happens to the rest of the body in a 30mph, or in a 60mph accident? Is it really going to be much comfort to your grieving relatives to learn that your hair-do survived OK? Do you believe that because your scalp is protected, your neck and your chest will be protected, too? Or, if not, that your magically preserved brain can be magically plugged into a new heart and lungs? Of course you don't. And of course you know that an impact which has enough force to do severe damage to your skull is likely to do severe damage to other vital systems too. In thinking about protection it is no use protecting one part. It's not enough to see the crescent: you have to look at the whole of the moon.

He's dead, Jim

But it's worse than that. Not only do helmets not provide adequate protection in road speed accidents: they may actually make things worse. In fact they must do so, because in whole populations, as helmet wearing rises, so does the rate of cyclist deaths. Yes, you read that right: the more cyclists wear helmets, the more get killed.
 I don't know why. No-one knows why. Two main mechanisms have been suggested: 'risk compensation', the willingness of people to do more risky things when they believe themselves protected, and rotational injury.

The fact that people do do riskier things when they think they're protected is to some extent obvious. Indeed, Bell cycle helmets have been sold with the slogan 'Courage for the Head'. Could cyclists really be using up all of the safety benefit that helmets provide by taking more risks? It's possible. Could drivers, thinking helmeted cyclists are protected, take more risks around them? That's possible too.

But the more worrying possibility is this: wearing a helmet makes your head bigger. It increases the diameter by about 50%, which means it increases the area by about 125%. Now, is it easier to hit a target if it's more than twice as big? You bet it is. Your head being effectively bigger means that it's more likely to get hit; but again it may be still worse than this. Because we have evolved over millions of years of falling to tuck our heads in. We have reflexes which know - without our thinking about it - just how far we need to tuck our heads in to avoid an impact. By making the head bigger we may possibly defeat that instinctive protection mechanism. And it gets worse: larger diameter means more leverage, more angular acceleration. It has been suggested - and so far this is no more than a suggestion - that helmet wearing may increase rotational injuries to the brain. Rotational acceleration tears brain tissue and causes much more severe brain damage than linear accelerations of the same magnitude.

So not only do helmets make you (very slightly) more likely to get injured; they may also - but this is not proven - significantly increase your risk of the most frightening sort of injury, brain damage.

So: it's junk, then?

Does all this mean you shouldn't wear a helmet? Not in my opinion, no. I have a helmet, a MET Parachute, and I do wear it. When I think it will do some good.

Accidents of the sort cycle helmets won't help with - high speed impacts with something solid - are, fortunately, incredibly rare. When people fall off bicycles, they mostly do so at low speed and very often on tricky, off-road tracks. On tricky, off-road tracks you're very rarely travelling at very high speed and what you hit usually isn't moving at all. Indeed, you mostly fall off in the trickiest sections (or at least, I do) and that's when you're going slowest. Of course, such a fall is unlikely to kill you, but it can leave you with nasty bruising, grazes or even concussion. And a cycle helmet will protect you from bruising and grazing on the part of the body it covers, and may help a bit with concussion, too. So I wear my helmet when I'm doing tricky off-road stuff, particularly if I haven't ridden the particular route before. I should say here that although I've fallen off mountain bikes by now literally thousands of times, I've never hit my head at all - it's almost always my hips and elbows that get it. So even on a mountain bike a helmet isn't essential, and I often don't wear one.

And I don't wear one on the road. Ever. There really isn't any point. I haven't fallen off a bike on the road since I was sixteen, and that's thirty-three years ago. I'm an experienced road rider, and I ride with good awareness of traffic; I know how to protect myself from many of the ways motorists can kill you. Of course I can't protect myself against a motorist who is driving too fast and genuinely doesn't see me, but in that case I do not believe a helmet offers any useful protection. Indeed, on the basis of the available statistics and the simple physics I've described above, I know it cannot.

So cycle helmets are not junk. They are genuinely useful under some circumstances. But pretending they can save your life in traffic accidents is at best mistaken and at worst dishonest. To be fair, helmet makers do not pretend this; but there are still ignorant or misguided people who do - indeed, the opinion that it isn't safe to cycle on the road without one is very common. This common misapprehension is what leads to occasional campaigns for the wearing
of cycle helmets to be made compulsory, by law. It's to counter this misapprehension that I've written this article.

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