The following extract is from the 2007 edition of Cyclecraft by John Franklin (Franklin, 2007), the definitive guide to safe and enjoyable cycling for adults and children, and recommended reading for the UK National Cyclist Training Standard.
Cycle helmets attempt to limit the consequences of a crash. They do nothing to prevent a crash taking place; indeed, if not used properly or if their limitations are not appreciated, they may actually increase that risk.
Helmet wearing by cyclists is a controversial and often emotional subject. It is important to keep the risk of head injury in perspective. The chance of suffering head injury when cycling is low, as it is when walking. Serious head injury when cycling is rare, and all the more so for people who learn to ride skilfully. People who cycle regularly live longer, on average, than those who do not, with less ill health, so cyclists are not especially vulnerable to any life-threatening injury. Whatever the merits of helmets, their promotion often unjustifiably scares people from cycling through making the activity appear much more hazardous than is really the case.
A helmet works through absorbing some of the force of an impact by itself deforming. The liner of shock-absorbing material acts as a buffer which reduces the acceleration forces that reach the skull. In this way helmets can prevent minor wounds to the head. Once the liner is fully compacted (which happens at quite low forces), it provides no further protection and all the residual energy passes directly to the skull. Thus the ability of helmets to afford useful protection in more serious crashes, such as those that involve motor vehicles, is much less certain.
Evidence in favour of helmet effectiveness comes from case-control studies, which have high standing among medical professionals. But in recent years, other similar studies have been found to give misleading results. The evidence on cycle helmets is inconsistent with regard to the protection afforded, and the research has attracted extensive peer criticism for methodological weaknesses and errors.
In stark contrast, other types of evidence, such as that based on hospital and traffic casualty data and trends, are much more sceptical, with no evidence that increased helmet wearing has reduced the actual risk of serious or fatal injury across cyclists as a whole. Some studies have concluded that risk has been increased.
Nevertheless, many helmet wearers believe that they have been saved from serious injury. This is a very common experience, out of all proportion to the actual number of head injuries suffered by bareheaded cyclists. There is some evidence that helmeted riders may crash more often, and are more likely to hit their (helmeted) heads if they do. The breaking of a helmet is not by itself evidence that it has provided useful protection to the wearer, as it is common for helmets to fail prematurely before the inner liner has been fully crushed.
It is a serious mistake to think that wearing a helmet is at all a substitute for having a safe bike and learning to cycle properly. Parents, in particular, should take heed of this. The limited protection offered by a helmet can be easily negated if a cyclist compensates by riding less carefully or in places where risk is greater, or if wearing one interferes in any way with the attention that is given to traffic. There is evidence that some people, especially children, will take greater risks when wearing a helmet and that drivers may take less care around helmet-wearing cyclists.
Buying and fitting a helmet
If you do decide to wear a helmet, make sure that you buy one that meets the Snell B90 or B95 standards. The more commonly available standards, such as the British/European EN1078, are no guarantee of quality, sample helmets having repeatedly failed independent tests, with some helmets shattering at very low impact forces.
The protection afforded by a helmet is very much dependent upon achieving a good fit. Heads are different, especially in the position of the chin relative to the skull, and a helmet which is suitable for one person may be quite unsatisfactory for someone else. Always buy a helmet from a shop where there is plenty of choice and where the sales staff have the knowledge to advise.
Check for a snug and comfortable fit around your head, after making any internal adjustments. The helmet should sit low on your forehead and you should be able to see the edge of the brim at the extreme upper range of your vision. Adjust the straps so that there is no slack in any of them (but the chin straps should not be uncomfortably tight), and then try to slide the helmet off. If it does not stay firmly in place, it is unsuitable or incorrectly adjusted. Keeping the straps tight in use is extremely important. Not only can loose straps reduce the protection given, by allowing the helmet to move on the head, but this in itself can lead to neck injury.
Ensure that the helmet will not interfere with your head movement, vision in any direction, hearing or the wearing of spectacles or sunglasses. Check also for general comfort, especially the adequacy of ventilation. Inadequate air circulation can impair your attentiveness on the road. Many cyclists who normally wear a helmet take it off when climbing hills in hot weather, and this is preferable to overheating the head in a way detrimental to safety.
Helmets have only a limited effective life, even with careful use, and damage is not always visible. It is the condition of the crushable inner liner that is most important, not the outer shell (if any). It is recommended that a helmet should be replaced at least every three years. If it is subjected to a hard drop or impact (inside or out) or becomes badly scratched, it must be replaced straight away. Chemicals, detergents, heat and sunlight can all reduce the strength of a helmet.
Franklin, J, . Cyclecraft. The Stationery Office UK, 2007. ISBN 978-0-11-703740-3.