A helmet works through absorbing some of the force of an impact by itself deforming. The core of shock-absorbing material acts as a buffer which reduces the acceleration forces that reach the head. In this way a helmet can provide an additional margin against injury, particularly following a simple fall or a glancing low-speed collision.
However, the protection afforded by a cycle helmet is quite limited. Over half of head injuries to cyclists affect parts of the head for which a helmet offers no protection. In most serious crashes, head injury is only one element of multiple serious injuries to various parts of the body, and is frequently not the sole cause of fatalities. The life-saving value of helmets is sometimes overstated, whilst the real-life experience of countries where the wearing of helmets has become more common suggests that, overall, head injury reduction is minimal.
British Standard BS6863 [2] sets out minimum requirements for helmets, and if you decide to buy one you should ensure that it meets this or the more demanding Snell standard. However 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 are able to offer advice. Check for a snug and comfortable fit around your head, after making any internal adjustments. Alter 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 for you.
Keeping the straps tight in use is extremely important. Not only can loose straps significantly reduce the protection given, by allowing the helmet to move on the head, but this in itself can also lead to serious neck injury. Research suggests that increased neck injury, due to badly fitted helmets, can cancel out any reduction in other types of head injury.
It is a particularly serious mistake to think that wearing a helmet is at all a substitute for learning to cycle properly. The protection offered by a helmet can easily be negated if you compensate by riding less carefully or if you find that wearing one interferes in any way with the attention that you are able to give to traffic. Ensure that a helmet will not interfere with your head movements, 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 could 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 certainly preferable to overheating in a way that reduces concentration.
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 – usually made of polystyrene – 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.
Quoted from
Cyclecraft by John Franklin
The Stationery Office, reprinted 2004