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Bicycle helmet

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File:Bicycle helmet.jpg
Bicycle helmet

A bicycle helmet is designed to attenuate impacts to the head of a cyclist in falls while minimizing side effects such as interference with peripheral vision[1]. Bicycle helmets are intended for use by pedal cyclists on ordinary roads, to give protection in the kind of accident in which the rider falls onto the road without other vehicles being involved[2].

A cycle helmet should be light in weight and should provide adequate ventilation, because cycling can be an intense aerobic activity which significantly raises body temperature and the head in particular needs to be able to regulate its temperature.

About helmets

How they work

There are two main types of helmet: hard shell and soft/micro shell (no-shell helmets are now rare). In both types impact energy is absorbed as a stiff foam liner is crushed, up to the point where the liner is crushed to its minimum thickness, or the helmet shatters, after which no further energy is absorbed. Collision energy varies with the square of impact speed: a typical helmet will absorb the energy of a fall from a stationary or slow-moving bicycle, an impact speed of around 12mph, but will reduce the energy of a 30 mph impact to only 27.5 mph, and even this will be compromised if the helmet fails. This energy calculation is based on the standards, which take no account of the weight of the rider's body.

As a subsidiary effect they also spread point impacts over a wider area of the skull. Hard shell helmets do this rather better, but they tend to be heavier and less well ventilated so are more common among stunt riders than road riders or mountain bikers. Additionally, the helmet (like any good hat) will reduce superficial injuries to the scalp. Hard shell helmets can also reduce the likelihood of penetrating impacts although these are said to be very rare.

The key component of most modern bicycle helmets is a layer of expanded polystyrene (EPS), essentially the plastic foam material used to make inexpensive picnic coolers. This material is sacrificed in an accident, being crushed as it absorbs a major impact. Bicycle helmets should always be discarded after any accident.

Helmets are most effective in straight line, or linear, blows to the head at moderate speed. Helmets are not well designed to deal with high speed impacts or rotational stresses (crashes that are not centred, and involve rotation of the head). They are not designed to provide adequate protection for a collision involving another moving vehicle, (e.g. a car).

A common misunderstanding is to assume that a broken helmet has prevented some serious injury. Helmets are designed to crush without breaking; EPS absorbs little energy in brittle failure and once it fails no further energy is absorbed.

History

Prior to the mid-1970s, the dominant form of helmet was the leather "hairnet" style, mainly used by racing cyclists. This offered minimal impact protection and acceptable protection from scrapes and cuts. Two of the first modern bicycle helmets were made by MSR, a manufacturer of mountaineering equipment, and Bell Sports, a manufacturer of helmets for auto racing and motorcycles. These helmets were a spinoff from the development of expanded polystyrene (EPS) foam liners for motorcycling and motorsport helmets, and had hard polycarbonate plastic shells. The bicycle helmet arm of Bell was split off in 1991 as Bell Sports, having completely overtaken the motorcycle and motor sports helmet business.

The first commercially successful purpose-designed bicycle helmet was the Bell Biker, a polystyrene-lined hard shell released in 1975. At the time there was no appropriate standard; the only applicable one, from Snell, would be passed only by a light open-face motorcycle helmet. Over time the design was refined and by 1983 Bell were making the V1-Pro, the first polystyrene helmet intended for racing use. In 1984 Bell produced the Li'l Bell Shell, a no-shell children's helmet. These early helmets had little ventilation.

1985 saw the introduction of Snell B85, the first widely-adopted standard for bicycle helmets; this has subsequently been refined into B90 and B95 (see Standards below). At this time helmets were almost all either hard shell or no-shell (perhaps with a vacuum-formed plastic cover). Ventilation was still minimal due mainly to technical limitations of the foams and shells in use.

Around 1990 a new construction technique was invented: in-mould microshell. A very thin shell was incorporated during the moulding process. This rapidly became the dominant technology, allowing for larger vents and more complex shapes than hard shells.

Hard shells declined rapidly among the general cyclist population during the 1990s, almost disappearing by the end of the decade, but remain popular with BMX riders as well as inline skaters and skateboarders.

The late 1990s and early 2000s saw advances in retention and fitting systems, replacing the old system of varying thickness pads with cradles which adjust quite precisely to the rider's head. This has also resulted in the back of the head being less covered by the helmet; impacts to this region are rare, but it does make a modern bike helmet much less suitable for activities such as unicycling, skateboarding and inline skating, where falling over backwards is relatively common. Other helmets will be more suitable for these activities.

Standards

In the United States the Snell Memorial Foundation, an organization initially established to create standards for motorcycle and auto-racing helmets, implemented one of the first standards. The American National Standards Institute (ANSI) created a standard called ANSI Z80.4 in 1984. Later, the United States Consumer Product Safety Commission (CPSC) created its own mandatory standard for all bicycle helmets sold in the United States, which took effect in March 1999.

In the UK the currently applicable standard is EN 1078:1997, which replaces BS 6863:1989.

The CPSC and EN1078 standards are lower than the Snell B95 (and B90) standard; Snell helmet standards are externally verified, with each helmet traceable by unique serial number. EN 1078 is also externally validated, but lacks Snell's traceability. The most common standard in the US, CPSC, is self-certified by the manufacturers. It is generally true to say that Snell standards are more exacting than other standards, and most helmets on sale these days will not meet them (no current Bell brand helmet is Snell certified, some Specialized ones are - the Snell Memorial Foundation website includes a list of certified helmets).

In 1990 the Consumers' Association (UK) market survey showed that around 90% of helmets on sale were Snell B90 certified. By their 1998 survey the number of Snell certified helmets was around zero. Hard shells declined rapidly among the general cyclist population over this period, almost disappearing by the end of the decade, but remained more popular with BMX riders as well as inline skaters and skateboarders.

Standards are generally getting weaker, driven by the market's desire for lighter and more ventilated helmets, but efficacy against minor injuries, which is the design purpose of most helmets, is not dissimilar. Nonetheless, all other things being equal, a Snell certified helmet is probably objectively safer than a non-Snell one. However a recent comparison of motorcycle helmet standards, with regard to the amount of force passed through to the head, found that Snell's emphasis on a harder shell, lead to a higher level of G-force being transmitted to the head. The idea behind helmets is to absorb energy, not reusability. Failure to meet Snell standards may not be a bad thing.

All helmets sold today must meet basic safety standards. The difference between inexpensive and expensive helmets will more likely reflect ventilation, comfort and convenience issues rather than safety.

Proper fit

It is important that a helmet should fit the cyclist properly - according to research up to 96% of helmets have been found to be incorrectly fitted, and an incorrectly fitted helmet puts you at up to three times more risk.

First, the correct size must be purchased. Most manufacturers provide a range of sizes ranging from children's to adult with additional variations from small to medium to large.

Helmets are held on the head with nylon straps, which must be adjusted to fit the individual. The ease with which adjustments can be made can be one of the major differences between a cheap helmet and a better quality one.

A common mistake is to fit the helmet so that it sits high on the forehead. The helmet should sit level on the cyclists head with only a couple of finger-widths between eyebrow and the helmet brim. It should not be possible to insert more than one finger between the strap and the throat, or to move the helmet more than a centimetre or so in any direction. The strap should be well back under the chin, close to the throat.

The helmet debate

There is a long-running argument over the use, promotion and compulsion of cycle helmets. Most heated controversy surrounds laws making helmet use compulsory, particularly regarding the substantial disparity between claimed injury savings in small-scale prospective studies (e.g. Thompson, Rivara and Thompson, 1989) and later, more comprehensive studies, particularly from jurisdictions which have used compulsion to substantially raise helmet use over a very short period. Helmet use in New Zealand, for example, rose from 43% to over 95% in under three years, with no measurable change in head injury rates (Scuffham, 1997).

Controversy is fuelled by support given to the pro-compulsion movement by Bell Sports in particular, and by the fact that many of the most vocal proponents of helmets are not themselves cyclists.

Overall, most cycling groups are opposed to mandatory helmet use, partly on grounds of equitability (cyclists are more often the victims in crashes than the cause) but largely because of the deterrent effect of laws on levels of cycling. Cycling gets safer the more people who do it.

Research evidence

File:Fahrradpark.jpg
Many areas with low helmet use have better bicycle safety.

Evidence for the efficacy of helmets in preventing serious injury is contradictory and inconclusive. In general, analyses of the relative merits of different bike safety interventions put helmets low down, because no helmet will reduce the probability of crashing (and there is some evidence that helmets may increase this likelihood). Proactive measures including bike maintenance and riding skills are far more important. Although the link is not causal it is observed that the countries with the best cycle safety records (Denmark and the Netherlands) have among the lowest levels of helmet use. Their bicycle safety record is generally attributed to public awareness and understanding of cyclists, education, and to some extent separation from motor traffic.

The evidence comes from two main types of observational study:

  • case-control studies, in which cyclists who have injured their heads ("cases"), and cyclists who have not ("controls"), are compared. Such studies consistently find that cases report a lower rate of helmet-wearing than controls. This has been taken as strong evidence that cycle helmets are beneficial in a crash and that all cyclists should be compelled to wear them. Known problems with this study design include: confounding (attributing benefits from differences in behaviour to differences in helmet choice); reporting errors (people falsely reporting helmet use)[3]. The most widely-quoted case-control study, by Thompson, Rivara, and Thompson, reported an 85% reduction in the risk of head injury by using a helmet. There are many criticisms of this study[4], including use of an unrepresentative control group. This study includes clear evidence that the injury profile of helmeted and unhelmeted cyclists in a population with voluntary wearing is significantly different, an effect first documented by Spaite et. al:

A striking finding was noted when the group of patients without major head injuries (246) was analyzed separately. Helmet users in this group still had a much lower mean ISS (3.6 vs. 12.9, p less than 0.001) and were much less likely to have an ISS greater than 15 (4.4% vs. 32.1%, p less than 0.0001) than were nonusers. In this group, 42 of 47 patients with an ISS greater than 15 (89.4%) were not wearing helmets. We conclude that helmet nonuse is strongly associated with severe injuries in this study population. This is true even when the patients without major head injuries are analyzed as a group[5]

  • Population studies compare changes in helmet use and injury rates in a single population over time, most notably where helmet laws have resulted in large changes in a short time. A review of jurisdictions where helmet use increased by 40% or more following compulsion showed no measurable change to head injury rates[6]. The largest study, covering eight million cyclist injuries over 15 years, showed no effect on serious injuries and a small but significant increase in risk of fatality[7]. Weaknesses of this type of study include: simultaneous changes in the road environment (e.g. drink-drive campaigns); inaccuracy of exposure estimates (numbers cycling, distance cycled etc.).

Different analyses of the same data can produce different results. For example, Scuffham analysed data on the New Zealand helmet law in 1995 and concluded that, after taking into account long-term trends, the laws had no measurable effect[8]. His subsequent re-analysis without accounting for the long-term trends showed a small benefit[9]. Re-analysis of the Thompson, Rivara and Thompson data substituting helmet wearing rates from co-author Rivara's contemporaneous street counts, reduces the calculated benefit to below the level of statistical significance. Another analysis of the source data form this study showed a 70% reduction in lower limb injuries from helmet use. One problem with all analyses is that the population of injured cyclists is generally very small, and it is difficult to collect sufficient incidents to form a statistically signifciant sample.

The definition of injury is also open to debate, and injury figures are acknowledged to be inacurate. Research by TRL and others shows that reporting of injuries is inversely related to severity. Fatal injuries are almost always reported, in the developed world, but 90% or more of lesser injuries go unreported. Helmets are most likely to be effective against lesser injuries. Pro-helmet studies routinely refer to prevention of traumatic brain injury, which has connotations of permanent intellectual disablement, but where sufficient data is provided it is found that the majority of the brain injuries in these studies are concussion. A study of fatally injured cyclists found injuries of fatal severity to multiple organ systems were in sixteen of twenty riders, including six with no significant head injury. Four riders died of fatal injury to head alone and one of these was the only rider know to be wearing a safety helmet. His death resulted from a fall from a bicycle at moderate speed rather than collision with a motor vehicle.[10]

Recent research on traumatic brain injury adds further confusion, suggesting that the major causes of permanent intellectual disablement and death may well be torsional forces leading to diffuse axonal injury (DAI), a form of injury which helmets cannot mitigate[11].

Much of the research is partisan in one way or another. Thompson, Rivara and Thompson were already committed advocates of helmet legislation before publishing their first study; their report for the Cochrane review has also been criticised for being dominated by their own work. Rodgers, who showed helmets to be associated with increased risk of fatality, was replying to criticism of CPSC for focusing on bicycle design and manufacture standards. One report concluding a 60% reduction in injuries was found to be in error due to a simple statistical error; correcting the error results in a claimed efficacy of 186%; despite this the authors continue to assert that the results stand[12]. A report commissioned by the UK Government was supportive of cycle helmet promotion[13] but dismissed out of hand much of the contradictory evidence, and the principal authors were associated with a programme of the Child Accident Prevention Trust (CAPT), which is strongly pro-helmet[14]. Curnow, author of papers on helmets and traumatic brain injury, has also published criticism of pro-helmet research[15].

The conflicting evidence and entrenched views have led to widespread and longstanding controversy. Various organizations have taken up definite positions on the issue, not always based on a full review of the evidence. For example, the British Medical Association used to be against helmet compulsion, following an extensive review of the evidence in 1999. In late 2004 the BMA's Board of Science and Education adopted a 'position' calling on the UK government to introduce cycle helmet legislation, and this was confirmed at the 2005 Annual Representative Meeting following fifteen minutes of debate(transcript). The BMA's new position use statistics provided by the British political lobby group, the Bicycle Helmet Initiative Trust, and exclude from consideration the majority of conflicting evidence, including the BMA's own previous work. Several provably wrong figures were removed after initial publication, but the review is still viewed as distorted, excluding not only references included in the 1999 BMA study, but the 1999 study itself. Debate continues within the BMA.

Reduction in bicycle participation

Mandatory bicycle helmet laws may lead to a reduction in the number of cyclists. The reduction in the number of cyclists may have a more negative impact on the health of a population than would have arisen from the head injuries that would have resulted from not using helmets since the reduction in injuries is apparently so small. The long term health benefits of bicycle use are well established so any reduction in bicycle activity will likely have a negative impact on the overall health of a population.

According to the "safety in numbers" theory, decreased cycling may have a negative effect on the safety of the remaining cyclists.

Cycle helmet promotion or high levels of use may deter cycling by reinforcing the misconception that bicycling is more dangerous than traveling by passenger car [1]. Such a reduction in cycling might cause an increased risk for remaining cyclists due to a "safety in numbers" effect.

Helmets and risk of injury

Many believe that a helmet can save a cyclist's life, an idea which is repeatedly asserted in debate. There is no sound evidential basis for this claim and there are no known cases where mass helmet use has actually reduced the number of cyclists' deaths or serious head injuries[16][17]. Association with increased risk of death has been reported[18][7][5]. It is likely that helmets could prevent a significant number of minor cycling injuries but the overall safety benefits are inconclusive; this is thought to be in part due to risk compensation behaviour[19][20][21]. A cost-benefit analysis of the New Zealand helmet law showed that the cost of helemts outweighed the savings in injuries even taking the most optimistic estimate of injuries prevented[22].

While a helmet may mitigate the effects of a fall or collision, other factors (such as maintenance, road conditions, and driver behaviour) are more important for reducing the chance of such accidents in the first place. In general, the value of bicycle helmets has been systematically overstated[23].

Some studies have even suggested that helmets increase risk. Although the head injury rate in the US rose by 40% as helmet use rose from 18% to 50%, this does not necessarily mean that helmets themselves increase risk. In fact, a range of theories exist to explain the observed disparity, including:

  • Risk compensation: helmeted cyclists may ride less carefully; this is well supported by evidence for other road safety interventions such as seat belts and antilock brakes.
  • Poor fitting: 96% of helmets not fitted correctly and incorrectly fitted helmets reportedly increase risk by a factor of 3.
  • Sampling bias in prospective studies: voluntary wearers may be more risk averse, skewing the results.

No research has yet been published which adequately addresses the reasons for the disparity.

Inadequate design

Some argue that the helmets that are currently on the market are not designed suitably to make a large reduction in fatalities for the types of injuries they are supposed to protect against. However, measures required to improve helmets would make the helmets commercial failures. Helmets designed to a higher standard have not sold well, while helmets designed to even lower standards have sold well[2]. For example, a helmet's ability to absorb energy could be improved by increasing the volume of polystyrene. However, a thicker helmet would be heavier, hotter to wear and could be considered unsightly; as such, there is a trend towards thin helmets with many large vents. This trend to lower standards has been noted in some of the studies[24] It is relatively common for helmets to fail on test, and some helmets on sale are not certified to any accepted standard.[25]

Design to inadequate standards

Most of the standards helmets may be certified by have been designed to be passable using current designs and materials rather than to set a certain minimun safety standard. For example, most tests involve weighting the helmets and dropping them onto anvils with flat, hemispherical and cornered (comparable to a kerbstone) shapes[3]. Since the hemispherical and cornered anvils present the most difficult tests to pass, they are tested with a shorter drop, although there is no reason in practical riding why a person falling onto a flat surface would not fall as far as someone hitting a round object.

Use, promotion, compulsion

Helmet use has increased significantly in many, but not most, jurisdictions since the 1980s, primarily because of helmet promotion and compulsion laws. These laws were designed to improve bicycle safety but are controversial because none has resulted in a measurable reduction in cyclist head injury ratesCite error: A <ref> tag is missing the closing </ref> (see the help page)., yet no country promotes helmets for either of these modes (although there was an experiment in Japan with walking helmets for children, which demonstrated no measurable benefit). Cycle helmet use correlates inversely with the level of cycling in a given country. Official zeal for cycle helmets is greatest where cycling is a minority activity.

Detailed analysis of hospital admissions data also fails to support the idea that cycling is unusually dangerous: a study in the UK found that the proportion of cyclist injuries which are head injuries is lower than the proportion for pedestrians.

Overall, cycling is beneficial to health - the benefits outweigh the risks by up to 20:1. Anything which jeopardises that benefit must be carefully weighed to ensure it is likely to achieve some meaningful benefit in turn. Thus far, no helmet law has been shown to do that.

References

  1. ^ Consumer Product Safety Commission. "Safety Standard for Bicycle Helmets" (pdf). Final Rule 16 CFR Part 1203. {{cite web}}: Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  2. ^ BS6863:1989, Pedal Cyclists' Helmets, British Standards Institution
  3. ^ British Medical Journal]
  4. ^ BHRF
  5. ^ a b A prospective analysis of injury severity among helmeted and non helmeted bicyclists involved in collisions with motor vehicles Spaite DW, Murphy M, Criss EA, Valenzuela TD, Meislin HW. 1991. Journal of Trauma: 1991 Nov;31(11):1510-6
  6. ^ British Medical Journal
  7. ^ a b Reducing Bicycle Accidents: A Reevaluation of the Impacts of the CPSC Bicycle Standard and Helmet Use Rodgers GB. 1988. Journal of Products Liability: 1988,11:307-317
  8. ^ Trends in cycle injury in New Zealand under voluntary helmet use Scuffham PA, Langley JD. 1997. Accident Analysis and Prevention: 1997 Jan;29(1):1-9
  9. ^ Head injuries to bicyclists and the New Zealand bicycle helmet law, Scuffham P, Alsop J, Cryer C, Langley JD. 2000. Accident Analysis and Prevention: 2000 Jul;32(4):565-73
  10. ^ Fatal injuries to bicycle riders in Auckland. Sage MD. 1985. NZ Med J: 25 Dec 1985 Vol 98 No 793
  11. ^ The efficacy of bicycle helmets against brain injury, Curnow WJ. 2003. Accident Analysis and Prevention: 2003,35:287-292
  12. ^ Trends in serious head injuries among English cyclists and pedestrians, Injury Prevention 2003; 9: 266-267 and responses
  13. ^ Bicycle helmets - a review of their effectiveness: a critical review of the literature Towner E, Dowswell T, Burkes M, Dickinson H, Towner J, Hayes M. 2002. Department for Transport: Road Safety Research Report 30
  14. ^ Critique of Road Safety Research Report 30
  15. ^ The Cochrane Collaboration and bicycle helmets Curnow WJ. 2005. Accident Analysis & Prevention: 2005;37(3):569-573
  16. ^ Robinson, D. L. (2006). "Do enforced bicycle helmet laws improve public health?" (PDF). British Medical Journal. 332: 722–5.
  17. ^ Letter from David Jamieson, MP, minister of state for transport, to Michael Jack, MP
  18. ^ BMJ 2000;321:1582-5
  19. ^ e.g. Mok et al., Risk compensation in children's activities: A pilot study, Paediatr Child Health: Vol 9 No 5 May/June 2004
  20. ^ John Adams, 1995, Risk, Routledge, ISBN 1857280687 — (Authoritative reference on risk compensation theory.)
  21. ^ Death on the Streets: Cars and the mythology of road safety, Davis, 1993, ISBN 0948135468
  22. ^ New Zealand bicycle helmet law-do the costs outweigh the benefits? Taylor M, Scuffham P. 2002. Injury Prevention: 2002;8:317-320
  23. ^ Hansen P, Scuffham PA, Aust J Public Health: 1995 Oct;19(5):450-4
  24. ^ e.g. Vulcan, A.P., Cameron, M.H. & Watson, W.L., "Mandatory Bicycle Helmet Use: Experience in Victoria, Australia", World Journal of Surgery, Vol.16, No.3, (May/June 1992), pp.389-397.
  25. ^ Heads Up, Walker B., Cycle magazine (Cyclists' Touring Club) June 2005

Case studies/risk

  • Thompson, R., Rivara, F. and Thompson, D. (1989), A Case-Control Study of the Effectiveness of Bicycle Safety Helmets, New England Journal of Medicine, 25 May, 320:21, 1361-67 Abstract — (The most widely cited pro-helmet study.)
  • Bicycle Helmet Research Foundation, "Commentary on A Case-Control Study of the Effectiveness of Bicycle Safety Helmets", accessed 21st June 2006
  • Scuffham Trends in cycle injury in New Zealand under voluntary helmet use, Langley. Accident Analysis and Prevention, Vol 29:1, 1997 &mdash (Showed no benefit from large-scale increases in helmet use.)
  • John Adams, 1995, Risk, Routledge, ISBN 1857280687 — (Authoritative reference on risk compensation theory.)

Helmet Fit

  • Parkinson, Gregory and Hike, Kelly E. (2003), Bicycle Helmet Assessment During Well Visits Reveals Severe Shortcomings in Condition and Fit, [4] Pediatrics, 2 August 2003 Vol. 112 No. 2, pp. 320-323 — (Showed that correct fitting is an exception.)

Reduction of fatalities or serious injuries

Compulsion Laws

Bicycling as traffic books

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