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.
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. In countries with long traditions of utility cycling, nearly all cyclists did not and still do not wear helmets. The use of helmet by non-racing cyclists began in the U.S. in the 1970s. After many decades when bicycles were regarded as children's toys only, many American adults took up cycling during and after the bike boom of the 1970s. 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 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 Inc., 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.
In 1985 the Snell B85 was introduced, 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.
Since more advanced helmets began being used in the Tour de France, Carbon Fiber inserts have started to be used to increase strength and protection of the helmet. The Giro Atmos and the Bell Alchera are among the first to use carbon fiber.
Some modern racing bicycle helmets have a long tapering back end for streamlining.
In the UK the current standard is BS EN 1078:1997, which is identical to the EU standard, and which replaced BS 6863:1989 in 1997.
In Australia and New Zealand, the current standard is AS/NZS 2063:1996. The performance requirements of this standard are slightly less strict than the Snell B95 standard but incorporate a quality assurance requirement. As a result, the AS/NZS can be argued to be safer.
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 (currently, Specialized is the only bicycle helmet brand in the world to meet the Snell standard. All of their helmets are Snell certified.)
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.
Although helmet standards have weakened over time there is no data on which to base an assessment of how this has affected the design goal of mitigating minor injuries. Minor injuries are substantially under-reported and it is difficult if not impossible to effectively measure such injuries on a meaningful scale.
A helmet's ability to absorb energy could be improved by increasing the volume of expanded polystyrene, but this would make it thicker, heavier, and hotter to wear. The trend is towards thinner helmets with many large vents. This trend to lower standards has been noted in some of the studies It is relatively common for helmets to fail on test, and some helmets on sale are not certified to any accepted standard. The most widely-cited pro-helmet studies were conducted when most helmets were of a hard-shell construction; these are now rare outside of niche applications such as BMX.
Collision energy varies with the square of impact speed; a typical helmet is designed to absorb the energy of a head falling from a bicycle, an impact speed of around 12mph or 20 km/h. This will only reduce the energy of a 30 mph or 50 km/h impact to the equivalent of 27.5 mph or 45 km/h, and even this will be compromised if the helmet fails. As a subsidiary effect they should also spread point impacts over a wider area of the skull. Hard shell helmets may do this better, but are heavier and less well ventilated. They are more common among stunt riders than road riders or mountain bikers. Additionally, the helmet should reduce superficial injuries to the scalp. Hard shell helmets may also reduce the likelihood of penetrating impacts although these are very rare.
In real accidents, while broken helmets are common, it is extremely unusual to see any helmet that has compressed foam and thus may have performed as intended. “Another source of field experience is our experience with damaged helmets returned to customer service... I collected damaged infant/toddler helmets for several months in 1995. Not only did I not see bottomed out helmets, I didn’t see any helmet showing signs of crushing on the inside.”
A new design of liner, the "cone-head", now being manufactured for motorcycle helmets but not yet available for bicycle helmets, has been designed in response to the 1987 study. It may provide better impact absorption.
Most helmets provide no protection against rotational injury and may make it worse. "The major discovery is that the skull plays an important role in protecting against rotational acceleration," says Phillips. He says almost all head injuries involve not just a direct blow to the skull but also damage to blood vessels caused by the brain rotating within the skull.
In mechanical terms, the head is an elliptical spheroid with a single universal joint, the neck. It is therefore almost impossible to hit it without causing it to rotate. The head tries to dampen these forces using a combination of built-in defences: the scalp, the hard skull and the cerebrospinal fluid beneath it. During an impact, the scalp acts as rotational shock absorber by both compressing and sliding over the skull. This absorbs energy from the impact."
The Phillips head protection system, also only available in motorbike helmets at present, is designed to reduce rotational injury.
Most manufacturers provide a range of sizes ranging from children's to adult with additional variations from small to medium to large. The correct size is important. Some adjustment can usually be made using different thickness foam pads. Helmets are held on the head with nylon straps, which must be adjusted to fit the individual. This can be difficult to achieve, depending on the design. Most helmets will have multiple adjustment points on the strap to allow both strap and helmet to be correctly positioned. Additionally, some helmets have adjustable cradles which fit the helmet to the occipital region of the skull. These provide no protection, only fit, so helmets with this type of adjustment are unsuitable for roller skating, stunts, skateboarding and unicycling.
The helmet should sit level on the cyclists head with only a couple of finger-widths between eyebrow and the helmet brim. The strap should sit at the back of the lower jaw, against the throat, and be sufficiently tight that the helmet does not move on the head. It should not be possible to insert more than one finger's thickness between the strap and the throat.
The first serious attempt by the UCI to introduce mandatory helmet use in 1991 was met with strong opposition from the riders. An attempt to enforce the rule at the 1991 Paris–Nice race resulted in riders' strike, forcing the UCI to abandon the idea.
While voluntary helmet use in professional ranks rose somewhat in the 1990s, the turning point in helmet policy was the March 2003 death of Kazakh Andrei Kivilev. The new rules were introduced on May 5, 2003, with the 2003 Giro d'Italia being the first major race affected. The 2003 rules allowed for discarding the helmets during final climbs of at least 5 kilometres in length; subsequent revisions made helmet use mandatory at all times.
No studies have been published yet into whether injuries have reduced as a result.
One pro-helmet website gives its "own pick of Basic Numbers from many sources": 773 bicyclists died on US roads in 2006, down just 11 from the year before. 92% (720) of them died in crashes with motor vehicles. About 540,000 bicyclists visit emergency rooms with injuries every year. Of those, about 67,000 have head injuries, and 27,000 have injuries serious enough to be hospitalized. Bicycle crashes and injuries are under-reported, since the majority are not serious enough for emergency room visits. 44,000 cyclists were reported injured in traffic crashes in 2006. In a campaign to make helmets compulsory for child cyclists, it has been stated that "in a three-year period from 2003, 17,786 children aged 14 and under were admitted to NHS hospitals in England because of injuries incurred while cycling
A UK opponent of compulsion has pointed out that it "still takes at least 8000 years of average cycling to produce one clinically severe head injury and 22,000 years for one death. Ordinary cycling is not demonstrably more dangerous than walking or driving, yet no country promotes helmets for either of these modes. "The inherent risks of road cycling are trivial... Six times as many pedestrians as cyclists are killed by motor traffic, yet travel surveys show annual mileage walked is only five times that cycled; a mile of walking must be more "dangerous" than a mile of cycling..." The proportion of cyclist injuries which are head injuries is essentially the same as the proportion for pedestrians at 30.0 % vs. 30.1 %. Overall, cycling is beneficial to health – the benefits outweigh the risks by up to 20:1..
Robinson's review of cyclists and control groups in jurisdictions where helmet use increased by 40 % or more following compulsion concluded that "enforced helmet laws discourage cycling but produce no obvious response in percentage of head injuries". Some of the data for this publication is available at This study has been the subject of vigorous debate. Authors do not agree on how studies should be selected for analysis, nor on what summary statistics are most relevant. A more recent review, by Macpherson and Spinks, includes two original papers (neither of which meet the criteria for inclusion in Robinson's review) and concludes that "Bicycle helmet legislation appears to be effective in increasing helmet use and decreasing head injury rates in the populations for which it is implemented. However, there are very few high quality evaluative studies that measure these outcomes, and none that reported data on an (sic) possible declines in bicycle use."
There are many other studies. The largest, covering eight million cyclist injuries over 15 years, showed no effect on serious injuries and a small but significant increase in risk of fatality. Although the head injury rate in the US rose in this study by 40 % as helmet use rose from 18 % to 50 %, this is a time-trend analysis with the potential weaknesses mentioned above; the correlation may not be causal. Association with increased risk has been reported in other studies. Different analyses of the same data can produce different results. For example, Scuffham analysed data on the increase of voluntary wearing in New Zealand to 1995; he concluded that, after taking into account long-term trends, helmets had no measurable effect. His subsequent re-analysis without accounting for the long-term trends suggested a small benefit. Scuffham's later cost-benefit analysis of the New Zealand helmet law showed that the cost of helmets outweighed the savings in injuries, even taking the most optimistic estimate of injuries prevented.
Such studies consistently find that cases of head injury report a lower rate of helmet-wearing than controls who have injured other parts of the body. This has been taken as strong evidence that cycle helmets are beneficial in a crash. 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, including use of a control group with very different risks. 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. This has been taken as evidence of confounding. In another study, helmet users also seemed to be protected against severe injuries to the lower body; "helmet non-use 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". It is possible that at least some of the 'protection' afforded helmet wearers in previous studies may be explained by safer riding habits rather than solely a direct effect of the helmets themselves.
Other case-control studies exist, all showing similar results. In Victoria, Australia, during 1977-1980, bicyclist casualties, then unhelmeted, sustained head injuries including severe head injuries, more than twice as frequently as the helmeted motorcyclist casualties.
A common misunderstanding is to assume that a broken helmet has prevented some serious injury. "the main impact was to my head. So much so, that my helmet broke in two (as it is designed to do). Without the helmet, it would have been my head that was broken and I wouldn’t be writing this blog entry! I’d be dead..."
Helmets are designed to crush without breaking; expanded polystyrene absorbs little energy in brittle failure and once it fails no further energy is absorbed. "cracks developing partly or fully through the thickness of the foam-slab renders it useless in crushing and absorbing impact forces" To prevent overt fragmentation, the foam in most helmets is reinforced inside with plastic netting to keep the foam together even after cracking.
Several mechanisms by which cycle helmet promotion or compulsion may deter cycling have been suggested. Helmets and their promotion may reinforce the misconception that bicycling is more dangerous than traveling by passenger car. Referring to the use of "human skull" images in a campaign, the CTC suggests that "this macabre imagery, with its associations of hospitals and death, is likely to reduce cycle use, thereby undermining efforts to realise the health and other benefits of increased cycling". Cycle helmets cost money and may make cycling less convenient; they are bulky and often cannot be stored securely with bikes. They are incompatible with some hairstyles, forcing bicycle users to recreate their hairstyle after each journey. Finally, bicycle helmets and other "safety" equipment have been seen as vexatious and ridiculous. For example, in the 2006 film The Benchwarmers, the character Clark—played by Jon Heder—sports a bicycle crash helmet as an accessory prop to highlight his lack of social skills and physical coordination.
Under the risk compensation theory, helmeted cyclists may be expected to ride less carefully; this is supported by evidence for other road safety interventions such as seat belts and anti-lock braking systems. There is some evidence for risk compensation by children in relation to safety equipment. Anecdotally, many riders report feeling safer with a helmet: "When I wear it, I feel safe..."
Motorists may also alter their behaviour towards helmeted cyclists. Recent evidence from England found that vehicles passed helmeted cyclists with measurably less clearance (8.5 cm) than that given to unhelmeted cyclists (out of an average total passing distance of 1.2 to 1.3 metres).
There are a few documented cases of young children, playing on bunkbeds, trees, jungle gyms, and so on, suffering death or severe brain damage as a result of strangulation by the straps of their bicycle helmets. One Swedish researcher commented of the Swedish Helmet Initiative: "We knew we'd killed, but didn't know we had saved anybody".
Helmet-wearing results in both benefits and costs for each individual. Material costs are the price of the helmet, its periodic replacement and any storage charges. Intangible costs include the time spent putting on, taking off, transporting and handling the helmet, curtailment of personal feelings of freedom or pleasure, more heat and sweat build-up in summer, difficulty in fitting head or ear insulation in winter. Personal feelings of safety may be classified as a benefit, or as a harm if, as Risk compensation theory suggests, it leads to greater risk of accidents. Benefits include the possibility of fitting additional ear protection, eye-shades, screens and mirrors. Every individual will perceive these costs and benefits differently.
In 1998 the European Cyclists' Federation adopted a position paper rejecting compulsory helmet laws as being likely to have greater negative rather than positive health effects. The UK cyclists' club, CTC, believes that the "overall health effects of compulsory helmets are negative. The UK minister of transport knew of no evidence to support the claim that helmets saved lives. The British National Children's Bureau has said "The 2004 BMA statement announcing its decision to support compulsory cycle helmets shows how the uncritical use of accident statistics can lead to poor conclusions. The same report estimated that, at most, universal helmet use would save the lives of three children aged 0 to 15 each year. That figure "assumes universal and correct use of helmets, it assumes that risk compensation does not occur and it assumes that no children die as a result of strangulation or other injuries caused by helmet use. These assumptions are most unlikely to be correct in the real world."
Dismissing concerns in 1996 that helmets should be shown to actually reduce injury rates, two pro-helmet doctors asked "How robust must the evidence be when the benefits of wearing helmets are so patently obvious? What is the downside to wearing a helmet, other than the mussing of Minerva's hair?". One of these, himself a cyclist, started his "career of advocacy" in 1972 and is now editor of an academic journal on injury prevention. In this position he has found "tiresome" academic argument that helmet wearing is useless.
Rivara was already engaged in surveying and lobbying for helmet use before the influential Thompson, Rivara and Thompson case-control study was commenced in 1989, while the report by Thompson, Rivara and Thompson for the Cochrane review has been criticised for being dominated by their own work.
Promotion of helmets raises further issues. Bell, the major helmet manufacturer, supports both helmet promotion and legislation.
From the point of view of cycling activists, the major problem with helmet promotion is that in order to present the idea of a "problem" to match the solution they present, promoters tend to overstate the dangers of cycling. Cycling is no more dangerous than being a pedestrian.
A study of cycling in major streets of Boston, Paris and Amsterdam illustrates the variation in cycling culture: Boston had far higher rates of helmet-wearing (32 % of cyclists, versus 2.4 % in Paris and 0.1 % in Amsterdam), Amsterdam had far more cyclists (242 passing bicycles per hour, versus 74 in Paris and 55 in Boston). Cycle helmet wearing rates in the Netherlands and Denmark are very low. An Australian journalist writes: "Rarities in Amsterdam seem to be stretch-fabric-clad cyclists and fat cyclists. Helmets are non-existent, and when people asked me where I was from, they would grimace and mutter: "Ah, yes, helmet laws." These had gained international notoriety on a par with our deadly sea animals. Despite the lack of helmets, cycling in the Netherlands is safer than in any other country, and the Dutch have one-third the number of cycling fatalities (per 100,000 people) that Australia has." The UK's CTC say that cycling in the Netherlands and Denmark is perceived as a "normal" activity requiring no special clothing or equipment.