thumb|right|Three styles of bicycle helmets: standard, full-face, and multi-sport

thumb|right|A typical bicycle helmet

A bicycle helmet is a type of helmet designed to attenuate impacts to the head of a cyclist in collisions while minimizing side effects such as interference with peripheral vision.

History

thumb|[[Nat Butler, 1910]]

thumb|Bicycle helmet, 1940s

thumb|[[Martin Wierstra (NED), 1959]]

thumb|[[Bernhard Eckstein, 1960]]

History of designs

thumb|upright|[[Uwe Raab wearing a "hairnet" helmet]]

A cycle helmet should generally be light in weight and provide ample 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 the helmet up to the 1970s was the "hairnet" style, an open construction made of rubber bars covered in leather. This offered acceptable protection from scrapes and cuts, but only minimal impact protection, and was mainly used by racing cyclists.

More widespread use of helmets began in the US in the 1970s. After many decades when bicycles were regarded largely as children's toys, 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 spin-off 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 motorsports 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 Lil Bell Shell, a no-shell children's helmet. These early helmets had little ventilation.

In 1985, 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.

thumb|left|A [[Giro (company)|Giro Atmos helmet, showing seamless in-mould micro shell construction]]

Around 1990 a new construction technique was invented: in-mould micro shell. 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.

Use of hard shells declined rapidly among the general cyclist population during the 1990s, almost disappearing from road and cross country mountain bike helmets by the end of the decade, but remaining popular with BMX riders and more aggressive mountain bike disciplines such as downhill riding.

The late 1990s and early 2000s saw advances in retention and fitting systems, with cradles which adjust precisely to the rider's head, replacing the old system of varying thickness pads. This resulted in the back of the head being less covered by the helmet, although more recent designs have largely addressed this.

Since more advanced helmets began being used in the Tour de France, carbon fiber inserts are often used to increase strength and protection of the helmet. The Giro Atmos and Ionos, as well as the Bell Alchera, were among the first to use carbon fiber, MET Helmets furthered the use of carbon fibre by in-moulding a complete cage during manufacturing.

Some modern road and track racing bicycle helmets have a long tapering back end for streamlining. This type of helmet is mainly dedicated to time trial racing and Triathlon as they lack significant ventilation, making them uncomfortable for long races.

History of standards

In the United States the Snell Memorial Foundation, an organisation initially established to create standards for motorcycle and auto racing helmets, implemented one of the first standards, since updated. Snell's standard includes testing of random samples. 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. There are two main types of helmet: hard shell and soft/micro shell (no-shell helmets are now rare). 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.

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. A 2004 report concluded that the performance requirements of the 1996 version of this standard was slightly less strict than the Snell B95 standard but incorporated a quality assurance requirement, making it arguably safer.

Design intentions and standards

The standards are intended to reduce acceleration to (and within) the head due to impact, as a stiff liner made of expanded polystyrene is crushed against the head. However, both the CPSC and the EN 1078 standards only look at linear accelerations and ignore rotational accelerations. The rotational accelerations that arise in bicycle accidents can be large enough to cause concussions, diffuse axonal injury and subdural haematoma. were found to be incorrectly fitted. Efficacy of incorrectly fitted helmets is reckoned to be much lower; one estimate states that risk is increased almost twofold.

History of use

Helmets use varies greatly between populations and between groups. Downhill mountain bikers and amateur sportive cyclists normally wear helmets, and helmet use is enforced in professional cycle sport and in a few legal jurisdictions. Utility cyclists and children are much less likely to wear helmets unless compelled.

Required helmet use in cycling sport

thumb|[[Joop Zoetemelk in 1979]]

Historically, road cycling regulations set by the sport's ruling body, Union Cycliste Internationale (UCI), did not require helmet use, leaving the matter to individual preferences and local traffic laws. The majority of professional cyclists chose not to wear helmets, citing discomfort and claiming that helmet weight would put them in a disadvantage during uphill sections of the race.

The first serious attempt by the UCI to introduce compulsory helmet use was 1991 Paris–Nice race, which resulted in a riders' strike, and UCI abandoned 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 Andrei Kivilev at the Paris–Nice. The new rules were introduced on 5 May 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.

right|thumb|Example of [[shock detector in bicycle helmet to signal when a crash has occurred]]

[[File:Bicycle Helmet Protection Technologies.png|thumb|Types of modern bicycle helmet design technologies. Energy absorbing Bontrager Wavecell (left) and Smith Koroyd (center);

MIPS (right)]]

Injury reduction

There is consistent scientific evidence that bicycle helmets reduce the severity of head injuries, particularly serious injuries, in accidents, although they are less useful when a motor vehicle is involved.

Health benefits of cycling

Studies from China, Denmark, the Netherlands and the United Kingdom show that regular cyclists live longer because the health effects far outweigh the risk of crashes. A reduction in the number of cyclists is likely to harm the health of the population more than any possible protection from injury. UK figures show that it takes at least 8,000 years of average cycling to produce one clinically severe head injury and 22,000 years for one death. De Jong developed a mathematical model to evaluate the health-risk trade-offs of all-age mandatory helmet laws, if they were to be introduced in various North American and Western European countries. He concluded that helmet laws appear to offer net health benefit only in those countries with more dangerous bicycling environments under optimistic assumptions of the efficacy of helmets.

Some researchers have suggested that a legal requirement to wear helmets there may have dissuaded people from cycling, and that repeal of these laws could lead to increased cycling. This suggestion has been criticised.