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 helmet up to the 1970s was the leather "hairnet" style. 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 U.S. in the 1970s. After many decades, when bicycles were regarded only 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 motor sports helmet business.
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.?
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.?