Helmetsprovide important head protection. The use of helmets, in one form or another, is almost universal amongst all mountain bikers. The main three types are cross country, rounded skateboarder style (nicknamed half shells or skate style) and full face. Cross country helmets tend to be light and well ventilated, and more comfortable to wear for long periods, especially while perspiring in hot weather. In XC competitions, most bikers use the usual road racing style helmets, for their lightweight and aerodynamic qualities. Skateboard helmets are simpler and cheaper than other helmet types; provide greater coverage of the head and resist minor scrapes and knocks. Unlike road biking helmets, skateboard helmets typically have a thicker, hard plastic shell which can take multiple impact before it needs to be replaced. The trade off for this is that they tend to be much heavier and less ventilated (sweatier), therefore not suitable for endurance based riding.Full face helmets (BMX style) provide the highest level of protection, being stronger again than skateboard style and including a jaw guard to protect the face. The weight is the main issue with this type but nowadays they are often relatively well ventilated and made of high tech materials, such as carbon fiber. Full face helmets with detachable chin guards are being developed but are not yet available in the U.S. As all helmets should meet minimum standards, SNELL B.95 (American Standard) BS EN 1078:1997 (European Standard), DOT or motorized ratings are making their way into the market. The choice of helmet often comes down to rider preference, likelihood of crashing and on what features or properties of a helmet they place emphasis. Helmets are mandatory at competitive events and almost without exception at bike parks, most organisations also stipulate when and where full face helmets must be used.