It is well known that helmets are very effective in preventing or reducing the severity of injuries to the head. A review of all (53) available studies on helmets  concludes that helmets are effective in reducing head injuries to motorcyclists who crash by 72%. To study this effect requires a sample of accidents with large numbers of riders with and without helmets. A recent study is from Greece where helmet wearing rates are still low . This sample consisted of 143 motorcycle riders with helmet and 1764 without killed or injured in accidents. The fatality rate of riders with helmet was 44% lower than for riders without a helmet.
The main purpose of helmets is to reduce the peak and duration of acceleration of the head by absorbing energy in case of a collision. There are two types of helmet: open face and full face. The full face helmet provides better protection to the face and chin area but is usually a little heavier. On the basis of research it is not possible to decide which type is better.
Tests at TRL with different energy absorbing liners and shells have shown that standard helmets have liners that are too stiff and shells that are too stiff and resilient . A more recent study showed that this may be correct when helmets are tested with existing standard tests. Tested with a more demanding test (which is recommended in that report) required an even stiffer shell. Anyway, this means that the design of helmets for PTW riders can be much improved. Eventually the motorcycle helmet may have a different design from a moped helmet. So far helmets for motorcycle riders and moped riders are the same and the standards for (testing) helmets make no difference between the two.
From the point of view of preventing injuries there is no reason why any group of PTW users should be exempted from compulsory wearing of a helmet.