Every head on a bicycle needs a helmet. And all bicycle helmets should be registered and approved. It has been shown that wearing a helmet reduces head injury by 70-88% and reduces facial injury by 65%.
In a world of statistics and numbers it becomes hard to discern what is true and what is not, but the above conclusions on how helmets greatly reduce head and face injury, is based on a thorough analysis of many different studies done by the prestigious Harborview Injury Prevention Research Center
In addition to that distillation of facts, the astounding number of studies amassed at the
Bicycle Helmet Safety Institute
site, is about as complete as they come. Here you will find statistical studies done about bicycle helmets and bicycle safety in general; quantities purchased, effectiveness of one design over another, effects of alcohol when on a bike, how different age groups are affected, and on and on and on.
So when you buy that helmet there are a few things to look for. Inside of the helmet you should see an assortment of tags certifying that it has been approved to be protective for your heard. The CPSC tag is probably the most important - but also look for ASTM, Snell, and ANSI certification as well. In many cases you will find multiple stickers from a combination of these organizations.
Most of all you need to pay attention to the four year rule. Your helmet ages over time. In that time, currently set at four years, the foam used to provide the protection for your head, loses it's resiliency and begins to get more brittle. It is recommended that after 4 years of the date of manufacture, that the helmet be replaced in order for it to offer the optimum protection.
In the photo above you can see just to the right of the model name on the white tag inside the helmet there is a date printed. Add four years to that date, and you have a good idea of when it's time to buy a new helmet.
Note too: When purchasing a helmet "on sale" that the date may already be one or two years past, effectively limiting the time that your helmet will provide optimum protection.