As a motorcyclist, these are some points to consider about helmets.
The rule of thumb the Motorcycle Saftey Foundation tells its members is that a helmet should be replaced every 3 years or so. The reasons are only obvious after someone points them out. If you happen to look inside the cover of a helmet, you'd see all the materials in there to absorb impact. These materials are only designed to absorb impact once. After that, they're compressed and not able to provide adequate impact energy dissipation in the event of another impact. This means that if you're ever in an accident where your head bangs into something, you should replace the helmet, even if the outer covering looks okay. This also means that you should be diligent when handling your helmet. For instance, dropping it from waist height onto a hard surface (like concrete) will cause some compression of the energy absorbing materials at the point of impact. Every time you drop your helmet or have it roll off the kitchen counter, it'll be slighty less effective at protecting your melon should you ever need it.
Remember, the cost of a helmet is insignificant to the cost of your head.
-Matt
