Part of the three-year recommendation has to do with the fact that sweat and UV radiation both have degratative properties (sweat is corrosive and UV radiation ages the material more quickly). I don't know about you, but I sweat like crazy when I ride, so after this season, I will be replacing my now three-year old helmet. Trust me, once you've crashed hard enough to nearly kill you WITH a helmet, you don't want to gamble on your safety.