How long do helmets last?

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Are there any materials scientists on the board? Here's a question that comes to my mind every now and then: what is the useful life of a helmet if you don't crash? They are mostly made of foam, after all, and their usefulness during an impact must be related to the shock absorption capacity of the foam. I assume that foam degrades over time, becoming stiffer, so perhaps it might be better to replace the helmet every couple of years. Since this is way out of my expertise, I have a tough time doing a good search of research papers.

Edited to add: The Bicycle Helmet Safety Institute recommends replacing the helmet every 5 years; Bell recommends every three years. There does not seem to be much testing out there. Here's the link: http://www.helmets.org/replace.htm