I found this link:
http://www.motorcycle.com/buyers-guide/buyers-guide-to-motorcycle-helmets-33366.htmlpartial quote:
"Did you know?
Most manufacturers recommend replacing your helmet every 5 years, whether it bounces off the pavement or not! (And if it does ever bounce on the road, it should automatically be traded in.)
Shell Construction
A helmet’s shell is generally made from either a molded polycarbonate plastic or a fiber composite that can consist of fiberglass, Kevlar and/or carbon fiber. A polycarb shell is cheaper to produce, so this construction is usually found at the lower end of the price spectrum. A fiberglass/Kevlar/carbon shell results in lighter weight and is usually found in pricier name brands.
Inner Liner
The stuff that actually absorbs the hit to a crashed rider’s flying head is the expanded polystyrene (EPS) liner. Yes, it’s the same kind of stuff as in your well-used beer cooler. This part crushes as it absorbs energy from a hit and it doesn’t bounce back, so this is a big reason why you should always replace your lid after a crash." end quote
