Originally Posted by wipfel
Supposedly the foam starts to get brittle, right?
Over time, and with exposure to UV, petroleum vapors, acids (from sweat mostly), and salt, the comfort padding, energy-absorbing liner, shell, and retention system can all suffer degradation.
AFAICT, the primary reason for recommending helmet replacement on a regular interval is that this is a standard risk management technique for personal protective equipment. Brand new equipment has a known history and the components can be confidently counted on to behave as designed. The older the equipment gets, the less certain one can be about whether (for example) it's suffered damage from too much UV exposure (how many hours in the sun was it exposed to? How many of those hours were of direct exposure of the interior?), petroleum vapors (Did the owner, or previous owner, store the helmet near a gasoline can? Did they routinely set it on their bike's fuel system vent?), or corrosive materials (have the rivets in the retention system started to rust?).
Most of this potential damage is not visible to the eye (for example, the EPS foam can be damaged in many ways not visible, particularly since it's almost never a solid block, and it's properties rely on it's configuration as well as it's chemistry), and non-destructive testing via other means is:
a) too expensive for most individuals
b) requires specific skills, again not held by most individuals
c) incurs a legal liability on the part of the individual/organization doing the testing
Recommending helmet replacement on a routine schedule minimizes user risk as part of a comprehensive approach to risk management. Other aspects of that approach are:
a) regular inspection of the condition of the helmet
b) careful attention to the storage environment of the helmet, so that exposure to degrading factors is less likely to go unnoticed.