Plastics/polymers used on helmets degrade both with UV exposure and outgassing. So although I think it is a slower process, it is possible that outgassing of plasticizers can possibly affect the helmet's performance as the rigid shell parts becom more brittle and might not provide the function intended in a crash, which is to keep the crushable expanded foam components of the helemet and the whole helmet in one piece. The expanded foam portion of the helmet degenerates similarly. Anyone can remember seeing simialr expanded foam materials eposed to the lelments for long periods of time, like as found in junk on beaches turns into something that is dried out and frangible into foam dust. Now whether one can pinpoint the amount of time when a helmet degrades to uselessness is a question, but the rough estimates put out there by helemt manufacturers or around 3 to 5 years is, I suspect, well within the safe side to minimize liability/litigation.....plus maybe flavored with a bit of marketing to urge some turnover and new sales.

Chombi