What I hate about this, is that every time it comes up, nobody ever really goes into details of why. What is no longer safe? What breaks down? Or is it updated standards? Just fucking get into the reasons so people can make rational choices, instead of just saying “trust me bro.”
UV light damages the plastics that the helmet is made of. Impacts, even small impacts, compress the foam. Temperature fluctuations burst the bubbles in the foam and cause stress which weakens the plastic.
The paragraph explains why even if it doesn’t explain how.