Originally Posted by cyccommute

It’s the combination of oxygen and UV radiation that causes the problem. The combination of the two results in photooxidation of the polystyrene molecules. UV causes oxygen to form free radicals in the polymer by splitting the bonds. Oxygen reacts with the radicals and weakens the polymer. It would be a mistake to assume that the damage is only done to the outer most parts of the polystyrene, however. The entire process causes embrittlement of the styrene throughout the helmet and reduces it’s ability to crush and absorb the energy of impact.