| UV will definately break down the plastic shell, though I have no idea how long it would take to have an appreciable effect on a helmet. Probably quite a long time. Sweat will have no effect on the styrofoam- you'll stink yourself out way before you sweat out the foam.
Basic helmet rule (of any sort) is that if there's a crack or a significant scratch (deep gouge rather than a cosmetic scrape), replace it. A gouge is a structural weakpoint, and may cause the shell to break under less than test pressures.
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"Man is an imperfect animal and never quite trustworthy in the dark."- HG Wells |