I know form personal experience that indeed styrofoam DOES degrade when improperly stored at least.

I bought a new helmet 20 years ago, used it till we moved 16 years ago. After we moved I cycled a little for a summer and then the bike and helmet went into the shed in the yard. Exposed to heat and cold, dampness and dryness and very LITTLE UV exposure. When I pulled the bike out after being in the shed for 15 years the helmet was flaky and shed powdery dust when I rubbed it. I was able to crumble the edges of the foam part of the helmet very easily and when I wore it a few times it left a powdery residue on my skin. It was shot. I bought a new one.

Question is: Why take a chance? They are cheap enough. if you value your brains then get a new one if at all in question. I would also question the integraty of the strap as well, especially being soaked in acidic sweat repeatedly.