I suggest getting a new helmet.

Manufacturers claim that excessive sunlight degrades the Styrofoam and suggest replacing helmets every three years. And, while you may not have crashed, how many times have you dropped your helmet on the ground, such as falling off bars or a shelf or when you go to put it away? All those little drops add stress and ultimately fatigue the helmet.

Whether you believe the manufacturers claims about replacing helmets after three years - why risk it? I like to believe my head is more important than the $200 a new helmet costs.

Finally, you can keep this helmet as a back up, but if you are going to throw it away, be sure to break it first, so that no one pulls it out of your garbage and uses it.