The Golden Rule of helmet safety- if you take a fall where you were glad you had your helmet on, you need to get a new one. What keeps you safe is not the hard outer shell- it is the EPS foam on the inside. This foam absorbs an impact so your skull doesn't. Once it compresses, it has done its job and you need to get a new one if you want to keep your head safe for the next fall.
WE ARE SKIING.