alright this is how it works.
Skate/Wake/Waterwater - Im talking about the plastic shell ones with some foam paddling on the inside (like a Capix or Pro tec). The difference between a wake and skate helmet is the foam in the wake one will not absorb water, and will protect you just as well as a stake helm. And people wear helmets in wakeboarding because you can smoke your head on a rail.
Ski/Bike - These are foam compression (hard styrotype foam) helmets, that usually have a really thin layer of plastic around it. For Appearnce reasons, cause styrofoam don;t look all that hot. ie one big hit and they supposed to be replaced style (your giro 9, burton hi fi etc) These offer MORE protection then a skate/wake helmet, and will give more protection in a larger impact.
I got a Capix Wakecap that i plan to use for whitewater, and riding around doing urban stuff on my suspension bike, i figure it offers more protection then wearing nothing at all which is usually the case. I can use my Capix wakecap skiing, but they are most det don;t insulated your head alot so you'll have to wear a hat underneath to keep warm, and i have also heard that in really cold temperatures the plastic shell can crack under a little bit of pressure (it gets brital), because it really wasn;t made for below freezing temps.
But i have  Burtin Hi fi helmet which will - keep my head warmer and protect me better, but maybe not look as dope.
I hope that made some sence
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