I have a Bell M2, which is pretty much the standard road racing/auto-x helmet. Its an SA2000 rated, which is basically the same as the Snell M2000 rating, except its fire proof and it designed to protect against multiple impacts. My question is, is this suitable for motorcycle use? Or will I go deaf from wind noise? There are vents in the top above the shield, and one where your mouth would be... Here's a pic:

 

you will go deaf but not before you're dizzy from the buffeting nd vibration. But don't take my word for it. Also the multiple impact design may not protect to the same degree as the single impact absorption design. Fire protection is nice but that means no ventilation. Foggy faceshield and sweaty head.

 

I think tony covered all the import points, but don't forget about peripheral vision. Generally helmets designed for auto-use have less peripheral vision than motorcycle helmets.