from the BHSI @

http://www.helmets.org/ventsqua.htm

Vents and Square Lines:
Problems with some designs

A major theme in the helmet market since 1997 has been more and larger air vents. All major manufacturers now have hyper-ventilated models following in the footsteps of Giro's 1997 Helios model (now discontinued). Manufacturers tout the number of vents in their helmets, a meaningless parameter. If all else were equal, more vents would be a Good Thing, but as usual, all else is not equal. Unfortunately opening up new vents usually requires harder, more dense foam and squaring off the edges of the remaining foam ribs to squeeze out the most impact protection possible from the narrower pieces still there. Since we believe that rounder shells and less dense foam are virtues in a crash, we don't recommend hyper-vented helmets unless you can't live without the added ventilation.