A major theme in the helmet market since 1997 has been more and larger air vents.
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. To provide impact protection with less foam the manufacturers normally have to harden the remaining foam, so that the force of a blow is transmitted to the rider's head with more pressure on one particular spot. Opening up more or larger vents often is achieved by molding the EPS part of the helmet with the plastic shell in the same mold.