One of the more interesting details in Mark Bowden's book "Blackhawk Down" is that the older, more experienced Special Forces soldiers did not wear ballistic Kevlar helmets, opting instead for plastic Pro-Tec skateboard helmets. The latter were more lightweight, more comfortable and did protect from the head bumps a Special Forces troop was bound to encounter while crawling around in tight spaces, although they offered zero protection from bullets. In short, they were willing to sacrifice safety for comfort.
Similarly, in New York during summer you'll often see messengers whizzing around on scooters with their helmets perched on top of their heads, like bulbous beanies. Obviously it offers little protection that way, but it is significantly cooler, temperature-wise.
Apparently messengers do the same thing in Brazil, and Sao Paulo design firm Questto Design noticed. Their design of the Capacete E8 helmet, produced by Brazilian firm EBF, has a hinged lower fascia that can be tilted up and out of the way, providing some temperature relief while ensconcing the rest of your noggin.