One of the best things I have found for keeping cool on hot summer days is a vented helmet. By having just a little bit of airflow inside my helmet really makes a world of difference when the temperature outside starts to rise. Even in the winter time a little bit of cool air on the head can keep your glasses from fogging up and keep you from overheating!
Personally I wear a Petzl Alveo helmet with vents. I have a pretty big head and have found that Petzl helmets fit a bit better than Kask or CT. Not that there is anything wrong with the others, they are just a bit more comfortable and everyone knows how much of a pain an uncomfortable helmet can be!
I also attached a visor to my helmet to keep the sawdust and chips from going in my eyes when doing large removals. The visor can really be a life saver when the wind is blowing in a wrong direction and the dust is flying. I can quickly reach up and slap the visor down to give my eyes an extra layer of protection. Be sure to always wear safety glasses when wearing a visor! Another thing that I find really comfortable about the Alveo helmet is the chin strap. Now I know that this might sound weird but with every other helmet I tried I couldn’t keep the chin strap from rubbing on the bottom of my ears. This would drive me crazy! The Petzl chin strap lines up really well and is also adjustable to get it in the right spot on almost anyone’s head.
Be sure to come into the shop and try out some of these helmets before buying any. You are going to be wearing the helmet all day long so you might as well be sure that it is going to fit you right and do what you need it to do!