I have a Bell Metro helmet. It's fine but even with all the holes, it is very hot. It also causes skin irritation on my forehead and causes (sorry people) pimples on my scalp. Are there specific features that I should look for to have a cooler (as in temperature) and less irritation experience for my head, as it exists under the helmet?
The most important thing to look for when shopping for a helmet that will keep you cool is that air flows all the way through it from front to back.
Put the helmet on and look in the mirror. If you can see all the way through the back of the helmet, it'll probably keep you relatively cool. And the more you can see through it, the better.
Higher end helmets will also sometimes have vents that scoop air down into the helmet. The vents are raised at the front and lower at the back to force air down into the helmet as you ride along. I don't know what your budget is, but you'll probably pay a pretty penny for one of these.
You could also try wet bandana on your head. That works wonders for me.
It's not necessarily about how many holes your helmet has as opposed to how the air flows through. It's important to try them on before you buy them, so I would look for the lightest helmet that still provides a good fit with comfort.
If your helmet is causing you irritation it may be fitted improperly.
Skin irritation could be heat and / or materials. Or both. My son gets rashes and bumps from synthetic fabrics on his skin in the summer, but not winter.
Try a 'do rag'.
But a better vented helmet will also add to the cool air coming in. Stick with lighter colors to reflect the sun more.