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Not much in the wya of ventilation, and it's probably foam lined like my petzl meteor III, which is basically insulation on your head.
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Most bicycle helmets operate on the concept of a one-hit helmet. Micro shell on an Expanded Polystyrene (EPS) shell. The EPS absorbs the energy of the impact by crushing or breaking and should be replaced afterward. However, this is usually a body hitting the deck with quite a bit of force. It's not intended for a bunch of small hits. Plus, the more-holes-than-helmet design
would not be well-suited to treework with all of the branches!
For multiple impacts, use the skateboard helmet concept: thick plastic shell with EPS energy absorbing layer.
The third concept is the industrial hardhat concept: thick plastic shell with suspension to absorb the energy.
Fourth concept is really just a modification of the third - climbing helmet: thick plastic shell with suspension to absorb the energy plus a chin strap.
I'm sure by way of ANSI and Snell etc., it's all much more complicated than this.
From the horse's mouth:
Salamander: ABS Shell, Polystyrene foam interior
http://www.grivelnorthamerica.com/products.php?gid=11&id=89
Oooh, purdy!
The hot new climbing helmet for 2008?
Now this is the cat's a**!
The Gladiator:
http://www.grivelnorthamerica.com/products.php?gid=11&id=90