The D.O.T. standard (Federal Motor Vehicle Safety Standard no. 218) requires, among other things, that a helmet soak up a significant amount of impact energy, prevent most penetration, and have a fastening system that will withstand significant force.
(more on choosing helmets...) (How to determine if a helmet is safe)

Vented Half Helmets 6 styles to choose from	Shorty 9 styles to choose from	German 2 styles to choose from
Lady Rider 12 styles to choose from	Polo 9 styles to choose from
Half Shell 2 styles to choose from	3/4 Face styles to choose from	MotoCross
Full Face 5 colors to choose from	Full Face Modular 4 colors to choose from	Open Face 3 colors to choose from

Airbrush Designs: Boneyard, Eagles, Flames, Skulls, and Rebels

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