According to OSHA, a hard hat must be worn when working in areas where there is a potential for injury to the head from falling objects or when there is the risk of exposure to electrical conductors that can potentially contact the head.
Hard Hats are divided into two categories:
Type I and Type II
Type I Hard Hats are intended to reduce the force of impact resulting from a blow to the top of the head, such as from a hammer or nail gun falling from above.
Type II Hard Hats are intended to reduce the force of lateral impact resulting from a blow which may be received off-center from the side, or to the top of the head. This form of impact may result from contact with the sharp corner of a side beam for example. Type II hard hats are lined on the inside with a thick, high density foam and suspension.