Dear Richard:
  The concern was for the exposed styrofoam helmets. All helmets (well, at 
least all the ones I can think of) contain styrofoam or expanded polystyrene 
as the substance that absorbs the forces of a collision. Styrofoam might be a 
trademark. Usually this material is covered by a hard shell. Some helmets cut 
down on weight by leaving out the hard shell. The hard shell is slippery, but 
the styrofoam is not; it looks as if it could snag on pavement, possibly 
snapping back one's head. All my helmets have hard shells, whether for 
motorcycling or bicycling.
  Blair
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