What Type of Helmet do I Need?
Most helmets are made of an EPS foam with a thin plastic outer shell. The shell helps the helmet skid easily on rough pavement to avoid jerking your neck. The shell also holds the EPS together after the first impact. Some excellent helmets are made by molding EPS in the shell rather than adding the shell later.
Ideally, you need a smoothly rounded outer shell, with no sharp ribs or snag points. Excessive vents mean less area contacting your head, which could concentrate force on one point. "Aero" helmets are not noticeably faster, and in a crash the "tail" could snag or knock the helmet aside. Skinny straps are less comfortable. Dark helmets are hard for motorists to see. Rigid visors can snag or shatter in a fall. Helmet standards do not address these problems, so it's up to you to be careful in this regard.