Two basic types of sport helmets

Helmets are usually found in two categories: one-use and multi-hit.

Single use helmets are mostly made with expanded polystyrene, or EPS, because it is cheap, light, easy to manufacture and has excellent crush characteristics with very little rebound. Once crushed it recovers some part of its thickness, but does not recover its protection. If you don't discard it after the first hit, you will be in for a nasty surprise if you happen to hit on the same spot for a second hard impact! Bicycle, motorcycle, roller skate and equestrian all season helmets normally use EPS for impact energy management.

The first multi-use bicycle helmets were made with expanded polypropylene, or EPP. EPP looks like EPS, but has a slightly rubbery feel. It recovers slowly after a blow and is good for more hits. Nobody can tell you how many more risks, but some. Its crush and manufacturing characteristics are not quite as good as EPS, so the helmet might have to be thicker, and it rebounds enough during the impact sequence to make it less than ideal, although the rebound occurs after the lab has measured the performance of the helmet and is missed in standards testing. EPP is used extensively in automotive padding, for things like the foam to back up a bumper. There are now on the market a few EPP helmets that meet both the bicycle helmet standard and the skateboard helmet standard – they have stickers inside telling you that.

All season multi-hit helmets are mostly made with butyl nitrate foam,”squishy" but dense foam that is good for many impacts. It is mostly black or gray. It is heavier than EPS and cannot manage as much impact energy for a given thickness. Hockey and football helmets are made this way, and so are white-water, old-style skateboard and aggressive trick skating helmets. You don't have to throw the helmet away after a hit, but it normally is not much thicker than an EPS helmet, and that means it will not manage as big an impact. Typical lab drops for multi-use helmets are one meter. For single-use EPS helmets the typical drop is two meters. That's a very large difference in impact protection.

Another "squishy" foam, but with superior impact characteristics is the foam marketed by THE SHOP. Behind the name is really good foam, tough for multi risks and "rate-sensitive" to make it stiffen up if the impact is really hard and ease up if the impact is lesser. It might be a good choice if avoiding concussions is your primary goal. Most helmets are designed to protect primarily against the high-end impacts that cause catastrophic brain injury, letting enough energy through to give you mild concussions. SHOP NAME helmets are heavy and soak up sweat, but some of them meet bike, ski and skateboard standards.

Usually you should not use a single use helmet for a multi-hit sport because it will not be replaced when it should be, i.e. after one hit, and when you hit again there will not be enough protection left. Looking at a crashed EPS helmet will make a believer out of you. And you should not use many of the multi-impact helmets for single-hit sports like bicycle riding because they will not have enough protection in that big hit. So you might use a skateboard helmet for another sport where the impacts are similar, repeated and less severe, but you would not use one for equestrian events. Roller-skaters are in luck, their activity is now included in its bike helmet standard, which is identical to the bicycle helmet standard.