All helmets have a thin exterior shell that's typically made out of ABS or other durable plastic. The big difference comes in the protective inner liner that is directly attached to the exterior liner. Note that when we discuss protective liners, we are not talking about the thin comfort/fit pads that rest directly on your head. Skateboard helmets typically have a protective inner liner made of soft foam. This liner is meant to slow the force of multiple low to moderate impacts. In other words, skateboard helmets are designed to sustain a series of small impacts at low speeds, like you might get at the skate park or in the driveway practicing a new trick.
A traditional bicycle helmet has a protective inner liner made of stiff expanded polystyrene (EPS) foam with a thinner liner of soft foam for comfort. Traditional bicycle helmets are designed to dissipate the force of a single major impact (like a crash) and often break apart in the process. You don’t want to wear a bike helmet while skateboarding for the simple fact that the bike helmet isn’t designed to be repeatedly knocked around like a skateboard helmet typically is. If you love the rounded skateboard helmet design but you also love your bike, all is not lost. Just because a skateboard helmet has the bucket shape doesn't necessarily mean you can't use it as a bicycle helmet. Any helmet sold in the United States as a bicycle helmet must meet CPSC (Consumer Products Safety Commission) safety standards. Some skateboard helmets meet the CPSC safety standard and can be used for biking.
In appearance, multi-sport helmets (sometimes called "dual-use helmets") look like a skateboard helmet, with a rounded, bucket-like shape that covers the back of your neck. In some ways, it’s like you’re biking with a skateboard helmet, however, a bike/skate offers a higher level of impact protection than a skateboard helmet. This is because dual use helmets have a protective interior liner made of stiff EPS foam (like a bicycle helmet). The stiff EPS liner has greater impact absorption properties than the soft foam, because bike crashes tend to occur at much faster speeds than skateboarding. A dual use bike/skate helmet will, at minimum, meet CPSC bicycle safety standards.
The other big difference between bike helmets and skate helmets is that any helmet sold in the United States as a bicycle helmet must meet CPSC (Consumer Products Safety Commission) safety standards. A bicycle helmet MUST bear an internal or external sticker (or both) stating that it complies with CPSC safety standards. Some helmets marked as "skateboard helmets" meet CPSC safety standards and can also be used for biking. ASTM F1492 specifications cover the design and performance requirements of helmets used for "skateboarding and trick roller skating" (yep, that's what they call it). A skateboard helmet may or may not have been tested against the ASTM 1492 safety specification. A skateboard helmet that has not achieved one of these minimum safety standards is not necessarily an unsafe helmet. ASTM certification is voluntary, so most manufacturers don't seek the ASTM F1492 skateboard helmet standard for their helmets because, like anything else, the testing process can add additional cost to the price of the skateboard helmet.
The kind of helmet you end up buying depends on what activities you plan on doing. If skateboarding is your only sport, you’ll be fine with a skateboard helmet. Remember that helmets sold as skateboard helmets are not required to meet ASTM safety standards; helmets without the ASTM standard can still protect your head. If you want a skateboard-style helmet that you can ALSO use for bicycling, you should purchase a skateboard helmet that also meets the CPSC safety standard for bicycle use. If you think you’ll be going from the skateboard to your mountain bike on a regular basis, you might think about getting a multi-sport (or dual-use) helmet.
No matter what type of helmet you choose, it will only protect your head if you're wearing it. If a helmet isn't fastened on your head, it's just a funny-looking hat.