Bicycle helmets typically have a very thin polycarbonate or other plastic exterior shell and a protective inner liner typically made of expanded polystyrene (EPS). The very thin foam padding that rests on your head is designed to make the helmet fit comfortably but does not have protective qualities.
One misconception is that EPS is synonymous with Styrofoam. They are two separate materials. Styrofoam is a trademark from Dow Chemical Company for a specific type of extruded polystyrene used frequently in building materials (those blue insulating boards), craft products, and other products. EPS is a generic term used to describe the foam products you see in various forms every day—foam drink coolers, foam cups, insulating materials, and, yes, bicycle helmets. While EPS has become the industry standard in bike helmet liners, other types of foam/plastic are used as well. The Bicycle Helmet Safety Institute (BHSI) has a great article on the different types of foam used in bike helmet liners. A helmet that works well for one sport may not be suitable for another sport. Bicycle helmets are required by federal law, to meet Consumer Products Safety Commission (CPSC) safety standards. If you’re biking, be sure you are using a cycling helmet that meets CPSC standards. For instance, most helmets designed only for skate use do not meet CPSC standards. Sometimes a helmet isn't just a helmet.
A number of helmet manufacturers recommend replacing helmets every three to five years. For instance, the VP of Corporate Affairs at Easton Bell Sports recommends replacing helmets after three years of use. He acknowledges that this is a conservative approach but notes: “True, many or most helmets ten years old may work fine. What we can’t say is that any particular helmet of that age will perform properly.”
Easton Bell was kind enough to send us some photos of their rooftop helmet garden (complete with Redwood duff.) Easton Bell noted that “One of the helmets in the front has had other environmental tests done to it, as we have a UV tank, oven, and freezer we use. It has been pretty messed up by exposure, but is not yet ready to impact. Three other helmets are off the roof right now in the hands of one of our engineers.”
The folks at the Bicycle Helmet Safety Institute (BHSI) also weigh in on when to replace a helmet. They are a bit more liberal in their replacement recommendations, going beyond the five-year mark if a helmet is not cracked or otherwise damaged. BHSI also conducted a study of the effect of cosmetics and other commonly used chemicals on helmets. Their initial visual observations were that helmets dosed with DEET (used in many insect repellents) and those dosed with sunscreen showed some visible degradation to the protective foam liner. However, test conducted by an independent laboratory showed that, which the exception of a helmet dosed with gasoline, all the helmets still passed the CPSC bike standard. The complete results of the BHSI test (with copious photos) is online on their website.
The one place where all these sources agree is that a helmet must be replaced if it’s taken a significant hit—an accident or fall hard enough to make you say, “Wow.” EPS foam is made to compress upon heavy impact. This disperses the force of the impact so that the helmet, not your skull, is taking the brunt of the blow. However, EPS foam does not completely regain its shape after a major impact. The bottom line from all these sources is that, if the helmet has been in an accident, replace it, even if it does not look damaged. Ultimately, the decision to replace a helmet or not rests with the wearer. We’re happy you’ve taken the precaution of wearing a bike helmet in the first place. When and how often to replace that helmet is your choice. Please remember that if your helmet has taken a significant hit—an accident or fall hard enough to make you say “Wow”—replace the helmet, even if it does not look visibly damaged. Just as you (should) give your bicycle a full check-up at the beginning of each season to see if anything needs to be replaced and a quick once-over before each ride to make sure the tires are properly inflated and nothing is loose, you should inspect your helmet regularly. Before you take your first ride of the season (preferably more than two minutes before you plan on riding, so you have time to replace if necessary), give your bike helmet a good inspection. And again, if you’ve sustained a heavy blow while wearing that helmet, it’s time to get a new one. If you’re able to read this, the helmet did its job, but now it’s time to retire the old soldier.
It’s a common (and environmentally responsible) practice for parents to trade off clothes, helmets, and sports equipment to other parents as their kids grow or to buy used sports equipment. And why not? A pair of soccer cleats that were only used for one or two seasons still have a lot of use left in them. The only case where this scenario might not be safe is in the case of helmets. Before your child wears a second-hand bike helmet, know where it came from and if the helmet ever sustained any significant hits. Just because a helmet looks okay on the outside does not mean the interior liner is still intact. Inspect any used helmet carefully. In general, XSportsProtective does not recommend used helmets. Like car safety seats, you simply don’t know the helmet’s history. And for only around $30-$40, why take the risk?