My kids grow out of their bike helmets quickly, and the helmets are very expensive to replace. Is it OK to buy them used at a garage sale?

First of all, it’s great that you insist that your kids wear bike helmets. Research shows that wearing one while riding reduces a child’s risk of brain injury by 88 percent. But, the truth is, it’s best to buy a helmet new. It could have been damaged in a crash -- even if you don’t see cracks -- and might not be able to withstand another one. When purchasing a helmet, look for the CPSC seal, which means it meets the standards of the Consumer Product Safety Commission. The helmet should sit flat on top of your child’s head and be snug enough so that it doesn’t slide down over the eyes when pushed or pulled. The chin strap should be snug. Many kids wear their helmets loose and tipped back, exposing their foreheads. But this doubles their chances of suffering a serious head injury. Never buy a helmet that’s too big so that your child can “grow into it.” It might not protect him in an accident. For an illustration of exactly how a helmet should fit your child, check the CPSC Web site.

Expert Frederick P. Rivara, M.D., is a bike helmet researcher and professor of pediatrics at the University of Washington.