Every three days, a bicycle accident in the U.S. kills a child. Every day, at least 100 children are treated in emergency rooms for bicycle-related head injuries. Experts are wondering precisely why only 21 states have laws requiring kids to wear helmets when biking, because in places where no such laws exist, fewer than half of all children ages 4-17 strap on a helmet when riding.

The startling percentage comes from the University of Michigan C.S. Mott Children's Hospital National Poll on Children's Health, Ann Arbor, Mich.

"These statistics underscore the importance of helmet laws to help prevent death and injury from children not wearing helmets while riding their bikes," says poll director Dr. Matthew Davis.

Meanwhile, the National Highway Traffic Safety Administration, Washington, D.C., predicts that universal use of helmets among bicyclists ages 4-15 would prevent more than 90,000 head and facial injuries per year.

Dr. Davis says not wearing a helmet comes down to a number cultural and economic factors. He recommends passing more helmet laws and more efficiently disseminating information about free or inexpensive helmets available to those in need. A third, more conceptual way to attack the issue is to educate families on the importance of wearing helmets in the same way they learned to accept wearing seatbelts in the 1980s.

"That is going to be perhaps the toughest," Dr. Davis says, "because it involves communicating the benefits of the health behavior and really trying to make a longstanding difference in the attitudes of parents and in the communities that may not yet be on board with the use of bicycle helmets."

The poll finds a sharp decline in kids who wear helmets from ages 4-11 (53 percent) to 12-17 (29 percent), as wearing one can be considered "uncool" for teens. In states that have helmet laws, 12-17-year-olds wear a helmet 54 percent of the time.

"There is a challenge here for health care providers and public health officials," Dr. Davis says, "to communicate that wearing a helmet is actually the cool thing to do besides being the healthy thing to do."