CATHERINE TALESE rides her bicycle everywhere — to work, to the theater, out to dinner — and always has a helmet on her head.

“There was a time when I didn’t wear a helmet; I thought I looked like a dork,” Ms. Talese, a freelance photography director who lives in Manhattan, told me recently. “But I’ve realized it’s not negotiable. Helmets are really your only safety gear in a city where pedestrians and drivers are still learning to share the road with bikers.”

Whether you ride on hectic city streets or bucolic back roads, helmets are essential armor. Bicycle helmets have been shown to reduce the risk of head injuries by up to 88 percent and facial injuries by 65 percent, according to a Cochrane Database Systemic Review published in 2000. Bike riders who play against those odds do not fare well in accidents. More than 90 percent of the 714 bicyclists killed in 2008 were not wearing helmets, according to the Insurance Institute for Highway Safety.

Even a light blow to the head can be serious.

“You don’t have to be going fast to hurt your brain,” said Dr. Angela F. Gardner, president of the American College of Emergency Physicians. A simple concussion can be debilitating, keeping you off the job or operating at half speed for weeks. “And every concussion increases the likelihood that you will have an injury to the brain if another concussion occurs,” Dr. Gardner said.