Question: I see some cyclists who do not wear a helmet. Since I am beginning to cycle for exercise, can you tell me how important a helmet is and what kind I should buy if I need one? Answer: There's no question about the importance of wearing a helmet when cycling. It has been established that about 75 percent of all bicycle fatalities result from head injuries and that most of these could have been prevented by a good helmet.

A friend of mine in California is a case in point. He went for a ride one noon hour without his helmet. Coming back to school he stopped at a stop sign to allow traffic to pass and lost his balance. He tried to pull his foot from the pedal clip to catch himself, but his foot caught and he fell, hitting his head on the cement curb. The fall caused a concussion that killed him. With a helmet, he would have been bruised on his shoulder, but his accident would not have been serious.

The April "Living and Fitness News" (American Running & Fitness Association) reviewed an article about cycling helmets. They said that a good helmet has a hard shell made of fiberglass, lexan or ABS plastic to disperse the energy of impact and to protect the soft inner lining from wear and tear. The inner lining is usually made from stiff, non-springy foam or polystyrene, an upgrade of the foam used to protect eggs and stereo equipment. Soft foam is used inside the inner liner to help the helmet fit your head comfortably. Helmets should be designed in such a way that air is allowed to circulate around the head, or they would be unbearable for summer riding.

All helmets should have a good strap system to keep them in place if you fall. They would be of little use if they turned around on your head and allowed it to contact the surface. The instructions that came with the cycling helmet I bought say to send it back if it is ever involved in a crash and they will send a new one. Apparently the foam can only absorb a blow one time, then it loses its protective value.

In the United States new helmets are measured by two standards and have stickers inside them stating their rating. The Snell Memorial Foundation crash-tests helmets, straps and fasteners and awards a rating meeting their 1984 standard.

The Bell Tourlite, Bell V-1 Pro, Shoei RC-2 helmets meet these standards. The American National Standards Institute also rates helmets. The BAILEN, Bell Stratos, Bell Windjammer, Cycle Products, Tiro, Maxon, Monarch Aerodyne, Monarch SuperTour, Nova and Vetta helmets all meet their "good" rating.

A bike shop or sporting goods store can help you fit your helmet properly. My helmet felt a little small when I first put it on, but it loosened up just the right amount after several wearings.

Prices range from $25-$50, and some cyclists order theirs through mail-order ads to save money. However, I like to try things on and have not always been successful when buying things through the mail. In any case, it is critical to wear this important piece of safety gear.

* Garth Fisher is director of the Human Performance Research Center at Brigham Young University.