FRIDAY, June 25 (HealthDay News) -- Sunscreen use has become routine for many Americans, but it's often applied incorrectly, according to a Saint Louis University expert.

"I've seen mothers at the park spraying sun block on babies from too far away to do any good. Others dab on too little to protect themselves adequately from the sun's rays," Dr. Quenby Erickson, an assistant professor of dermatology, said in a university news release.

"If you've gone through the admirable effort of buying and applying sunscreen, make sure it's working while you wear it," she added.

Erickson offered the following advice:

• Use enough sunscreen, for starters. Adults need the equivalent of a shot glass full of sunscreen to cover their whole body. Cover all the skin that's exposed to the sun, including your ears, the back of your neck, the tops of your feet and, if you're balding, the top of your head.
• Apply sunscreen 30 minutes before going outdoors, and reapply every two hours and after swimming.
• When using spray sunscreens, hold the bottle two to three inches away from the body. Rub spray sunscreens into the skin for full coverage.
• Check your sunscreen's expiration date -- it can be less effective after that time.

More information

The U.S. Food and Drug Administration has more about sun safety.