People often search for shade to avoid direct exposure to the sun, assuming their skin is fully protected as long as they are out of direct burning rays. But seeking shelter under a beach umbrella won't protect from damaging UV rays. Even sunscreens with SPF 100 — the highest rating possible — aren't completely protective.

A new randomized clinical trial published in JAMA Dermatology examined people who spent three-and-a-half hours on a sunny beach. Some were protected by an umbrella and others wore SPF 100 sunscreen. They were examined for sunburn on their bodies (face, back of neck, upper chest, arms and legs) about a day after sun exposure.

Researchers found that 78 percent of participants who were under the shade of a beach umbrella developed sunburn compared with 25 percent of participants who used SPF 100 sunscreen. There were 142 sunburn incidences in the umbrella group and 17 in the sunscreen group, according to the study.

"Umbrella shade alone may not provide sufficient sun protection during extended exposure to UV rays," the authors wrote. "Although the SPF 100 sunscreen was more efficacious than the umbrella, neither method alone prevented sunburn completely under actual use conditions, highlighting the importance of using combinations of sun protection practices to optimize protection against UV rays."

You can lower your risk of sunburn by following these guidelines:

• Avoid direct sun between 10 a.m. and 4 p.m., and stay in the shade when possible.

• Don't be fooled by clouds; you can get sunburn on cloudy days.

• Wear protective clothing, such as long-sleeved shirts and wide-brimmed hats that shade scalp — especially important if you are bald or have thinning hair — and your neck and ears as well as your face.

• Protect your eyes with sunglasses, preferably wraparound, that have broad UV protection which absorbs at least 99 percent of UV rays.

• Use a sunscreen with at least a 30 SPF, and apply it at least 15 minutes before going into the water. Reapply after swimming or sweating.