Planning a sunny escape? Staying under a beach umbrella may not be enough to prevent sunburn.

Researchers conducted an experiment in Lakeville, Tex., in August 2014, assigning 81 men and women with skin highly sensitive to burning to spend three and a half hours on a sunny beach at midday. Half used only a beach umbrella, and half used only sunscreen with a sun protection factor, or S.P.F., of 100.

(According to the Food and Drug Administration, there is insufficient data to show that an S.P.F. over 50 provides added protection.)

For each of seven exposed areas, researchers assigned a score from 0 to 4 indicating the amount of sunburn, from none to pus-filled blisters. The umbrella group averaged 0.75, and the sunscreen group 0.05.

Neither group got complete protection from burning — there were 142 sunburn incidents in the umbrella group and 17 in the sunscreen group.

“The study shows that you can’t rely on a single protection method alone,” said the lead author, Hao Ou-Yang, a researcher at Johnson & Johnson. “Reduce overall time in the sun, wear clothes and a hat, seek shade, and absolutely think about using high-S.P.F. sunscreen to protect yourself.”

Several of the authors are employed by or paid consultants for Johnson & Johnson, the manufacturer of the sunscreen used in the study, published in JAMA Dermatology.