Sunscreen can be a valuable tool for skin cancer prevention — but only if it’s used correctly. When applying sunscreen, many people make mistakes that could compromise their protection from the sun’s harmful ultraviolet rays, which may increase their risk of skin cancer.

Some of those mistakes are highlighted in new research published online in the Journal of the American Academy of Dermatology. Researchers set up free sunscreen dispensers at the Minnesota State Fair and observed 2,187 people using them over the course of 93 hours.

Only one–third (33 percent) of people applied sunscreen to all exposed skin, and just 38 percent were wearing sun–protective clothing, hats or sunglasses. Additionally, utilization of the free sunscreen dispensers decreased significantly on cloudy days.

“These results highlight some of the ways people use sunscreen incorrectly,” says board–certified dermatologist Ingrid Polcari, MD, FAAD, an assistant professor in the department of dermatology at the University of Minnesota Medical School in Minneapolis and one of the study authors. “To get the best possible sun protection, it’s important to wear protective clothing, such as long–sleeved shirts and pants, and to apply sunscreen to all exposed skin, not just your face and arms.”

“Everyone should apply sunscreen every time they go outside,” Dr. Polcari adds. “Even on cloudy days, up to 80 percent of the sun’s harmful UV rays can reach your skin.”

The researchers also observed that more women than men utilized the free sunscreen dispensers at the state fair; while 51 percent of the fair attendees were women, they accounted for 57 percent of the sunscreen users.

“Research has shown that women are more likely than men to use sunscreen, but it’s vital that men use it too,” says board–certified dermatologist Darrell S. Rigel, MD, FAAD, a clinical professor in the Ronald O. Perelman Department of Dermatology at New York University. “Men over 50 have a higher risk than the general population of developing melanoma, the deadliest form of skin cancer, and UV exposure is the most preventable skin cancer risk factor, so it’s important for men of all ages to protect themselves from the sun’s harmful rays by seeking shade, wearing protective clothing and applying sunscreen.”

Dr. Rigel offers the following tips for choosing a sunscreen:

• Choose a sunscreen with an SPF 30 or higher. While no sunscreen can filter out all of the sun’s UVB rays, SPF 30 sunscreens block 97 percent of the sun’s UVB rays.
• Look for the words “broad spectrum.” This means the sunscreen will protect against both UVA rays (which cause premature skin aging) and UVB rays (which cause sunburn). Both types of UV rays can lead to skin cancer.
• Look for the words “water resistant.” No sunscreen is completely waterproof, but water–resistant sunscreens can provide protection for wet or sweaty skin for 40 or 80 minutes, as indicated on the label. All sunscreens should be reapplied every two hours, or after swimming or sweating.
• For sensitive skin, choose a sunscreen with the active ingredients zinc oxide or titanium dioxide. Those with sensitive skin also should avoid sunscreens that contain fragrance, oils and para–aminobenzoic acid, also known as PABA.