Ask Well: Reapply Your Sunscreen
By CATHERINE SAINT LOUIS
Q. We are told that sunscreen breaks down after two hours. Does it break down at the same rate out of the sun? How many times should it be reapplied to be effective?
Asked by Anna Louise • 4 votes
The main reason to reapply a broad-spectrum sunscreen every two hours isn’t that it breaks down; most of today’s sunscreens are stable in sunlight. Rather, reapplication is crucial because most people don’t apply enough sunscreen in the first place.
A full ounce — or the amount in a shot glass — should be slathered on 15 minutes before exposure, then reapplied. Use products that offer “broad-spectrum protection,” meaning they protect against both UVA and burning UVB rays. Both kinds of radiation can lead to skin cancer.
“In the past sunscreen broke down, but now technology has bypassed that,” said Dr. Steven Q. Wang, the director of dermatology at Memorial Sloan-Kettering Cancer Center in Basking Ridge, N.J. “Unfortunately people’s behavior hasn’t changed, so that’s why the two-hour rule is still a good principle.”
Dr. Wang, who is a spokesman for the Skin Cancer Foundation, which gets funding from sunscreen manufacturers, tells his patients to apply sunscreen as if they were adding a second coat of paint to a house. First, he advises starting at the left ear, then methodically going from left arm to left trunk to left leg to left foot to right foot and back up. Reapply sooner than every two hours if you’re sweating, swimming or exercising through a scorcher.
Remember, there’s no such thing as a complete sunblock. A sunscreen offers only some protection. “People have a false sense of security after they put sunscreen on,” Dr. Wang said, “Then they stay in the sun five or six hours.”
It’s a problem worsened, in a way, by sunscreen itself. Applied copiously, a cream with a high S.P.F. prevents a lobster-red burn, a hard-to-miss signal to head inside.
But don’t be fooled. Even without a burn, your skin is still being damaged by the sun, Dr. Wang explained, “increasing your risk of developing skin cancer as well as signs of aging, like wrinkles.”