Unlike sunburns and suntans, which appear after a few hours or days, UV-induced photoaging—the external and internal changes in sun-exposed skin—develops over decades, and it is irreversible.

Avoiding photoaging

People who want to protect their skin must engage in sun-safe behavior: avoid intense sun exposure, wear a hat, sunglasses and sun-blocking clothing and make judicious use of a balanced sunscreen that protects against all types of UV radiation.

Judicious use of a balanced sunscreen requires an understanding of a sunscreen’s Sun Protection Factor (SPF). One might assume that applying sunscreen with an SPF of 100 would allow you to bare your skin 100 times longer before suffering sunburn. This simply isn’t the case.

By the numbers

For high-SPF sunscreens, theory and reality are two different things. When properly applied, SPF 50 sunscreen blocks 98 percent of sunburn rays, while SPF 100 blocks 99 percent. When used correctly, a sunscreen with SPF value in the range of 30 to 50 will offer strong sunburn protection, even for people most sensitive to sunburn.

"One might assume that applying sunscreen with an SPF of 100 would allow you to bare your skin 100 times longer before suffering sunburn. This simply isn’t the case"

For all SPF levels, applying more is better. Numerous studies show that sunscreen users apply between one-fifth and one-half the quantity of sunscreen recommended by makers. By applying only 25 percent of the ideal amount of SPF 30, the sunburn protection results in a functional SPF of only 2.3.

Someone who applies SPF 100 sparingly can wind up with a level as low as 3.2. Used incorrectly, these products are less effective than T-shirts, which generally have an SPF of 5.

Sun safety

In the battle against photoaging, remember to practice safe sun: avoid UV rays during the hottest part of the day, wear sun-protective clothing, a hat and sunglasses; liberally apply a balanced sunscreen to any and all exposed skin. And, if you’re planning on staying out for long periods or spending time in the water, don’t forget to reapply.