If you regularly read Sun & Skin News, you may figure we’ve said everything we can about sunscreen. However, a recent talk with Zoe Draelos, MD, clinical associate professor, Department of Dermatology, Wake Forest University School of Medicine, gave us some fascinating facts about the structure and function of contemporary sunscreens, how to choose them, and how to use them. We can’t wait to pass them on to you:

Sun Protection Factor (SPF) is not the only way to gauge sunscreen’s protective ability.

SPF represents a sunscreen’s ability to shield against the sun’s shortwave, ultraviolet B (UVB) rays. It measures the length of time a product prevents skin reddening from UVB compared to how long the skin takes to redden without protection. However, SPF does not measure protection against the sun’s long-wave, UVA rays, which accelerate skin aging, and which recently were strongly implicated in skin cancer along with UVB. The U.S. Food and Drug Administration has not yet approved a standard to measure UVA protection. Still, we know that the chemical sunscreen ingredient avobenzone (Parsol 1789) and the physical sunscreen ingredients (formerly called “sunblocks”) zinc oxide and titanium dioxide offer significant protection against UVA.

The way ingredients mix in a sunscreen may be as important as its individual ingredients.

Knowing a sunscreen’s individual ingredients doesn’t tell you everything. The way the ingredients interact with one another and with UVA and UVB is also important in determining protection.

“The art in sunscreens is mixing active ingredients for broadest protection,” notes Dr. Draelos. “While avobenzone provides excellent protection for longer-wavelength UVA radiation, it must be combined with UVB actives and ideally some shorterwavelength UVA actives.” Usually, at least three active ingredients are called for. These generally include PABA derivatives, salicylates, and/or cinnamates (octylmethoxycinnamate and cinoxate) for UVB absorption; benzophenones (such as oxybenzone and sulisobenzone) for shorter-wavelength UVA protection; and avobenzone, titanium dioxide, or zinc oxide for the remaining UVA spectrum.

Some sunscreens last longer than others.

Another factor in determining sunscreen’s effectiveness is how well it maintains potency. All sunscreens eventually lose effectiveness on the skin. Some chemical sunscreens stay active only a short period; they can absorb only so much UV before it breaks down their molecular structure.

But zinc oxide and titanium dioxide, which have a much smaller chemical reaction to UV, stay active longer. They also stay in place better on the skin. “All sunscreen migrates somewhat when sebum (oil from the skin) and perspiration mix with it, and the less sunscreen adheres, the more uneven its protection,” Dr. Draelos points out. “Physical sunscreens create a film that adheres well. Many newer gels also create a waterresistant film that stays in place well.”

For everyday pursuits involving a few minutes in the sun, sunscreens with moisturizers are comfortable and can be effective. However, if sitting at length perspiring in the sun, you need stronger, water-resistant, beachwear-type sunscreens that hold together on your skin. Conversely, those sunscreens may not be as good for everyday incidental exposure. They are stickier, don’t go as well with makeup, and need to be reapplied every two hours.

Choosing a sunscreen is individual.

No single sunscreen is right for everyone’s needs. It depends partly on your own biology and partly on what you’re using the sunscreen for. But follow a few standards: Buy a high-quality product with an SPF of 15 or higher; check its ingredients to make sure it offers broad-spectrum protection; and decide whether it works better for everyday incidental use or extended outdoor use. Finally, look for The Skin Cancer Foundation’s Seal of Recommendation, which guarantees that a sunscreen product meets the highest standards for safety and effectiveness.

Once you choose the right sunscreen, use it the right way. “Spread it on liberally at least 30 minutes before going out in the sun, so that the skin fully absorbs it,” advises Arnold W. Klein, MD, professor of medicine/dermatology at UCLA’s David Geffen School of Medicine. “I also personally recommend reapplying after 30 minutes in the sun, then every two hours thereafter or whenever you get wet.”