I'm a transplant to Southern California. The sun does wonders for my disposition, but a recent trip to Dr. Sandra Morimoto, my dermatologist, got me wondering about what it does for my skin. So back to Morimoto I went.

"The skin is composed of multiple layers," she began. "The outmost layer functions as a mechanical barrier to outside chemicals and microbes, and prevents fluid loss. Underneath that is the epidermis, which consists of multiple layers of living keratinocyte cells. Scattered in a random pattern throughout the epidermis are the melanocytes, the cells that produce melanin pigment. They give skin its color and its ability to tan. Below the epidermis is the dermis, which contains collagen and elastin proteins. These elastic fibers are some of the first things to be destroyed by chronic sun exposure. Loss of them causes the skin to sag."

Tanning, said Morimoto, "is the skin's response to damage from ultraviolet light. You cannot tan without injuring your skin. A sunburn occurs when excess ultraviolet radiation damages cell proteins. As the cells die off, they set off an inflammatory immune response, which causes the blood vessels to dilate and become leaky. That results in redness, swelling, and blistering of the skin. Then the immune system goes into repair mode."

Ultraviolet light, she said, "consists of three bands, or wavelengths. The shortest, called UVC, is filtered by the ozone layer, and doesn't reach the earth. The middle and longest wavelengths, UVB and UVA, do make it through. UVB causes skin reddening, sunburn, and tanning. Chronic UVB exposure increases the risk of skin cancer. UVA rays, which comprise 96% of ultraviolet light, penetrate into the lower dermis and augment UVB damage in sunburning and tanning. Both UVB and UVA damage cell DNA and can cause mutation. Those damaged cells can replicate into a tumor. That's why it's important to use a broad-spectrum sunscreen that blocks both UVB and UVA."

Morimoto sought to explain sunscreen's protective action beyond the SPF numbers. "Sunscreens work in one of two ways. They are either physical blockers which block or scatter radiation the way a lead blanket would, or chemical blockers, which bind to the skin and absorb UV radiation. Most sunscreens block well in the UVB range, but are lacking in the UVA range." Morimoto preferred the physical blockers, which "block a wide range of light." Plus, they're hypoallergenic. "Titanium dioxide and zinc oxide are the ingredients in physical sunscreens. Micronized zinc oxide (which has smaller particles than ordinary zinc oxide) has the broadest spectrum of protection of any product on the market. And it is more translucent than regular zinc oxide, so it's more cosmetically acceptable" than a layer of white goop.

The divisions among the chemical sunscreens were legion, but Morimoto dove right in. The first division was between "those that absorb primarily UVB, and those that absorb UVA as well." A product called PABA was the first commonly used UVB blocker, "but due to its propensity for causing allergic reactions, it's rarely used." Cinnamates and salicylates tend to cause fewer reactions, and are more water-resistant to boot.

UVA-absorbing sunscreens were also divided. "Benzophenes contain Oxybenzene, which is used in 20-30 percent of sunscreens. It's a very effective UBA blocker, but it can cause dermatitis. The second UVA blocker, Parsol 1789, is the best available," but it too can cause allergic reactions.

Now it was time for the number on the bottle: SPF. "The Sun Protection Factor is a measurement of the product's ability to prevent UVB reddening. If a person normally burns after 10 minutes, an SPF of 15 would mean that the person could stay in the sun 15 times longer before burning. The numbers, however, measure only protection against UVB, not UVA. Plus, they're valid only if the person applies the sunscreen very thickly. Most people apply only a third of the correct amount. Further, many people think that SPF 30 provides double the protection of SPF 15. Actually, SPF 15 gives 93% protection; SPF 30, 96%; and SPF 50, 98%." What to use? "Use the highest number tolerated, in a form you're comfortable using. It's no good picking a high-SPF sunscreen if you don't like how it feels on your skin and you hate using it."

Morimoto offered a few parting tips, a general regimen for doing battle with the ultraviolet. "Apply a broadband-spectrum sunscreen with the highest SPF you can tolerate as thickly as you can. One ounce -- roughly enough to fill a shot glass -- is considered the amount needed to cover exposed areas. And chemical sunscreens need 15 to 30 minutes to bind to the skin before they're effective. And reapply sunscreen every two hours, even when using ones marked 'waterproof' or 'all day.' Children and people with sensitive skin or allergies do best with physical blockers, and people prone to acne should look for words like 'non-cosmetic' and 'won't clog pores.' Aerosols and sprays have problems with uneven distribution and offer less protection. Don't put sunscreen on infants under six months; it's best to just keep them out of the sun."

And even if you slather on the zinc oxide, "other protective measures are necessary; no sunscreen gives complete protection. Try to avoid the midday sun, between 10 a.m. and 4 p.m. Typical summer weight T-shirts provide an SPF of only five to nine, and even less when wet. Adding Rit Sunguard laundry treatment to the wash cycle can increase the SPF of regular clothing from 5 to 30. One treatment is good for 20 washes, and it can be purchased in many grocery stores. And a wide-brim hat gives protection equivalent to an SPF of five."

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