Summer, contrary to recent evidence in this part of the country, is usually considered the time for fun in the sun. But although some sun is nice, too much sun isn’t, because only 60 percent of sunlight is visible, and only 25 percent is heat. The remaining 15 percent falls in an invisible part of the electromagnetic spectrum called “ultraviolet”–and ultraviolet and living tissue don’t get along. Our skin is well aware of this, and reacts to ultraviolet radiation by producing more of the pigment melanin, which darkens our skin and helps keep ultraviolet radiation from penetrating.

The sun’s ultraviolet is divided into three types: “near” ultraviolet or UVA, “far” ultraviolet or UVB, and “vacuum” ultraviolet or UVC. UVC, the shortest, most energetic type, is almost completely blocked by the atmosphere, which is a good thing, because it packs almost as much energy as X-rays. UVB is the radiation that causes sunburn. UVA, which accounts for most of the sun’s ultraviolet radiation, for a long time it was seen as fairly benign. However, the growth of indoor tanning parlors, which relied heavily on UVA, prompted scientists to take a closer look at these wavelengths. They discovered that UVA could cause premature aging of the skin.

An even bigger concern than sunburns and premature aging is the fact that ultraviolet’s energetic rays can damage cells, causing damage that eventually triggers uncontrolled growth–cancer. Fortunately, most ultraviolet is blocked by ozone in the upper atmosphere. Unfortunately, man-made chemicals are depleting that ozone.

The amount and type of ultraviolet you’re exposed to varies, too, with the position of the sun in the sky, because that determines the amount of atmosphere the sun’s rays have to pass through. That’s why you’re at greater risk at noon or in the summer, when the sun is directly overhead, than in evening or in the winter, when it’s low in the sky.

Since people aren’t willing to live their lives entirely in dark caves, sunscreens were developed. These sunscreens work either by reflecting the ultraviolet (“physical” sunscreens), or by absorbing it (“chemical” sunscreens).

Physical sunscreens are usually opaque and contain talcum, zinc oxide, titanium dioxide or some combination: that white pasty cream lifeguards cover their noses with is a familiar example. Physical sunscreens are highly effective, but they have to be reapplied often and thickly, since they tend to melt off in the heat, and they’re also, well, ugly–even though you can now get them in bright, party colours. (Yecch!)

The more popular chemical sunscreens are made of substances that absorb ultraviolet radiation, and come as lotions, creams, gels, oils and even in waxy sticks.

Every sunscreen has a Sun Protection Factor (SPF) designation, which indicates its relative ability to provide protection against UVB. For example, a product labeled SPF 15 will permit a person who would normally sunburn in 10 minutes to remain outdoors for 15 times that long, or about 2 1/2 hours, before burning.

Most experts recommend you use a sunscreen with no less than SPF 15 protection. But the SPF rating only refers to UVB protection, not UVA protection. To find a sunscreen that protects against both kinds of radiation, you have to look at the label.

The most common ingredients in chemical sunscreens include a chemical called PABA (no, I don’t know exactly what it is) or its derivatives. About seven percent of the population is allergic to PABA, but fortunately there are other chemicals that work as well, especially derivatives of benzophenone, cinnamates, and salicylates. (I don’t know what they are, either, but they sound impressive, don’t they?)

Most of these are good mainly against UVB. Only the relatively new ingredient Parsol 1789, also known as avobenzone, provides significant protection against nearly the full range of UVA–but it has to be mixed with a UVB-blocking agent, usually PABA.

Someday we may see lotions containing microsponges filled with melanin, or even pills that cause our skins to produce more melanin: a literal “tan-in-a-bottle.”

But the best protection will always be to stay out of the sun. It’s particularly important for children, because most people get half of their lifetime exposure before the age of 20, and it’s this cumulative damage that can lead to cancer or premature aging of the skin.

Even clouds aren’t necessarily protection–you can burn under a thin cloud layer–but when the clouds are thick and rain is pouring, you don’t have much to worry about.

Which proves, I guess, that even this summer’s soggy clouds have a silver lining.

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