For many years, the idea that the earth is threatened by life from outer space has been the inspirational source for radio, TV, movies, and books. In fact, the earth is being bombarded every day by an invisible threat that should keep us running for cover. It's the sun. The source of the sun's harmful rays is not its visible light. It's the sun's invisible and harmful ultraviolet rays that trigger the changes in the skin that produced sunburn and/or a tan in our younger years and set the stage for wrinkles, sagging skin, liver spots, and skin cancer later in life.

There are three kinds of ultraviolet light: UVA, UVB, and UVC. UVC, the shortest ray, usually does not reach us thanks to the earth's ozone layer. UVB, commonly considered the "tanning" ray, penetrates only the top layer, or epidermis, of our skin. That layer contains melanocytes, cells that determine skin color, and keratinocytes, cells that give skin it's strength, flexibility, and makes it waterproof. UVB rays stimulate melanocytes that promote tanning. UVB rays also stimulate keratinocytes that trigger wrinkles and sagging skin. In North America UVB rays are stronger in summer than in winter. Until the 1980s UVA rays, which are the same strength all year round, were believed to be safe. These still often-ignored rays can go through glass or clothing and skin epidermis to the second layer of skin, the dermis. This is where the supportive collagen, elastin, blood vessels, and nerve endings reside, providing support for the epidermis. Prolonged UVA exposure cracks and shrinks the collagen and elastin. Without support, the epidermis sags. With sufficient damage liver spots and skin cancer can occur.

Ignorance is bliss. Without knowledge of skin damage, many of us have spent long hours in the sun-sometimes because we were eager to get the tan we believed made us look healthy. We enhanced tan potential with oils, lotions, and suntan creams to enhance tanning. Some of us (I'm guilty) went so far as to use reflectors, similar to those we use to keep the sun from overheating our car, in hopes of quickly getting a deeper tan. Once science revealed the source of sun-triggered skin disease and that there was no such thing as a harmless tan, dermatologists began warning us to stay out of the sun, especially at midday during hot seasons or at high altitudes. Soon products to prevent sunburn were introduced. Some work by scattering sunlight-reflecting it away from your body. Others absorb the UV rays before they reach your skin.

Most of us purchase products based on the SPF or Sun Protection Factor. This is the number that tells us how long we can stay in the sun without suffering the effects of UVB (not UVA) rays. If your sunscreen has an SPF of 15, it means that if you normally get a sunburn in ten minutes, it will take 15 times 10 or 150 minutes before you burn. However, this formula isn't foolproof because as SPF gets higher, the benefit decreases. Despite the above formula, an SPF of 30, which filters out 97 percent of UVB rays, is not twice as strong as an SPF of 15, which filters out 93 percent of UVB. Products with an SPF of 30 are adequate-especially if reapplied often as most skin professionals recommend. Most dermatologists urge a minimum SPF of 15-including as an added ingredient in women's makeup.

Meantime, there's no way to tell how effective a product is at filtering UVA's wrinkle producing rays. You must look at the ingredients list. Use a product that contains ecamsule, avobenzone, oxybenzone, or the much less expensive but also adequate titanium dioxide, or zinc oxide. If you are exercising or in the water, look for the words, "water resistant". The FDA defines "water resistant" sunscreen as meaning that the SPF level stays effective after 40 minutes in the water. "Very water resistant" means it holds after 80 minutes of swimming. These sunscreens are in no way "water-proof," so they need to be reapplied regularly.

Sun protection, especially in sunny Arizona, requires year-round vigilance. It's particularly important for those of us who enjoy the many outdoor activities available to us. Put sunscreen at the top of your self-care list. Regardless of what an expert or an advertisement says, buy and use the product that you like so you'll be sure to use it. You won't regret it.