It's All About The Skin
Your skin is the biggest organ of your body. Its function is to separate and protect you from all the nasty things out there in the world. One of those nasty things is ultraviolet (UV) radiation from the sun. Getting out in the sun exposes your skin to two wavelengths of harmful UV rays: UVA and UVB. In simple terms, overexposure to UVA will age the skin (wrinkles and spots) while overexposure to UVB will burn the skin.
Your skin has an absolute arsenal of protective tools: nerve endings, sweat glands, hair follicles and melanocyte cells. When skin is exposed to UV light the melanocytes produce a chemical pigment called melanin. The melanin absorbs UV radiation like a sponge, thereby protecting the cells. In the process the melanin darkens the appearance of the skin. The only problem with this process is that melanin is produced very slowly. That is why you can't get a tan in a day. By repeating the short term exposure over the course of 5 to 7 days, the level of melanin gradually increases. This increases the protective level and deepens the tan.
Unfortunately we're not all patient enough to gradually build up our bodies' natural sun protection. Expose your skin to excessive UV before your melanin level is built up and you subject your skin cells to DNA attack. Your body responds by increasing blood flow to the capillaries of the skin to bring new cells and nourishment in order to repair or replace the damaged cells. The rush of blood delivers the sunburn's distinctive color and the accompanying swelling triggers the pain receptors. You can see the blood rush for yourself. Press sunburned skin gently with a finger and the skin immediately turns white as blood is pushed out of the capillaries. Let go and the skin turns red again as the blood flows back in.
Sunscreens are chemical compounds that bond with the skin and filter light so that less UV radiation penetrates the skin. The various agents in sunscreens can reflect UV, disperse it, or absorb it. Think of sunscreen as a chemical sunglass lens, allowing some light through, but not all. Since some UV rays are still passing into the cells, the skin's natural melanin production continues. Therefore tanning still occurs, just at a slower rate. How much tanning is retarded depends on how much UV the sunscreen blocks.
Sun Protection Factor
Sunscreen effectiveness is rated by a multiplier called Sun Protection Factor (SPF). It compares the time required for burning for protected and unprotected skin. For example, if your skin type typically burns in 15 minutes, and you apply a sunscreen with an SPF of 8, you should be able to stay in the sun for 120 minutes (15 times 8) without burning. One problem with SPF is that it assumes proper application and maintenance of the sunscreen. Most people don't use and reapply as much as directed. Using half the suggested amount of a sunscreen with an SPF of 50 would yield an SPF of 7. Another problem with SPF is that it measures the blocking ability for UVB only. To date, there is no requirement to measure or show a sunscreen's ability to block UVA.
The threat from UV is greater at high altitudes, at lower latitudes, and in reflective environments such as water, snow and sand.
Up to 80 percent of UVA and UVB reaches the earth on cloudy days.
Look for a "Broad Spectrum" sunscreen that blocks both UVA and UVB. The American Academy of Dermatology recommends using a broadband waterproof sunscreen with SPF of at least 15.
Apply liberally--the AAD recommendation is at least 1 oz. for the entire body.
Apply 20 to 30 minutes before exposure to the sun to give proper time for the sunscreen to adhere to the skin.
Reapply 2 hours after exposure or sooner if you go in water or perspire heavily.
In an ironic twist, wearing sunglasses, which diminishes light entering the eyes, represses the formation of melanin and therefore increases the risks of sunburn and the need for sunscreen.