Research has shown that prolonged exposure to the Sun's ultraviolet (UV) rays increases the risk of developing certain health problems. There are two major types of UV rays. Shorter-wave UVB rays are responsible for damaging the skin cells at the surface and producing sunburns. Longer-wave UVA rays, which account for the vast majority of UV radiation your skin is exposed to, penetrate deeper into the skin. Although they contribute little to sunburns, UVA rays are responsible for prematurely aging the skin, making it leathery, spotted, and wrinkled. Recent evidence tells us that both UVB and UVA rays can lead to skin cancer. They can also cause eye damage, including cataracts, and play a role in suppressing the immune system, reducing the body's ability to fight off disease.

When you must be out in sunlight, especially between the hours of 10 a.m. and 4 p.m., public health experts advise that you use a sunscreen to protect exposed parts of your body from sun damage. As the video explains, all sunscreens have a sun protection factor (SPF) rating. The SPF rating indicates how long the sunscreen remains effective on the skin. You can determine this time by multiplying the SPF factor by the number of minutes it normally takes for your skin to burn without sunscreen. For instance, if you normally develop sunburn in 10 minutes, an SPF 15 sunscreen will protect you for 150 minutes. Studies have shown that SPF 15 sunscreens block about 93 percent of UV rays, and that SPF 30 sunscreens block about 97 percent. Many experts note that sunscreens with an SPF higher than 30 provide little, if any, additional protection.

Although sunscreen use helps minimize sun damage, no sunscreen completely blocks all wavelengths of UV light. For this reason, there are different formulations. Chemical blockers, such as oxybenzone and methyl anthranilate, work by absorbing the energy of UV radiation before it affects your skin; these protect well against UVB radiation. Physical blockers, such as zinc oxide and titanium dioxide, are metals that remain on the surface of the skin when applied and reflect UV radiation before it penetrates; these effectively block UVA radiation. Because no single chemical ingredient blocks the entire UV spectrum, more and more sunscreens today are composed of several ingredients, with each one blocking a different region of UV light and providing "broad spectrum" protection.

Whether sunscreen comes as a lotion, a roll-on, or a spray, it will only be effective if used properly. All sunscreens should be applied at least 15 to 20 minutes before sun exposure. This allows the ingredients to bind together, forming a protective film on the skin surface. Some sunscreens can lose effectiveness after two hours or if they come in contact with water or sweat, so reapplication is required. Research has shown that people apply much less sunscreen than they need to—often just 10 percent of the amount recommended. As a result, they receive far less protection than the SPF would indicate.

It's worth noting that even scientists are uncertain about which type of UV radiation—UVA or UVB—is more dangerous. Also, some ingredients found in many formulations, including oxybenzone and a form of vitamin A called retinyl palmitate, are being studied for possibly increasing the risk of skin cancer. The type of sunscreen application may be cause for concern as well. Many experts believe that creams are safer to use than sprays and powders, whose superfine particles might be absorbed into the body or inhaled, causing damage to cells and organs.

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