By Maren Dunn, D.O. Explore Big Sky Health Writer

How bad is the sun for my skin? Does sunscreen really work to reduce aging or cancer?
– Rita, Bozeman

The human body needs sunlight. It boosts mood and is essential for the synthesis of vitamin D. However the ultraviolet (UV) radiation in sunlight causes skin damage, including sunburn, advanced skin aging and higher risk of skin cancers like melanoma.

UV radiation is divided into two types: UVA and UVB. UVA represents 95 percent of the UV radiation reaching the Earth and causes skin cancer, while UVB represents 5 percent and commonly causes sunburn and skin cancer. Both are damaging to human skin causing premature aging. Protection from UV exposure can be accomplished with sun-protective clothing, sunscreen and sun avoidance during peak hours.

Sunscreens protect the skin by reflecting or absorbing the UV radiation. The term “broad spectrum” typically refers to sunscreens that block both UVA and UVB. The U.S. Food and Drug Administration has made sure this will be the case as of December 2013, when only sunscreens that block both wavelengths can be labeled “broad spectrum.” Additionally, only the terms “water resistant” or “very water resistant” will be allowed on labels. These terms indicate that the product’s protection lasts for 40 or 80 minutes of water activity. The FDA is discontinuing use of the terms “sweat proof” or “waterproof” on all sunscreen labels.

SPF, or sun protection factor, measures a sunscreen’s ability to block UVB. It’s a scientifically measured ratio of the radiation dose required to produce redness on the skin. SPF 15 absorbs 93 percent of the UVB rays, while SPF 30 and 50 absorb 97 and 98 percent, respectively. It’s false to think you can stay in the sun 15 times longer when wearing SPF 15 sunscreen. Your skin will still be affected by 7 percent of the UV radiation that’s reaching the Earth, which during the summer months can still be a large and damaging dose.

Since there is strong evidence that sunscreens prevent squamous cell carcinoma, melanoma and aging, SPF 15 should be used daily, during all months of the year and luckily can be found in many cosmetics and daily moisturizers. “Broad spectrum” is always preferred to assure more complete UVA and B protection. SPF 30 plus is recommended for people performing outdoor work, sports or recreational activities, or for those who are unlikely to reapply often during prolonged periods in the sun.

Proper application is paramount. For lotions, an adult should apply at least 1 ounce, or a shot-glass full, to sun-exposed areas 15 minutes before sun exposure. Reapplication every 2-4 hours is extremely important; sooner if the skin has been exposed to water.

Since infants’ skin is immature, it is not advised to apply sunscreen on babies younger than 6 months old. If it’s the only form of sun protection available, sunscreen can be placed on limited areas like the cheeks and hands. The American Association of Pediatrics recommends shade and sun-protective clothing for infants.

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New FDA sunscreen label rules:

Broad Spectrum: provides UVA and UVB protection

Water Resistant: provides 40 minutes of protection when in water

Very Water Resistant: provides 80 minutes of protection in water

Percent protection from UVB radiation:

SPF 15: 93 percent

SPF 30: 97 percent

SPF 50+ : 98 percent