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Decoding the SPF of Sunscreen

By Alix Charles, MD, FAAD

What do SPF numbers really mean? SPF stands for “sun protection factor”. It is a measurement of how much ultraviolet B light (UVB) a sunscreen blocks from reaching your skin. Because UVB is responsible for sunburns and skin cancer, it is really important to decrease how much you are exposed to these harmful rays. 

To calculate a sunscreen’s SPF number, scientists test the product in a lab to determine what percentage of UVB can still penetrate to the skin. This percentage is then divided into 100 and what you get is a SPF number. For example, a sunscreen that lets 50% of UVB reach the skin has a SPF of 2 (100 ÷ 50 = 2). A sunscreen that lets only 2% of UVB reach the skin would have a higher (and better) rating of SPF 50 (100 ÷ 2= 50). 

Dermatologists routinely recommend you use a sunscreen with at least a SPF 30, because you are only letting about 3% of UVB reach your skin. You can see that the amount of UVB that gets past an SPF 30 versus SPF 50 sunscreen is small, so wearing at least SPF 30 should leave you well covered! However, know that this equation is only accurate if you apply enough sunscreen lotion.  

The average sunbather applies half the amount of sunscreen used in laboratory testing, which means a burn occurs in half time. One ounce (about the amount that fits into a standard “shot” glass) of lotion is recommended for full body coverage. Sweat, water and excessive toweling can decrease sunscreen’s effectiveness, even if it is advertised as waterproof or sweat proof, so make sure you reapply throughout the day to provide full protection. In addition, sunscreen losses its effectiveness over time, so throw out anything more than three years old or past expiration date.

It is important to note that although SPF levels indicate protection from harmful UVB rays, it does not measure protection from the sun’s other ultraviolet rays: ultraviolet A light (UVA). UVA rays have more long-term damaging effects on the skin, such as premature aging. Although UVA rays do not cause burns, scientists have linked excess UVA exposure to malignant melanoma, damage to the immune system and other skin cancers like squamous cell carcinoma. To protect yourself from UVA damage, make sure your sunscreen states that it is a “complete blocker”; that ensures protection against both UVB and UVA.

For maximum protection whenever you are in the sun, always wear SPF 30 sunscreen at a minimum, even if only outdoors for a short period of time. Remember, long term sun exposure can lead to skin cancer, cataracts, and suppression of the immune system.

For more information about the sun and your skin, contact your dermatologist.

Topics and Subtopics: Skin Health

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