May 15, 2019

Six Things Everyone Should Know About Sunscreen

By Dr. Benjamin Carter

A few months ago, I diagnosed a patient with skin cancer. The patient was accompanied by a family member who asked me, “How can the sun, which is good for us, cause skin cancer?”  

The answer to this question is too long to explain in detail in this article. Her question made me think in broader terms about things that are good for us but can cause damage if exposure is excessive. For instance, too much thyroid hormone causes hair loss, tremors, rapid heart rate, and other health problems. Too much ibuprofen can lead to kidney failure. Too much calcium can cause abdominal pain, kidney stones, memory loss, and bone fractures. All of these things, when produced or consumed in appropriate amounts, are beneficial to our bodies. You can, it would seem, have “too much of a good thing.”

Some things are within our control, such as our consumption of ibuprofen and our exposure to the sun. Some things are not within our control, such as our production of thyroid hormone (or in my case, my consumption of chocolate). We should pay particular attention to the things we can control or the behaviors we can modify.

My patient required surgery to treat her skin cancer. While surgical removal of skin cancer is commonplace for me in my practice, for many of my patients, this experience causes anxiety and discomfort. I would prefer that people avoid the sun damage that can cause cancer in the first place so that they can avoid biopsies and surgeries later on. Hopefully, the information presented below will help us improve our interactions with the sun and its rays and reduce our risk of skin cancer.


Here are six things everyone should know about sunscreen:

1. SPF stands for sun protection factor. SPF is based on a sunscreen’s ability to reflect or absorb ultraviolet B (UVB) rays. It does not refer to ultraviolet A (UVA) rays.  Both UVB and UVA rays from the sun can cause cancer.

There are two ways to think about SPF.

  • SPF is a way to measure how long it will take for someone to get sunburned. A good way to calculate how much protection you will get from your sunscreen is to use the following formula: Amount of time it normally takes to sunburn without sunscreen x SPF Factor = Amount of time it takes to burn with sunscreen. For example, if it normally takes me 10 minutes to sunburn (have my skin turn light pink) and I apply SPF 15 sunscreen, it will take 150 minutes for me to sunburn.
  • SPF is based on blocking power as a percentage: SPF 15 blocks 93 percent of UVB rays; SPF 30 blocks 97 percent of UVB rays; SPF 50 blocks 98 percent of UVB rays. Anything over SPF 50 is likely providing marginal benefit at substantially increased cost.  

2. Not all sunscreens are created equal. The term “broad spectrum” refers to the fact that a particular sunscreen can block both UVB and UVA rays. However, even amongst broad spectrum sunscreens, there are differences. Perhaps the most important thing to understand is whether or not you are using a physical blocker (zinc) or a chemical blocker (avobenzone). They are both effective, but physical blockers offer a more complete barrier to UVB and UVA rays. Newer zinc sunscreens can be both effective and virtually invisible. I recommend zinc oxide concentrations above 7%. Some of my favorite sunscreens include Blue Lizard and ELTA MD, which both have zinc concentrations as high as 10% but do not leave my face feeling pasty white. For a daily moisturizer, I recommend Cerave AM.

3. You have to reapply, particularly between the hours of 10:00 a.m. and 3:00 p.m. Some of the blocking ingredients may degrade over time (60–120 minutes), making reapplication of sunscreen every two hours a must.

4. There is no such thing as waterproof, sweat proof, or all day protection when you are talking about sunscreen. In 2013, the FDA issued formal regulations prohibiting sunscreen manufacturers from making these claims. Sunscreens must now use the terms water resistant, sweat resistant, or long-lasting.

5. Vitamin D deficiency has not been associated with sunscreen use. There is some controversy regarding this topic, but to date, the large scale medical studies indicate no significant reduction in vitamin D production following application of sunscreen. This is particularly true for the majority of people who use far less than the recommended amount of sunscreen (recommended amount is one shot glass for full body coverage). Alternative vitamin D sources include salmon, eggs, fortified milk and orange juice, and oral supplementation.

6. In addition to preventing skin cancer, sunscreens also prevent wrinkling, discoloration of the skin, and aging of the skin. Many of these effects are the result of UVA rays not UVB rays, making broad spectrum coverage even more important.  


Dr. Ben’s Tips for Enjoying the Sunshine

  • Apply sunscreen 15 minutes prior to exposure.
  • Use one shot glass of sunscreen to cover your body (quarter of a bottle).
  • Use SPF 30 to SPF 50 (nothing more and nothing less).
  • Reduce sunscreen applications considerably by wearing sun protective clothing.
  • Remember the high-priced real estate: Protect the ears, the nose, and the rest of the face.
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