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UNF public health professor shares sunscreen advice to reduce skin cancer risk

merten headshotDr. Julie Merten, University of North Florida associate professor of public health, studies a hot topic especially during the summer - wearing sunscreen. Merten is the chair of the Skin Cancer Prevention Task Force in Northeast Florida, working to reduce skin cancer through advocacy, policy and education efforts. She has done extensive research on sunscreen and minimizing skin cancer risks.

Here are some important tips for correct sunscreen usage:

  • Say yes to sunscreen and say no to cancer. According to the Skin Cancer Foundation, more people are diagnosed with skin cancer each year in the U.S. than all other cancers combined. And the sun is usually the primary culprit. About 90% of nonmelanoma skin cancers are associated with exposure to ultraviolet (UV) radiation from the sun or tanning beds. 
  • Use an FDA-approved sunscreen. The American Academy of Dermatology recommends that everyone 6 months and older wear sunscreen. Make sure the sunscreen has these characteristics:
  • Broad spectrum, which protects against UVA and UVB sunrays
  • Water-resistant (effective for up to 40 minutes in water) or very water resistant (effective for up to 80 minutes in water)
  • Sun Protection Factor (SPF) of 30 or higher
  • Find a sunscreen formula that works for you. Sunscreens all have different ingredients beyond the ingredients meant to ward off the sun. Just because one sunscreen doesn’t feel good on your skin, doesn’t mean all will feel bad. Try different brands and formulations.
  • Apply early and often. Sunscreen should be applied in a thick layer 30 minutes before heading outside and reapplied every two hours.
  • Throw out expired or old sunscreen. Look for an expiration date on the bottle and throw out expired sunscreen. If there is no expiration date, throw out sunscreen three years after opening. If your sunscreen looks or feels different — being much thicker or thinner or the color has changed — toss it.

Commercial sunscreens have recently come under fire due to concerns about some of the chemicals in the sunscreens. Concerns including coral reef destruction, endocrine disruption, and Vitamin D deficiency have increased consumer interest in natural, organic and non-toxic sunscreen products. This demand has led more people to make their own homemade recipes, often found online.

In 2019, Merten and UNF researchers randomly sampled 189 homemade sunscreen pins on Pinterest and found that nearly all pins positively portrayed the effectiveness of the product, although most of these recommended recipes offered insufficient UV radiation protection.

When it comes to sun protection, homemade is not always the best. Most homemade sunscreens have not been tested for their true UV protection, water resistance or photostability. However, most commercially available sunscreens are highly regulated and tested for safety and efficacy.

There are two types of sunscreens: chemical absorbers and mineral blockers. Chemical sunscreens, which absorb into the skin, have received the most criticism for absorbing into the bloodstream as well as potentially damaging coral reefs. By contrast, mineral blockers, such as zinc oxide or titanium dioxide-based sunscreens, sit on the surface of the skin to create a physical barrier to reflect the dangerous rays from the sun. With the mineral blockers, there is no absorption into the skin or body and no association with environmental damage.


Marten is currently conducting a second homemade sunscreen study. If you are over the age of 18 and use or make homemade sunscreen, please visit the study website to complete a brief, three-minute Homemade Sunscreen Survey by Saturday, July 31.