Sunscreen is a commonly used protectant against the sun. Exposure to direct sunlight has the potential to cause skin cancer, which is often indicated by carcinomas or melanomas or unusually pigmented patches of skin that show up in sun-exposed areas. Despite the risk, people still spend plenty of time in the sun intentionally for a variety of reasons with sunscreen as their only protective measure. While the intention is there, there is a lot of variety when it comes to how much sunscreen is applied and how often it is applied per day, which can both be important when exposure to sun is long.
How Often? What Type?
An experimental study by van der Pols et. al. explored the effectiveness of consistent sunscreen usage altogether through a four and a half year study with follow up done in a subtropical zone of Australia. There were a total of 1,621 participants with half tasked with daily application of sunscreen to the head, neck, arms, and hands every morning and the rest were instructed to use sunscreen as they usually would. There were two main carcinoma types tracked in the study. Daily usage showed little effect on the development of basal cell carcinomas, but a decreased incidence of tumors were observed 5 years post-intervention. Squamous cell carcinomas on the other hand were shown to be significantly reduced and prevented, with both carcinomas and tumor numbers decreased in the long term. These positive effects were not seen as much in the discretionary application participants. This study provides evidence that daily application of sunscreen provides the best protection against the sun. But how many times a day works the best?
A study done by Bodekær et. al. looked at the accumulation of SPF in the skin over a period of 5 days and concluded that three daily applications yielded the best SPF protection, in contrast to the daily application that did not have a significant amount of accumulation. While accumulation of SPF is not required to be protected against the sun, it is just an added layer of prevention. This addition can also be useful when going on vacation or during times where exposure to sun is increased consistently, to not only protect from cancer but prevent some sunburns. This study also observed significantly better SPF accumulation in the skin when using mineral sunscreen compared to non-mineral.
How Much?
The guidelines from the FDA and the International Organization for Standardization recommend about 2 milligrams (mg) per centimeter of area of skin (cm^2). Estimating 2 mg of sunscreen is dependent on the sunscreen’s viscosity and must be determined per type. It is also important to be intentional about the amount of sunscreen used daily. A study by Petersen and Wulf that specifically explores that, and across the board it has been shown that people tend to use less sunscreen than recommended on average. Petersen and Wulf quantified it to find an average of 0.39 to 1.0 mg/cm^2 of skin, which is concerning when it comes to effectiveness. Most studies looking at the effects of wearing sunscreen in the long term use closer to exactly 2 mg/cm^2, so those positive effects shown in the previous studies may not be observed at the same levels if less sunscreen is used.
Conclusion
Finally, it is important to keep in mind the other factors that play into sun intensity and protection such as latitude and just common sense. Sunscreen is able to protect a limited amount, but an interesting pattern observed was that those who wore sunscreen more consistently were sunburned, which was likely due to an overestimation of the protection strength of the sunlight. In summary, sunscreen is used best when applied three times a day and using about 2 mg/cm^2 which must be self-determined. Mineral sunscreen is also best but can be less desirable in certain cases due to the appearance of the residue it leaves on the skin.
References
Bodekær and Wulf (2012). Accumulation of sunscreen in human skin after daily applications: a study of sunscreens with different ultraviolet radiation filters. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1600-0781.2012.00651.x
Petersen and Wulf (2014). Application of sunscreen − theory and reality. https://onlinelibrary.wiley.com/doi/pdf/10.1111/phpp.12099.
Perugini et. al. (2019). Topical Sunscreen Application Preventing Skin Cancer:
Systematic Review. https://www.mdpi.com/2079-9284/6/3/42?type=check_update&version=1.
Van der Pols et. al. (2006). Prolonged Prevention of Squamous Cell Carcinoma of the Skin by Regular Sunscreen Use. https://d1wqtxts1xzle7.cloudfront.net/45193045/2546.full-libre.pdf?1461923745=&response-content-disposition=inline%3B+filename%3DProlonged_Prevention_of_Squamous_Cell_Ca.pdf&Expires=1717188706&Signature=TqzgmiinJVMeevJYJkWONOuHDBI5W-1YOxja54WkxyRu6b8ErOVXu7lPBeDgN5mXgas-oLAiFHOdiRY57nXhPxPA6BzjopSjjFKIhixs7P6pBJb20TwVLIeRz7Sks0hH4fz6u9RwECkKDp-xlbtpjE95Q~YB6I3XAjAOKthfNlXpqbCmKeM1guToj7ld8WcA23eXVudZANJou~9yrimMZJgFeJij4Xy6JX1mlA1KmEvm0u39LW1wnwZQRjmp-vEdZqv2Fw3tp9GVIolVU0pxgfTsxtmyDcfVzUQnNKeHOu70cKs50s7eh2vD5Df2d8YxKqf33ERJhX8yhhRQfoUvbA__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA. Assessed and Endorsed by the MedReport Medical Review Board