The Confounding Case of Skin Cancer and Sunscreen

Randomized Controlled Trials are considered the gold standard of medical research for a good reason: they are specifically designed to isolate the influence of an investigational medicine, device, or technique as much as possible. This is critical because of the potential effects of confounding variables. Confounding variables are things that alter the results of an experiment but aren’t being measured and accounted for, creating the illusion that one thing causes another. One area this has been amazingly prevalent in has been the case of sunscreen and skin cancer.

I remember distinctly, as a youth, being told that sunscreen increases your chance of getting skin cancer. Even today, many internet threads about sunscreen proclaim it is dangerous and cancerous. This doesn’t make sense to me, but has stuck in my mind for years and influences what kinds of sunscreen I buy. The initial worry started in large part due to observational studies in the early 1990s that linked increasing rates of skin cancer to widespread use of sunscreen.^[1,2] These studies were not randomized controlled trials, but instead observational studies that saw two simultaneous upward trends and theorized that they may be related.^[2] Due to the “telephone effect” of sensationalist scientific reporting and word of mouth, that quickly changed to a vague idea that the ingredients in sunscreen must cause cancer. Let’s shine a light on skin cancer, sunscreen, and the real link between the two.

Skin cancer is the most common cancer, with studies showing that around 1 in 5 adults have had skin cancer by age 70 and that it has a higher prevalence than all other cancers combined.^[3] Many types of skin cancer can be effectively treated with surgery, but prevention remains the best defense against the sun.^[4] The main cause of skin cancer is ultraviolet (UV) light, the high-energy part of the light spectrum.^[5] The link between UV light exposure and skin cancer is unambiguous and, when you look closely, obvious.^[6] UV light is a proven carcinogen, with UVA and UVB light passing into skin cells and damaging DNA.^[6] The high-energy light causes pieces of DNA to spontaneously link together, making the DNA unreadable.^[6] At the same time, the light can also reduce the ability of innate DNA repair tools, making long exposure to sunlight problematic.^[6] The upshot is that long exposure to UV light from sources like the sun or tanning booths causes precancer and skin cancer, aging of the skin, and damage to the eyes.^[5]

The bright idea of sunscreen is to reduce the amount of UV light absorbed by the skin.^[5] Ideally, this reduces the damage done by sunlight, reducing the chance of DNA damage, skin cancer, and premature aging.^[5] When properly applied, sunscreen is very efficient at protecting DNA.^[6] Sunscreen works in one of two ways: by blocking the UV light entirely (such as with mineral sunscreens) or by absorbing the UV light and converting it to heat (such as with chemical sunscreens).

So what about those studies in the 1990s? The problem, of course, was confounding variables. One influential study found that those who wore sunscreen were more likely to develop skin cancer.^[2] This makes sense, of course, because those who have skin cancer are more likely to be in the sun in the first place! The problem is that there was no control for time spent in the sun, a critical confounding variable.^[1] So what about the overall increase in skin cancer rates over that time? Two explanations were put forth almost immediately, but seem to have been missed by the general public.^[2] First, some early sunscreens did not have broad-spectrum sun protection; they only protected against UVB radiation.^[1] This meant that people using the sunscreen may have felt protected against sunburn, but were still receiving significant amounts of dangerous light. Again, the problem is that the time spent in the sun was likely higher for people using this insufficient protection, and studies were not controlled against the time people spent outside.^[1] Finally, sun protection campaigns increased the awareness of skin cancer, increasing diagnosis rates of those who would otherwise have suffered without a diagnosis.^[1] The increased testing rates were not accounted for in the initial studies, and appeared to show an increase in incidence. By missing these important confounding variables, a dark cloud formed over public perception of sunscreen.

Since these early reports, people have continued to “find” evidence of danger in sunscreens. Worries about the environmental effects of certain chemical sunscreens, including damage from oxybenzone and octinoxate, have led some to believe that these are dangerous for people as well, though there is not a strong causal link between these chemicals and poor health outcomes.^[7] This is not to detract from the very real environmental concerns of using these sunscreens in water, especially in sensitive locations near reefs, but to emphasize that worry around sunscreen continues.^[7]

Recently, there have also been concerns about benzene, a manufacturing chemical that was found to have contaminated some sunscreens in the US. At high concentrations, benzene is carcinogenic and can cause damage to bone marrow, leading to anemia, increased risk of infection, and leukemia.^[8] The question then is whether sunscreens increase this risk. A study of 10,000 Americans found that those who reported they “always” use sunscreen had about ½ as much benzene in their blood compared with those who reported they “never” use sunscreen.^[9] Does this mean that sunscreen protects against benzene? No! It means there is likely another confounding variable. Benzene is released from car exhaust, cigarette smoke, chemicals, and occupational sources, and it’s likely that people who use sunscreen are not exposed to these sources at the same rates as those who don’t.^[8,9] One example would be that those who live at the beach might live further from industrial zones, lowering their overall exposure.

Regardless of ongoing concerns about sunscreen ingredients, one thing is clear: no cancer risk is higher than skin cancer, and sunscreen unambiguously lowers the risk of skin cancer.^[6] This does not mean all sunscreens are equal or that we should fail to hold manufacturers accountable for contamination! It also doesn’t mean sunscreen is a magical shield against the giant fusion bomb in the sky we call the sun. In fact, when looking for ways to avoid DNA and skin damage from the sun, sunscreen is the last line of defense. Experts recommend that to avoid sunburn, you should:^[5,10]

1. Seek shade and avoid the sun
2. Wear protective clothing, including sunglasses and a hat
3. Wear sunscreen, especially with an SPF of 30 or higher

So this summer, it’s a bright idea to lower your cancer risk by staying cool, covered, and slathered up. Taken together, these measures can confound even the harshest sun.

Creative Director Benton Lowey-Ball, MWC, BS, BFA

References

[1] Lapides R, Saravi B, Mueller A, Wang-Evers M, Maul LV, Németh I, Navarini A, Manstein D, Roider E. Possible explanations for rising melanoma rates despite increased sunscreen use over the past several decades. Cancers. 2023 Dec 16;15(24):5868. https://doi.org/10.3390/cancers15245868

[2] Garland CF, Garland FC, Gorham ED. Rising trends in melanoma an hypothesis concerning sunscreen effectiveness. Annals of epidemiology. 1993 Jan 1;3(1):103-10. https://doi.org/10.1016/1047-2797(93)90017-x

[3] Stern RS. Prevalence of a history of skin cancer in 2007: results of an incidence-based model. Archives of dermatology. 2010 Mar 1;146(3):279-82. https://doi.org/10.1001/archdermatol.2010.4

[4] Olsen CM, Wilson LF, Green AC, Bain CJ, Fritschi L, Neale RE, Whiteman DC. Cancers in Australia attributable to exposure to solar ultraviolet radiation and prevented by regular sunscreen use. Australian and New Zealand journal of public health. 2015 Oct 1;39(5):471-6. https://doi.org/10.1111/1753-6405.12470

[5] Neale R, Janda M. Sun Exposure Summit Position Statement. 2023. http://assc.org.au/wp-content/uploads/2023/01/Sun-Exposure-Summit-PositionStatement_V1.9.pdf

[6] Cadet J, Douki T. Formation of UV-induced DNA damage contributing to skin cancer development. Photochemical & Photobiological Sciences. 2018;17(12):1816-41. https://doi.org/10.1039/C7PP00395A

[7] Suh S, Pham C, Smith J, Mesinkovska NA. The banned sunscreen ingredients and their impact on human health: a systematic review. International journal of dermatology. 2020 Sep;59(9):1033-42. https://doi.org/10.1111/ijd.14824

[8] Kaden DA, Gentry R, Ledger BM. Comparative Health Risks from Benzene in Sunscreens. Brief. 2024 Jul 1;53(4). 

[9] Chang MS, Moore KJ, Trepanowski N, Koru-Sengul T, Hartman RI. Sunscreen use is not associated with increased blood concentrations of benzene among adults in the United States: data from the National Health and Nutrition Examination Survey 2003-2006 and 2009-2018. Journal of the American Academy of Dermatology. 2022 Aug 1;87(2):440-3. https://doi.org/10.1016/j.jaad.2021.09.003

[10] DiNardo JC, Downs CA. Dermatological and environmental toxicological impact of the sunscreen ingredient oxybenzone/benzophenone‐3. Journal of cosmetic dermatology. 2018 Feb;17(1):15-9. https://doi.org/10.1111/jocd.12449