Shining Some Light on Sunscreen and Skin Cancer
Summer is here, so what are the facts regarding sunscreen and skin cancer?
We have all heard the caution that “correlation does not prove causation.” It means in essence that finding that two things go together does not necessarily mean one causes the other. And yet we are constantly confronted with challenges to our understanding of this notion.
Recently, Critica Art and Social Media Director Catherine DiDesidero called our attention to a posting on the website of an organization called Midlands Directory that began “Did you know that despite the invention of sunscreen, cases of skin cancers are on the rise every year?”. The article correctly notes that both the rates of melanoma and other skin cancers and the use of sunscreen have increased in tandem over recent decades. The conclusion they reached? Toxic ingredients in sunscreen must be the cause of potentially deadly skin cancers like melanoma.
The headline of this post, “Recent Study Shows How Sunscreen Causes Cancer, Not the Sun,” promises to add even more scientific fuel to the anti-sunscreen fire. We are promised research further proving that the longstanding recommendation to protect our skin from the sun’s dangerous ultraviolet radiation is wrong and itself dangerous.
But it turns out that there are innumerable problems with the inferences the article draws from various facts and studies and these issues highlight how difficult it is for us to decide whether to trust any claim about something causing an injury or disease. Let’s start with the association between increased use of sunscreen and increased rates of skin cancer.
The simplest statistical tool to establish that one thing is associated with another is called the correlation coefficient. My statistics professor often used the example of height and weight to illustrate this. In general, taller people weigh more than shorter people. If the relationship were perfect and all tall people were heavier than all short people, the correlation coefficient would be 1.0. If there were absolutely no relationship between height and weight, the correlation coefficient would be 0. If taller people were lighter than shorter people, the correlation coefficient would be a negative number between 0 and -1.0. It turns out that the correlation coefficient for height and weight is a positive number between 0 and 1.0 that varies depending on what population and age group you are examining. It is greater than 0 because there really is a relationship between height and weight, but not 1.0 because in any group there are always unusually heavy short people and very light tall people.
This is an easy example to grasp because it seems intuitive. And we are probably justified in leaping to the conclusion that being tall is a cause of weighing more. There simply is more of a tall person than there is of a short person and the extra length has to weigh something. But even here, the correlation coefficient just tells us that height and weight are associated: in any population, as one goes up so does the other. The correlation coefficient doesn’t actually tell us what causes what.
As the wonderful book Spurious Correlations by Tyler Vigen (Hatchette Books, 2015) shows us, all kinds of things generate correlation coefficients that are greater than zero, even though it is obvious they have no relationship whatsoever with each other. Some of the more amusing examples in the book are:
- Margarine consumption and divorce rate in Maine
- Customer satisfaction with Taco Bell and international oil production
- Physical retail sales of video games and UFO sightings in Massachusetts
It just so happens that in each case when one goes up or down, so does the other. By pure coincidence, both customer satisfaction with the fast-food chain Taco Bell and the number of barrels of oil produced around the world both rose at about the same rate between 1997 and 2003. Plug those numbers in into the correlation coefficient mathematical equation and it generates a correlation coefficient of about 0.8, meaning the relationship is almost a perfect match. Yet we do not for a minute think that either one is the cause of the other.
And yet, for whatever reason, when some individuals notice that there is a positive correlation between sunscreen use and skin cancer, they immediately assume that sunscreen use causes skin cancer. Then they leap to the next step and try to figure out what the biological mechanism behind such a phenomenon might be. In this case, anti-sunscreen types like California scientist Elizabeth Plourde have decided that there are ingredients in sunscreen that must be carcinogenic to humans. They grasp at all kinds of straws, yank together disparate bits and pieces of information from studies, and weave a tall tale about sunscreen’s nefarious effects on our health.
It turns out, of course, that all of this is wrong. It is in fact settled science that sunscreen, when used properly, protects against and certainly does not cause skin cancer. The vocal group of people who insist that sunscreen causes cancer have fallen into the trap of believing that a spurious association between two things means that one causes the other. We have seen the devastating effects of this kind of thinking before, as when anti-vaxxers took the fact that autism rates and the introduction of a mercury-based preservative called thimerosal to vaccines occurred together to leap to the erroneous conclusion that vaccines cause autism. We have to hope that the “sunscreen causes skin cancer” is not as tenacious as that one, or even more people will die from malignant melanoma, the deadliest form of skin cancer and one that has been shown to be caused by exposure to the sun’s ultraviolet radiation.
What could be the real reasons that skin cancer rates are rising? First, an anecdotal guess. When I was about 15, some 50 years ago, it was considered cool to lay on the beach without any sunscreen for hours. Painful sunburns were tolerated at the beginning of each summer so that a deep and lasting tan would ultimately set in and carry us through until school resumed. All of that sun exposure, we now know, increases the opportunity for ultraviolet radiation to cause mutations in skin cells that ultimately lead to cancer. But the process of actually developing cancer takes decades, just as cigarette smokers may start smoking in their teenage years, quit at age 40, and develop lung cancer at age 70. So some of the increase in skin cancer we are now seeing may be due to this delayed effect.
Other reasons for the increase are articulated by dermatologist Francesca Fusco as including : “longer lifespans (the sun damage that leads to skin cancers generally accumulates over time); the thinning ozone layer, which allows greater amounts of harmful ultraviolet radiation (UVR) to penetrate the earth’s atmosphere; the increased popularity of outdoor activities; clothing styles that leave more skin exposed; and the advent and popularity of tanning booths. Improved diagnostic techniques also allow doctors to detect more skin cancers at an early stage.” All of these reasons are biologically plausible and taken together can explain why skin cancer incidence is rising. All experts are clear that using sunscreen alone is not sufficient to protect us from skin cancer. But all include sunscreen as part of the protection strategy.
But what about the “recent study” mentioned in the Midlands Director article that supposedly shows that sunscreen causes cancer? Is there in fact some new evidence that does implicate sunscreen as a human carcinogen?
Click on the link to that study and you see it was actually published four years ago. A group of scientists from the Karolinska Institute in Sweden administered a questionnaire to nearly 30,000 Swedish women, asking about their exposure to sunlight. Those who avoided the sun most had twice the rate of dying during the study’s 20-year follow-up period compared to those with the highest sun exposure. It should be noted that although the web article pounces on the “twice as great” figure, the absolute difference was actually only 3%. We will cover what that difference means in a forthcoming Critica article.
Now there are many, many reasons that the study does not in fact document that sunscreen causes skin cancer. In fact, the study itself never mentions sunscreen as a potential carcinogen; it doesn’t even state that lack of sun exposure is the culprit. Once again, it is really based on an association, in this case between sun exposure and skin cancer. But perhaps the most important clue that it isn’t sunscreen that is causing cancer in this study comes from a follow-up analysis of the data published two years later by the same Karolinska group. In this second paper, published in 2016, they found that the excess deaths in the low sun exposure group of women were mostly due to cardiovascular disease and other non-cancer illnesses. Because the high sun exposure women lived longer, they actually had a higher rate of cancer deaths. So one suspects that understanding why women who don’t go outside much die more often from heart disease is the important factor here. It might be that women who stay indoors a lot are more sedentary, eat poorly, and are more prone to obesity and diabetes, all risk factors for cardiovascular disease. Nothing in the two Swedish papers ever comes close to implicating sunscreen as a cause of skin cancer, the website article’s headline notwithstanding.
Sometimes associations do lead to finding causes of diseases. The association of gay men, people who use IV drugs, and people who had blood transfusions with AIDS was the clue that led epidemiologists to figure out that the cause must be an infectious agent transmitted by contact with infected bodily fluids and then to the discovery of the causative virus, HIV. The association between smoking and lung cancer ultimately allowed scientists to prove that nicotine causes cancer. In each case, finding an association was only a clue; establishing cause and effect required a great deal more work.
Most associations turn out to be artifacts that tell us nothing about cause and effect. The trick is to figure out how we explain this to people when we are constantly bombarded with exaggerated claims based on correlations. Sunscreen doesn’t cause skin cancer any more than eating margarine causes people in Maine to get divorced. It is critical that we help people understand what we really mean when we say “correlation doesn’t prove causality.” We must shine some sunlight on every instance in which the claim is made that because X and Y are associated, X must be causing Y. Most of the time, that association is just a coincidence.
 Green AC, Williams GM, Logan V, Strutton GM: Reduced melanoma after regular sunscreen use: randomized trial follow-up. J Clin Oncol 2011; 29(3):257-63; Huncharek M, Kupelnick B: Use of topical sunscreens and the risk of malignant melanoma: a meta-analysis of 9067 patients from 11 case-control studies. Am J Public Health 2002; 92:1173-1177; Dennis LK, Beane Freeman LE, VanBeek MJ: Sunscreen use and the risk for melanoma: a quantitative review. Ann Intern Med 2003; 139:966-978.
 Lindqvist PG, Epstein E, Landin-Olsson M, et al: Avoidance of sun exposure is a risk factor for all-cause mortality: results from the Melanoma in Southern Sweden cohort. J Intern Med 2014;276:77-86.
 Lindqvist PG, Epstein E, Nielsen K, et al: Avoidance of sun exposure as a risk factor for major causes of death: a competing risk analysis of the Melanoma in Southern Sweden cohort. J Intern Med 2016;280:375-387