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FiveThirtyEight

Science

The past 50 years have seen rapid evolution in medical opinion on sun exposure. My mother tells stories of spending entire summers lying on the beach coated in baby oil. I recall using sunscreen as a kid, but I also remember that I typically got one really bad sunburn per summer. In contrast, my 3-year-old daughter is not permitted to leave the house without a heavy coating of sunscreen and ideally a large, floppy hat. When she was a baby, I forced her to wear a “bathing suit” with long sleeves and pants.

This change in behavior has been prompted in part by the growth in skin cancer rates, and the increasing recognition that sun exposure and skin cancer are linked. According to the Skin Cancer Foundation, treatment of non-melanoma skin cancers in the U.S. increased nearly 77 percent between 1992 and 2006, and melanoma diagnosis increased by 1.9 percent annually between 2000 and 2009.

Dermatologists and pediatricians alike emphasize the importance of avoiding the sun and using sunscreen. But lately we’ve seen the inevitable backlash. Some parents note that they turned out fine despite rarely wearing sunscreen in their youth, and wonder if it’s really necessary to be quite so aggressive in applying it to their own children. Companies hawk sunscreen with ever-higher SPF values, despite the fact that the U.S. Food and Drug Administration has acknowledged there is no evidence that raising SPF above 50 makes any difference.

Layered on top of this general skepticism are further health concerns. Sunscreen is made of chemicals, and a selective read of media reports would have one believe that these chemicals cause both cancer and hormone disruption. (“The Dr. Oz Show’s” take on the issue warned, “Your sunscreen might be poisoning you.”)

So does sunscreen prevent cancer or cause it?

The fact that sun exposure and skin cancer are linked is well accepted and the evidence is compelling. Skin cancer rates vary across the United States: Those areas with more year-round UV exposure have higher skin cancer rates. People who are fair-skinned have higher skin cancer rates, and their pigmentation matters more than where they live.

What is less well established, at least in the data, is whether sunscreen mitigates these risks. The theory behind sunscreen is very sound. Sunscreen absorbs UV light, blocking it from reaching the skin, and UV light exposure is linked with cancer. In practice, however, the evidence on the relationship between sunscreen use and skin cancer is more limited.

One randomized trial in Australia evaluated the effects of daily sunscreen usage among adults over a period of four and a half years on the diagnoses of basal cell carcinoma and squamous cell carcinoma, which both describe abnormal cell growth in the upper layer of the skin. (Basal cell is the less dangerous of the two, since growth is typically more limited.) Participants were mainly of fair or medium skin color, and those in the treatment group were told to apply sunscreen every day. Participants in the control group were not given this instruction. By the end of the period there was no difference in the share of people diagnosed with either type of skin cancer, though there was evidence of a smaller number of squamous cell tumors in those using sunscreen.

In the case of melanoma, the more dangerous form of skin cancer, there’s no data that tells us whether sunscreen works.

Again, it’s worth saying that the theory behind sunscreen is pretty compelling, but the evidence just isn’t as strong. One possibility is that it takes a lifetime of sunscreen use to make a difference. Another possibility is that people who use sunscreen spend more time in the sun, which mitigates some of the benefits.

Now let’s turn to the risks. There are many chemicals in sunscreen but popular concerns revolve around two: oxybenzone and retinyl palmitate.1

Concerns about oxybenzone surround its possible disruption of the endocrine system, the system in the body that manages hormones. Endocrine disruption can cause developmental issues, both cognitive and sexual. Concern about oxybenzone in the case of sunscreen can largely be traced to this particular paper on an experiment in rats. In the experiment, young rats were fed a diet of food mixed with oxybenzone and other chemicals. After several days they were killed and their uteruses were removed and weighed. (Higher uterine weight is indicative of endocrine disruption.) The study found that at high doses of oxybenzone, uterine size increased.

An obvious initial complaint in applying these findings to humans is that people typically do not eat sunscreen. Our skin can absorb about 1 percent to 2 percent of the oxybenzone in sunscreen, meaning the results of the rat study are applicable in principle. However, studies of rats like this one are often better for proving possibilities than for allowing us to make actual risk statements, since the rats were exposed to doses of oxybenzone much higher than humans ever see.

In a helpful follow-up study, several authors showed that it would take between 35 and 70 years of daily full-body sunscreen usage to achieve the dosage experienced by the rats. And small studies in humans do not show evidence of hormonal changes in response to daily sunscreen use. So yes, in principle oxybenzone is an endocrine-disrupter, but practically this finding isn’t important enough to inform sunscreen use.

The second chemical — retinyl palmitate — has prompted concerns about links with cancer (the debate is nicely summarized here). In studies that looked directly at skin cells, a combination of retinyl palmitate and UV light was shown to generate free oxygen radicals (the opposite of antioxidants, these have long been linked to cell disruption and cancer). No direct links with skin cancer have been seen, however. Some limited evidence from studies of mice produced conflicting results.

The cellular evidence may be a little concerning but, again, applying this to humans directly is difficult. And probably the strongest reason to believe in the safety of retinyl palmitate is that this chemical has been in use in many makeup and facial products for 40 years or more with no published evidence suggesting it is risky.

The bottom line is that although there are holes in our knowledge, there is no concrete evidence of any chemical risks of sunscreen at typical doses.

As to whether I should be slathering my kid with sunscreen or not, the good news is that I’m not causing any damage by doing so, and I’m certainly sparing her the painful sunburns of my youth. On the other hand, it may be dangerous to be lulled into thinking that sun exposure is without risk when she wears sunscreen. Protective clothing, hats and shade may have as much — or more — of a direct role to play. Perhaps it’s time for another full-body bathing suit.

Footnotes

  1. It is worth saying that not all sunscreens contain these chemicals, and “all-natural” sunscreens (with their higher price tags) typically do not. ^

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