(This is the second in a 2-part guest post by Jen Phillips. Jen holds a PhD in biology and is a Senior Research Associate at Westerfield Laboratory Institute of Neuroscience at the University of Oregon.)
Making sense of sunscreen controversies
Hello again! In Part I of this series of posts, we explored the science behind sunlight, sunscreen and skin cancer. That post left us with some pretty compelling reasons to cover up and slather on, but in this continuation we’ll be examining a few common claims out there that suggest sunscreen isn’t all it’s cracked up to be and may, in fact, be bad for you. To put your mind at ease up front, there are no legitimate health concerns associated with sunscreen use, and there are demonstrable health benefits to regular application. So what’s all the fuss about? Let’s take a look:
Does sunscreen use prevent cancer?
At first, this might seem like a silly question. UV rays causes cancer, sunscreen blocks UV rays, therefore sunscreen must reduce skin cancer, right? Sure, but scientific evaluation to back up this conclusion has had some mixed results.
The biggest and most well-designed study to date was conducted in Australia, beginning in 1992. The study included over a thousand participants, divided into an experimental group who applied SPF 16 sunscreen daily for four years, and a control group who used sunscreen at their own discretion. Initial results were reported after the four-year testing period, and additional data were obtained by tracking participants in both groups for another decade. Overall, the study showed clear benefit of daily sunscreen use in prevention of squamous cell carcinoma (SCC), a benefit in prevention of Melanoma that was only noted at the end of the 10 year follow-up, and only a slight, statistically insignificant benefit in prevention of basal cell carcinoma (BCC.)
Why weren’t the results more spectacular, you might wonder, given the obvious correlation between UV radiation and skin cancer? Based on its prevalence, one could hypothesize that BCC is easier to trigger than the other forms of skin cancer, and thus the SPF 16 sunscreen did not block enough UV rays to significantly reduce BCC rates. It’s also possible that UVA rays play a role in triggering BCC. When this study was initiated in 1992, the sunscreen provided to participants—and indeed, all sunscreens on the market at that time—were UVB blocking only.
Still, even imperfect protection is better than none at all, right? So using sunscreen would seem like a pretty straightforward choice. Nevertheless, recent concerns over the safety of the products themselves have complicated the picture.
Does sunscreen disrupt hormones?
This claim is specific to oxybenzone–an organic UV filter that has been an active sunscreen ingredient for over 40 years. The worry stems from in vitro studies showing that oxybenzone influences expression of estrogens and androgens in cultured breast cancer cells. A study using mice also showed hormone related changes in uterine tissue when animals were fed oxybenzone daily. Further potential for concern came from reports of oxybenzone being detected in urine and breast milk.
While it all sounds scary, the Paracelsus maxim (“the dosage makes the poison”) is important to keep in mind. In vitro studies are notoriously lousy at predicting effects in functioning, multicellular systems like humans. Further, the mice in the oxybenzone study were fed massive doses of the compound. It would take several hundred years of daily sunscreen application to achieve similar exposure in humans, so it’s unlikely to be an issue for most of us.
What about the levels detected in urine and breast milk? Well, oxybenzone is a popular ingredient in lots of products and materials beyond sunscreen. As such, it’s not surprising that it’s detectable in human excretions, and it’s not unique in that regard. The key questions here are: are those levels attributable to sunscreen use and, more importantly, are those levels harmful? To the first question, the urine study found no correlation between sunscreen use and oxybenzone levels. To the second, a number of human studies specifically testing whether oxybenzone exposure from sunscreen use had any physiological effects showed no significant problems. And finally, irrespective of the source of exposure, there have been no health problems of any kind linked to oxybenzone. While it’s definitely valid to continue to monitor the chemicals in our environment, it’s also important to note that in nearly 5 decades of widespread use, no safety issues have come up.
Does sunscreen cause cancer?
This concern is primarily based on some in vitro studies showing that retinyl palmitate can generate biologically reactive molecules when exposed to UV radiation. Retinyl palmitate isn’t a UV filter at all, but rather a Vitamin A precursor molecule that is a common additive to sunscreen formulas, as well as a great many other skincare and food products. The factors generated when retinyl palmitate is exposed to light, called free radicals or reactive oxygen species, have a well-known potential to induce cancer.
Again, this a reasonable thing to test, but of the numerous studies conducted to assess the carcinogenic potential of retinyl palmitate, only a minority showed an effect, whereas most showed no adverse outcomes at all. When weighed against the fact that widespread use of retinyl palmitate for years has resulted in no health problems, the few small in vitro studies suggesting harm are not convincing.
An additional cancer concern was raised by a couple of small studies that showed an increase in Melanoma in low-SPF sunscreen users compared to non-users. The methods of these studies were relatively poor, compared to others that showed either no difference or a protective effect of sunscreen use. Importantly the researchers didn’t evaluate other potential causes for the increase in Melanoma that they observed. Stacked up against the better methodologies of larger studies, most particularly the comprehensive Australian study mentioned above, the data just don’t support a causative link between sunscreen use and Melanoma.
Is sunscreen toxic?
This concern arises from recent modifications to the inorganic sunscreen ingredients, zinc oxide and titanium dioxide, both of which filter UVB and some UVA rays. Zinc oxide has been around for a while—you’ll probably recognize it as that opaque white stuff that beachgoers slather all over their noses. Titanium dioxide is a more recent addition to the sunscreen arsenal, but is similarly pasty in appearance.
While such products are well-suited for small, high profile areas like the nose and cheekbones, their thick, pasty formulas are so difficult to spread that they’re not generally used for whole body protection, which is unfortunate because in addition to being great UV filters they’re also far less likely to cause skin reactions or allergies than some of the organic compounds.
To get around this problem, recent formulations use microsized” or “nanosized” titanium dioxide or zinc oxide mineral particles. These preparations have the dual advantage of making the product easier to spread and less opaque. However, the small size of these particles has raised concerns about their ability to penetrate the skin and reach toxic levels in the living cells beneath. Fortunately, all the evidence indicates that these particles aren’t able to enter the body through the skin. Most were even tested on broken skin and showed no tendency to penetrate to the living cell layers. This seems to be due, at least in part, to the fact that although the individual particles are indeed very small, they tend to aggregate together in solution, forming chemical bonds that keep them from moving around independently.
This concern seems reasonable on its face. Skin cells in the epidermis produce a vitamin D precursor that is activated by sunlight (by UVB rays, specifically). Block the sunlight, and you could potentially block vitamin D synthesis. Studies have shown that sunscreen can indeed reduce vitamin D production. Vitamin D screening is common in primary health care, and is now a cornerstone of the supplement industry, as well as a popular scapegoat for a wide range of maladies in the ‘wellness’ community.
There is a great deal of variation in the definitions of what might constitute a clinically normal vitamin D level in adults, so much so that experts don’t actually recommend routine screening at all. However, legitimate vitamin D deficiency can lead to some well-characterized developmental problems, like rickets, so it is important to do due diligence when considering the effect of sunblock for children. Fortunately, research has shown that the use of sunscreen doesn’t contribute to vitamin D deficiency.
Why the discrepancy? In short, it seems that no one is perfectly compliant when applying sunscreen. A little sun exposure, literally 15-30 minutes a week, is all an otherwise healthy fair skinned person needs to synthesize sufficient amounts of vitamin D. A few minutes in the sun without sunscreen, or maybe just on that one spot you couldn’t quite reach, appears to be enough. This is borne out by both the lack of correlation between sunscreen use and vitamin D deficiency, and also the low incidence of rickets and other known outcomes of vitamin D deficiency, in the sunscreen-using population.
Are spray-on sunscreens safe?
A couple of years ago, the FDA announced that it was collecting more data on spray-on sunscreen, which at the time was relatively new to the market. This was interpreted as an ominous development by some, and a number of unofficial advisories were issued about avoiding use on children, risk of inhalation, etc. The FDA didn’t issue any sort of follow up on their findings, but based on the number of consumer bulletins about spray-on sunscreen on the FDA website, it seems pretty clear that there are no specific health concerns regarding the sunscreen part of the ingredients. Much more emphasis has been placed on regulating the ‘dosage’, if you will, since it is applied so much differently than sunscreen lotion. There are also a number of common-sense precautions that also apply to many other spray-on products. For example, it’s a terrible idea to apply an alcohol-based spray around an open flame, no matter what the other ingredients are. It’s probably not a good idea to spray sunscreen into your kid’s face, any more than it would be a good idea to spray insect repellent or hairspray. Inhaling aerosol products in lieu of air is typically an unpleasant experience. However, aside from these fairly broad safety guidelines, there isn’t any specific reason to shy away from spray on sunscreen.
-sunscreen continues to be recommended by healthcare professionals to prevent burns as well as to protect skin from premature aging and from cancer.
-sunscreen safety has been well established. No health issues have been linked to sunscreen ingredients, despite extensive testing.
-sunscreen provides maximum protection when used appropriately—applying a thick, even layer and reapplying regularly.
One last thing: the FDA requires sunscreen to be tested for stability and durability, and the expiration dates are usually around 3 years from the date of manufacturing. These dates are backed by sufficient testing so as to be trustworthy, but also require the liberal application of common sense. If your sunscreen is stored in extreme temperatures for a long period, for example, or if it changes consistency from one year to the next, it’s a good idea to replace it even if it’s not technically expired.
So, be smart about sun exposure, wear hats and water shirts when possible, seek out a broad-spectrum product with high SPF, slather up, and have a beautiful summer. I’ll see you at the pool.
For more tips and information, see this sunscreen FAQ from the Skin Cancer Foundation
Also, see the Skin Cancer Foundation’s response to some of the recently renewed claims.
Burnett and Wang, 2011. Current sunscreen controversies: a critical review. Photodermatology, Photoimmunity & Photomedicine. 27, 58-67
Jansen, et al., 2013. Photoprotection: Part II. Sunscreen: Development, efficacy and controversies. Journal of the American Academy of Dermatology 69(6). 867.e1-867.e14.
LeFevre, et al., 2015. Screening for Vitamin D Deficiency in Adults: U.S. Preventative Services Task Force Recommendation Statement. Annals of Internal Medicine.162(2): 133-140.
Green, et al., 2011. Reduced melanoma after regular sunscreen use: randomized trial follow-up. Journal of Clinical Oncology 29(3):257-63.