Meditations of an oncology geek

Can sunscreen cause cancer?

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28 May 2011

Josh Soto from Encinitas, CA asks:

Hey Ryon,

Here’s something that I’ve been pondering for some time. I think it would be interesting to see if over the last thirty years incidences of skin cancer have risen and/or continued to climb. Even with skin cancer awareness and sunblock usage it would seem like these numbers are still climbing… which makes me want to ask this question: is it possible that the chemicals in sunblock could be causing skin cancer?

Hey Josh,

It’s actually very hard to comment on trends for most types of skin cancer (basal cell carcinoma and squamous cell carcinoma) because there are very little statistics on incidence. Unlike other cancers, basal cell carcinoma and squamous cell carcinoma are not required to be reported to cancer registries (1). That being said, squamous and basal cell carcinomas are extremely common and almost always benign when discovered and subsequently removed.

The third main type of skin cancer is malignant melanoma, which is one of the most aggressive cancer types if it is not caught early (2). Due to the rise in man-made chlorofluorocarbon (CFC) use, the late 20th century saw a reduction in the ozone layer, the layer of the earth’s atmosphere that inhibits UV rays reaching the surface. Given the same amount of sunlight, humans are currently exposed to more UV rays than (arguably) at any other point in history.

CFC’s were banned in the historic 1987 Montreal Protocol, but the ozone layer is not expected to completely recover until the year 2050 (3). UV exposure is directly linked to carcinogenesis in the skin, so UV exposure could be implicated in any observed rise in skin cancer, and any other causative model of skin cancer must take this link into consideration.

But, can sunscreen CAUSE cancer?

According to the popular media, yes! In 2010, there was a huge scare regarding certain sunscreens causing oxidative damage and/or speeding the growth of tumors. I figured there must be some pretty substantial data for such hysteria! I went through dozens of online commentary articles that discuss the issue in great detail, except that they all lacked any details about the research that started the discussion concerning the supposed causal link between sunscreen use and skin cancer. Very few of the articles actually cite original research studies, or the scientists, making it very difficult to find the studies and scientific papers that they are talking about.

So, how do you distinguish legitimate health advice from the hordes of fake experts and doctors? Online “experts” like Dr. Mercola even base entire articles off of data presented on AOL news (4). The best way to sift through online health advice is to see if the author’s claims come from peer-reviewed articles in scientific journals.

Alas, I continued my intrepid search of cyberspace for that elusive phantom data that started all this hysteria…and I finally found a peer-reviewed study in a scientific journal from 2006 conducted at the University of California, Riverside that describes an increased amount of reactive oxygen species (cancer-causing agents) caused by application of certain chemicals found in sunscreens! (5). However, the authors were careful to note that the amount of reactive oxygen species might be negligible in scale next to the photoprotective effect of modern sunscreens, and were very careful not to jump to conclusions.

And yet, the popular media did anyway! Because what’s more boring than a highly nuanced study? People don’t want to read about that. What gets articles read is sensationalism and edgy writing, while subtlety insinuating conspiracies and cover-ups!

It turns out that the most cited source, cited by the source, cited by AOL News, is the Environmental Workers Group (6), which compiled data from a preliminary study done on mice that was not peer-reviewed:

In the NTP study, groups of 36 male and 36 female SKH-1 mice were irradiated five days per week (Monday through Friday, in the morning) for 40 weeks with solar-simulating light (SSL) with a UVA/UVB ratio of 20.5:1, similar to natural sunlight. Two levels of light intensity were tested, 6.75 and 13.7 mJ CIE/cm2, equivalent to 0.3 and 0.6 of a minimal erythemal (sunburn) dose (MED). According to an FDA publication, 0.6 MED is equivalent to nine minutes of unprotected skin UV exposure on a sunny day with a UV index of 10 on the World Health Organization UV scale (WHO 2002; Yan 2007). The mice received topical applications of control cream or creams containing 0.1%, 0.5%, 1.0%, or 2.0% (w/w) retinyl palmitate on the dorsal skin region in the afternoon on the days of irradiance exposures. These doses of retinyl palmitate correspond to real-life concentrations found in personal care products on the market (Cosmetics Ingredient Review 2009; NTP 2000).

No statistical analysis was performed, and sample sizes of animals per condition were not enough to assume a standard distribution. Also, the creams used on the mice were NOT photoprotective, meaning that there is no way of knowing if the vitamin A additive would be able to have an effect if the severity of the sun’s rays were abrogated anyway. The mice also developed squamous cell neoplasias, a very low-grade cancer – not malignant melanoma, which is the skin cancer type that is most troubling and leads to the most deaths in humans.

The Environmental Workers Group also employed a method of data crunching that is not particularly clear, making it very hard to discern if they did their math correctly. I was happy to see the San Francisco Gate catch this nuance and call their data presenting practices into question as well (7).

Let me put it this way: I would be laughed at if I presented this data to my colleagues and drew the same conclusions. If I went so far as to cry “wolf” over such thin data, I would probably get thrown out of my PhD program.

So, could Vitamin A in topically applied sunscreens actually speed low-grade tumor progression? My verdict: maybe. But the data is far from convincing, and it might pale in comparison to the anti-UV effect of the sunscreen, but we would have no way of teasing that out with the current data.

However, a REAL issue with sunscreen is that North American labels give SPF ratings against UVB rays, not UVA. UVB causes sunburn and aging appearance, but UVA (slightly different wavelength) is the one that really damages DNA (I previously explained how radiation can cause cancer here). A lot of sunscreens have added UVA protection, but the FDA does not monitor this as tightly. The controversy was summarized fairly well in a NY Times article (8).

As a result, there could be a generation of sunbathers that have received much more sun damage than they thought, which is troubling news given the fact that UVA rays are the main cause of melanoma (the most dangerous skin cancer).

So, will this cancer researcher and avid outdoor athlete continue to wear sunscreen? Absolutely! All things considered, even if the vitamin A (retinyl palmitate) DOES speed the development of some skin lesions, the protective UVA anti-melanoma effect is enough for me to keep applying sunscreen.


3) Speth, J. G. 2004. Red Sky at Morning: America and the Crisis of the Global Environment New Haven: Yale University Press, pp 95

Written by Ryon

May 29th, 2011 at 7:26 pm

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