An FDA-run study shows that those chemicals in sunscreen get into your blood… and stay there for a while.

“Be nice. Wear your sunscreen. I love you”. 

I uttered this mantra each day as my kids were whisked off to day camp last summer, cognizant of the burns I suffered as a free-wheeling and UV-exposed youth and aware of the ample biological evidence that sunscreen, by preventing severe burns, would reduce their risk of skin cancer in the future.  This is #GoodParenting.

And then I came across this article, appearing in the Journal of the American Medical Association that kicked the parental guilt (usually reserved for being away at academic conferences) into high gear.  The study, in 24 healthy controls, showed that the active ingredients in many chemical sunscreens had significant systemic absorption. And, the truth is, we have no idea if that matters.

Let’s step back for a minute and discuss why this study was done at all. Sunscreens are treated as over-the-counter medicines in the US, but they were put on the market before the modern era of FDA oversight, meaning the FDA has (for now) not evaluated the potential toxicity of the active ingredients. This is because, well, sunscreen is topical – maybe we don’t need to worry about all that toxicology stuff.

But last year, the FDA created a guidance document with a specific suggestion – if any of the active ingredients in sunscreen achieve a steady state blood level of greater than 0.5 ng/ml, formal toxicology studies should be performed.

That 0.5 ng/ml level is not arbitrary. It’s the same level that the FDA uses to determine if impurities introduced in drug manufacturing need to be specifically toxicologically evaluated. It’s the “ok, do we need to worry about this?” level.

The researchers conducted a formal pharmacokinetic study. 24 healthy volunteers were cooped up (indoors, without sunlight) for a week. According to manufacturer instructions, sunscreen was applied four times a day for the first four days – mimicking what you might do on a typical vacation somewhere warm and sunny. Throughout their stay, blood levels of avobenzone, oxybenzone, octocrylene, and ecamsule (four of the most common active ingredients in chemical sunscreen) were measured.

All four active ingredients achieved blood levels higher than that 0.5 ng/ml threshold. In fact, with the exception of ecamsule, they blew right through it.  The average peak oxybenzone level was around 200 ng/ml. That’s 400 times higher than the level to trigger a formal toxicology review.

This study was preliminary but if you take a look at the drug concentration curves, you can make a couple of inferences. First, the average drug level increased each day during the four days that sunscreen was applied. This means that there is drug accumulation (at least when you are applying it four times a day). You can also see that drug washout was rather slow. Though the authors don’t provide the specific numbers, the half-life of the drug appears to be on the order of a few days – meaning over a summer of use this could really add up. More formal, longer-term studies are needed and the FDA has stated that they have asked industry and “other interested parties” to conduct them.

Sunscreens will stay on the market for now.

And that’s fine. Thinking about it, is it any surprise that these chemicals are absorbed? A quick glance at their chemical structures show interconnected benzene rings that just scream “fat soluble”.

But just because something gets into your body doesn’t mean its bad for you. I know these compounds have chemical names, but the assumption that artificial compounds are worse for you than all the other stuff we put in our bodies is known as the “naturalistic fallacy”. By way of comparison, the average blood level of caffeine after a cup of coffee is 50 times higher than the peak concentration of oxybenzone seen in this study. But that oxybenzone level is about 7 times higher than the blood nicotine level seen after smoking a cigarette.

In other words, the fact that you can measure something in the blood doesn’t tell you anything about whether it is bad for you. We simply don’t know what the risk is. And we need to find out.

Personally, I wouldn’t call for a freeze on chemical sunscreens. These drugs have been used for decades and there have been no strong epidemiologic signals of harm. Quite the opposite, they have likely prevented uncounted cases of skin cancer. The problem with studies like these is that the fear they engender may do more harm than the good science that results from them.  Nothing has changed about the harm of UV-rays since the publication of this study in JAMA – you don’t want your skin exposed to them.

But it may be worth remembering that chemical sunscreens are not the only option.  Clothing and hats provide excellent sun protection, and the systemic absorption of a sombrero is pretty low. And of course, there are barrier sunscreens (formulated with zinc and titanium dioxide) which, yes, leave you looking like an 18th century consumptive, but which are generally regarded as safe.

This summer, my mantra may change a bit.  “Be nice. Wear your embarrassingly white sunscreen. I love you”.

This commentary first appeared on medscape.com