Thread by @AdeAdamson

Effect of Sunscreen Application on Plasma Concentration of Active Ingredients bit.ly/2vD3w9V This provocative study in JAMA about sunscreen deserves a #tweetorial. Bear with me as I take you through the controversy and discuss sunscreen in general. 👇🏾👇🏾👇🏾

TL; DR – certain chemical sunscreens are absorbed after application; we don’t know what this means for human health. This does not mean you should abandon sunscreen use nor does it mean sunscreens are unsafe.

For those of you still here...there is a lot to unpack, here we go….

Let’s start with: What are the 2 types of sunscreen? There are physical/mineral blockers and chemical blockers (a.k.a. Inorganic and organic, respectively) sunscreens.

Physical blockers are comprised of zinc oxide or titanium oxide, they work by reflecting light off the skin. You can think of them as fine particles of sand that sit on top of the skin. FYI: They can leave a chalky look, particularly on dark skin.

Chemical blockers absorb UV radiation and are made up of several different chemicals including: octocrylene, avobenzone, oxybenzone, octinoxate, homosalate, octisalata, and others).

Physical sunscreens show no evidence of absorption through the skin, and no evidence of adverse environmental effects.

However, chemical sunscreens have been shown in this study (and previously) to be absorbed to the skin. The has also been shown to damage coral (more on this later).

Given the above, the FDA deemed physical blockers as “generally regarded as safe and effective” (GRASE). Whereas certain chemical sunscreens have been deemed as having insufficient data for safety of use.

🚨🚨🚨This does NOT mean that the chemical sunscreens are unsafe, its just that there is not enough data. In fact the FDA tried to get manufacturers conduct safety studies, but they manufacturers dragged their feet!

As a result, the FDA took it upon themselves to do the study, hence this JAMA paper.

This JAMA study was a small randomized control trial in which 24 healthy participants applied one of 4 commercially available sunscreen several times a day for several days, then blood levels of sunscreen ingredients were measured.

Even on day 1 of application, levels of specific chemical sunscreen could be measured in the blood of participants. Ultimately, participants had levels of sunscreen ingredients in their blood at higher than the established FDA threshold levels.

IMPORTANTLY the study says NOTHING about health effects of chemical sunscreen, but it does suggest that this needs to be studied.

Also, this is maximum dosing, meaning in the real world because of poor application, sweating, swimming, sun exposure, levels in the blood are likely lower.

Let’s switch gears and approach an even more provocative question, does sunscreen prevent skin cancer?

You’d think as heavily as sunscreen is marketed with billion dollars in sales that this question was completely settled. Interestingly, there has only been 4 prospective studies that examine sunscreen’s role in skin cancer prevention.

None of these studies assess the role in healthy individuals!!! 🤯🤯🤯 See the systematic review here: jaad.org/article/S0190-… As you can see for AKs and SCCs it seems to help, the data is less robust for BCCs and melanoma.

This is not to say that sunscreen doesn't work, it is very bioplausible that sunscreen could benefit in melanoma and BCC prevention, its just that there aren’t well done trials to support that conclusion.

Here is the only prospective trial to study whether sunscreen reduced incidence of melanoma. The absolute risk reduction is ~1% (meaning 100 people would have to apply regularly to avoid 1 melanoma), and the result was of “borderline” significance: ascopubs.org/doi/10.1200/JC…

This does not mean you shouldn’t wear sunscreen. The above study was old, and the SPF level of sunscreen used in the study was much lower than what is recommended now. However, this is the BEST data we have.

Again to be crystal clear 🗣️🗣️🗣️: I'm not suggesting sunscreen doesn't work and is not a useful skin caner prevention strategy. I'm just pointing out how thin the data is in this department. That shouldn't be controversial, its just facts.

Let’s switch it up again and talk about coral reef and Oxybenzone , a chemical sunscreen ingredient that has been implicated in coral reef bleaching, which prompted Hawaiian state legislature (and others) to pass legislation banning certain sunscreens. What’s the evidence?

Oxybenzone is a UVA and UVB filter used in 2/3 of chemical sunscreens. The CDC estimates that 96.8% of the US population is exposed to it. See: ncbi.nlm.nih.gov/pubmed/18629311

Oxybenzone has been shown to be an endocrine disruptor in RATS, but the amount one would have to put on in order to achieve the levels used in the lab would take 35 years of daily application to entire body: jamanetwork.com/journals/jamad…

To date there have been no safety issues in humans since oxybenzone has been used in sunscreen in the US since 1978.

What about coral reef? Oxybenzone is not easily removed from water treatment facilities and the chemical can damage coral reef. This has led to certain communities banning sunscreen with the chemical in it.

However, the studies on coral reef and oxybenzone were ALL done in the laboratory setting, and detection in wildlife is extremely low in concentration. Global warming is a VASTLY larger contributor to coral bleaching than sunscreen.

Bottom line: this study shows that more work needs to be done to elucidate the potential health implications of chemical sunscreen absorption. However, there is little/weak evidence to suggest chemical sunscreen is harmful to health.

However, the “safest” way to play it would be to use physical blocker sunscreen when possible until more is known.

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