Oxybenzone

Oxybenzone absorbs ultraviolet light and it is absorbed through skin. Both statements are true, both are measured, and the distance between the second one and the word harm is the whole of this page.

The number, and where it comes from

FDA has guidance that a sunscreen active reaching plasma concentrations “greater than 0.5 ng/mL or with safety concerns should undergo nonclinical toxicology assessment including systemic carcinogenicity and additional developmental and reproductive studies”.

So there is a threshold, and it is a testing trigger rather than a danger line. In 2019 FDA investigators ran a trial to find out whether ordinary sunscreens cross it. Twenty-four participants applied one of four commercial products under maximal use conditions, meaning the full labelled application over most of the body, four times a day.

Maximum plasma concentrations for oxybenzone were 209.6 nanograms per millilitre for one spray, 194.9 for a second, and 169.3 for a lotion. Against a threshold of 0.5.

“Systemic concentrations greater than 0.5 ng/mL were reached for all 4 products after 4 applications on day 1.”

That is a large multiple and it is worth taking seriously, because it is exactly the kind of measurement that should have existed before the products were on shelves and did not.

What the authors said next

This is where most coverage of this study stops and where the study itself does not.

The investigators concluded that the plasma concentrations “exceeded the threshold established by the FDA for potentially waiving some nonclinical toxicology studies for sunscreens”, that this “supports the need for further studies to determine the clinical significance of these findings”, and then, in the same paragraph:

“These results do not indicate that individuals should refrain from the use of sunscreen.”

Scientists rarely write a sentence like that into a conclusion. They wrote it because they could see how the number would be read and because the number does not support the reading.

Crossing a testing threshold means the testing has not been done. It does not mean the testing was done and came back badly. Those are different states of knowledge and only one of them justifies changing behaviour, which is the distinction set out on the guide to hazard and exposure.

Against that sits a known quantity on the other side of the ledger. Ultraviolet exposure causes skin cancer, and that is not an open question.

What to do if you would rather not

Wanting a filter that stays on the surface is a perfectly reasonable preference, particularly in pregnancy, and you do not need to prove harm to hold it.

The mineral filters are the answer. Zinc oxide and Titanium dioxide sit on the skin and scatter light rather than absorbing into it, and zinc oxide alone gives broad protection across UVA and UVB. In the same FDA trial the products tested were all organic filters, because the question being asked was about absorption.

The trade is cosmetic rather than protective: mineral filters leave more visible residue, and that is the reason most people give for not using them.

On the reef question

Oxybenzone has also been restricted in several jurisdictions on coral reef grounds rather than human health ones. Those are separate arguments resting on separate evidence, and a restriction of that kind is not a statement about what the ingredient does to a person.

Worth knowing because the two get merged in coverage, and a reader who avoided oxybenzone for reef reasons and then reads a health reassurance is entitled to notice that the reassurance did not address what they were asking.