You put it on to protect yourself. You probably never thought about where it goes after.

Every time someone swims, showers, or sweats, a little of their sunscreen washes off. Multiply that by every beach, every reef, every summer. The numbers add up fast — the National Park Service estimates that up to 6,000 tons of sunscreen wash through U.S. reef areas alone every year (Stanford Woods Institute, 2022).

But "sunscreen" is too broad a word for what's actually doing the damage. The harm traces back to a specific set of ingredients: the chemical UV filters — above all oxybenzone and octinoxate — that a lot of conventional sunscreens use as their active protection. Not the bottle. The chemistry inside a particular kind of bottle. That distinction is the whole story, so keep it in mind as we go.

Here's what the science actually says.

A chemical filter called oxybenzone, and how it kills coral

For years, "sunscreen kills coral" got passed around as a slogan — vague enough to be easy to dismiss. In 2022, a team at Stanford published the how in the journal Science, and it points at one chemical, not sunscreen in general. Coral and sea anemones absorb oxybenzone — one of the most widely used chemical UV filters — and their own cells chemically modify it, swapping part of the molecule for a sugar. That change flips oxybenzone from a UV blocker into a phototoxin: a compound that becomes actively poisonous when hit by sunlight (Vuckovic et al., 2022). In the experiments, every anemone exposed to oxybenzone under simulated sunlight died within 17 days, while those kept in the dark stayed alive.

There's a bitter irony buried in that finding. The chemical designed to protect against sunlight becomes lethal because of sunlight — in exactly the shallow, bright water where reefs live.

It gets worse for reefs that are already struggling. When coral bleaches from heat stress, it expels the symbiotic algae living in its tissue. Those same algae, it turns out, soak up some of the toxin and blunt the damage. So a bleached reef — already the most vulnerable — loses its chemical defense right when it needs it most (Vuckovic et al., 2022). Sunscreen pollution and climate change don't just add up. They compound.

This built on earlier work. A 2016 study from a NOAA-affiliated research team found that oxybenzone was toxic to juvenile coral at low concentrations, causing increased bleaching susceptibility, DNA damage, and deformed skeletal growth in developing coral (Downs et al., 2016). The damage shows up worst in the young — the recruits a reef needs to replace itself.

It's not just coral, and it's not just oxybenzone — but it is still the chemical filters

The reef story gets the headlines, but the chemical UV filters reach a lot further down the food chain. And it's a whole class of them, not one bad apple.

Octinoxate (ethylhexyl methoxycinnamate), the other ingredient most often named alongside oxybenzone, has been shown to disrupt thyroid hormone systems in fish. In zebrafish larvae, waterborne exposure to octinoxate and avobenzone measurably interfered with the hormonal axis that governs development (Ka & Ji, 2022). Thyroid disruption in a developing organism isn't a minor thing — it's the system that regulates growth itself.

These compounds also don't just pass through. UV filters bioaccumulate — they build up in the tissue of aquatic organisms rather than flushing out. In one controlled study, all five chemical UV filters tested — including oxybenzone and octinoxate — accumulated in the tissue of crayfish, an invertebrate that sits at an important link in the freshwater food web, and the heavier filters persisted in that tissue even after exposure stopped (He et al., 2021). Because these chemicals are washed into the water continuously, scientists have taken to calling them "pseudo-persistent" pollutants: even though they break down over time, they're replenished faster than they disappear. Octinoxate specifically has been flagged as persistent, bioaccumulative, and toxic, and damaging even to seagrass — the meadows that anchor entire coastal ecosystems (de los Santos et al., 2024).

None of these are trace-lab-only concerns. A 2022 National Academies review found sunscreen chemicals at concentrations between roughly 1 and 10 micrograms per liter at popular swimming spots (National Academies of Sciences, Engineering, and Medicine, 2022) — low, but squarely in the range where the effects above start showing up.

Why governments started banning them

This isn't fringe science anymore — it's on the books. Hawaii became the first U.S. state to ban the sale of sunscreens containing oxybenzone and octinoxate, a law signed in 2018 that took effect January 1, 2021 (Hawaii SB2571, 2018). The U.S. Virgin Islands, Palau, Aruba, Bonaire, and Thailand have passed their own restrictions on the same chemical filters, and some go further — the Virgin Islands also bars travelers from bringing those sunscreens in, and Maui County now requires mineral-only sunscreens (de los Santos et al., 2024). When this many governments independently pull the same ingredients off the shelf to protect their coastlines, the precautionary read is hard to miss.

Chemical filters wash off. Mineral ones don't work that way.

Here's the part worth sitting with. Every ingredient named in this entire piece — oxybenzone, octinoxate, avobenzone, the rest — belongs to one category: chemical UV filters. They protect by soaking into a thin layer of whatever they're on and converting UV into heat. That same solubility is exactly what lets them rinse off in the water, get absorbed by coral cells, and build up in the tissue of marine life. The property that makes them work is the property that makes them a pollutant.

Mineral filters are a different kind of thing entirely. Zinc oxide and titanium dioxide are minerals, not synthetic organic compounds. They sit on the surface and physically reflect and scatter light rather than being absorbed into anything — so there's no organic compound for coral cells to metabolize into a phototoxin the way they do with oxybenzone. That's a real chemical difference, and it's why every one of these bans names the chemical filters specifically and leaves the non-nano minerals alone.

One honest caveat, because it matters: the Stanford team behind the phototoxin discovery is careful to note that mineral sunscreens haven't yet been proven harmless to coral either — that research is still ongoing (Stanford Woods Institute, 2022). "Not the ingredient the science and the law keep pointing at" is not the same as "proven perfectly safe," and anyone selling you the second claim is getting ahead of the evidence. What's solid today is the contrast: the documented reef damage traces to a specific set of chemical UV filters, and minerals aren't among them.

We built Clā around mineral protection — zinc oxide and titanium dioxide — for the way it wears and the way it reads on skin. It also happens to sit on the right side of this whole story: not the chemistry the reef bans are worried about. That wasn't the reason we made it. But it's a good thing to be.

Sun protection you'll actually wear every day shouldn't cost the ocean anything. Turns out it doesn't have to.

References

de los Santos, C. B., et al. (2024). Potential hazards of octinoxate (ethylhexyl methoxycinnamate) exposure in the seagrass Posidonia oceanica (L.) Delile: Experimental evidence. Science of the Total Environment. https://www.sciencedirect.com/science/article/pii/S0048969724075545

Downs, C. A., Kramarsky-Winter, E., Segal, R., Fauth, J., Knutson, S., Bronstein, O., et al. (2016). Toxicopathological effects of the sunscreen UV filter, oxybenzone (benzophenone-3), on coral planulae and cultured primary cells and its environmental contamination in Hawaii and the U.S. Virgin Islands. Archives of Environmental Contamination and Toxicology, 70, 265–288. Summary: https://oceanservice.noaa.gov/news/sunscreen-corals-noaa-studies.html

Hawaii State Legislature. (2018). Senate Bill 2571 (Act 104): Relating to Water Pollution. https://www.capitol.hawaii.gov/sessions/session2018/bills/SB2571_CD1_.htm

He, K., Hain, E., Timm, A., & Blaney, L. (2021). Bioaccumulation of estrogenic hormones and UV-filters in red swamp crayfish (Procambarus clarkii). Science of the Total Environment, 764, 142871. https://www.fs.usda.gov/nrs/pubs/jrnl/2021/nrs_2021_he_001.pdf

Ka, Y., & Ji, K. (2022). Waterborne exposure to avobenzone and octinoxate induces thyroid endocrine disruption in wild-type and thrαa−/− zebrafish larvae. Ecotoxicology, 31(6), 948–955. https://link.springer.com/article/10.1007/s10646-022-02555-1

National Academies of Sciences, Engineering, and Medicine. (2022). Review of Fate, Exposure, and Effects of Sunscreens in Aquatic Environments and Implications for Sunscreen Usage and Human Health. Figures via Smithsonian Ocean: https://ocean.si.edu/ecosystems/coral-reefs/truth-about-corals-and-sunscreen

Stanford Woods Institute for the Environment. (2022, May 5). Understanding how sunscreens damage coral (R. Jordan, reporting on Vuckovic et al., Science). Stanford Report. https://news.stanford.edu/stories/2022/05/coral-killing-sunscreens

Vuckovic, D., Tinoco, A. I., Ling, L., Renicke, C., Pringle, J. R., & Mitch, W. A. (2022). Conversion of oxybenzone sunscreen to phototoxic glucoside conjugates by sea anemones and corals. Science, 376(6593), 644–648. https://www.science.org/doi/10.1126/science.abn2600