How Sunscreen Chemicals Harm Coral Reefs, and What "Reef Safe" Really Means

What washes off

Sunscreen does not always stay on the skin. It comes off in the water when people swim, and it goes down the drain when they shower.¹ At a busy beach, that happens thousands of times a day, usually in the shallow, sheltered water where corals and people both gather.

The National Oceanic and Atmospheric Administration (NOAA) states the concern directly: some chemicals found in sunscreen and other personal-care products threaten coral reefs and other marine life. NOAA also describes this as an area of continuing research.¹ Both points belong in any account of the subject.

The chemicals in question

Sunscreens protect skin with UV filters, which fall into two groups. Chemical (or organic) filters, such as oxybenzone, octinoxate, octocrylene, and avobenzone, absorb ultraviolet light. Mineral filters, zinc oxide and titanium dioxide, are particles that sit on the skin.

Oxybenzone and octinoxate have drawn the most attention from researchers and lawmakers. NOAA's list of chemicals in sunscreens that can harm marine life includes both, along with octocrylene, benzophenone-1, benzophenone-8, OD-PABA, 4-methylbenzylidene camphor, and 3-benzylidene camphor. The list also includes nano-sized titanium dioxide and nano-sized zinc oxide, a detail that complicates the simple idea that "mineral" always means harmless.¹

What these chemicals can do

NOAA summarizes the possible effects by organism. In corals, the chemicals accumulate in tissues and can induce bleaching, damage DNA, deform young, and even kill. In green algae, they can impair growth and photosynthesis. They can induce defects in young mussels, damage the immune and reproductive systems of sea urchins and deform their young, and decrease fertility and reproduction in fish. In dolphins, they can accumulate in tissue and be transferred to young.¹

The wording matters. NOAA says these chemicals can do these things. Much of the underlying evidence comes from laboratory studies in which organisms are exposed to measured concentrations under controlled conditions. That establishes that a hazard exists. It does not by itself establish how often those effects occur on a particular reef.

What the National Academies found

In August 2022, the National Academies of Sciences, Engineering, and Medicine published a review requested by Congress and sponsored by the Environmental Protection Agency. NOAA summarizes its conclusion this way: specific chemical UV filters found in sunscreen can harm aquatic life, including corals.^1,2

The report was equally clear about what remains unknown. UV filters have been found in water, sediment, and animal tissue in many aquatic environments, with levels that vary by location and time and tend to be higher near recreational areas. But toxicity data for many species, particularly marine species, are limited; there are few studies of longer-term effects; and information on how quickly UV filters dissipate and degrade is incomplete.²

For that reason the committee did not recommend banning particular ingredients. It called on the EPA to conduct an ecological risk assessment of all UV filters currently on the market, mineral filters included, and described that assessment as urgently needed. It asked that the work focus on the settings where exposure is likely to be highest, such as shallow coral reefs near heavily used shorelines, and that it examine UV filters in mixtures and alongside other pressures, including climate change and rising ocean temperatures.²

What has been measured in the water

Field measurements show that these chemicals do reach reef waters, sometimes at substantial levels. A study of Hanauma Bay, a heavily visited marine preserve on Oʻahu, analyzed water samples collected in 2017 and found oxybenzone in near-shore waters at concentrations ranging from 30 to 27,880 nanograms per liter. The authors concluded that those levels could threaten the bay's coral reef, identified beach showers as a source alongside swimmers, and estimated from modeling that a day's contamination could remain in the bay for up to 50 hours.^3,4

A later University of Hawaiʻi study sampled four South Shore beaches and Hanauma Bay repeatedly across the day. It detected both oxybenzone and octocrylene and found high variation within a single day, which the author attributed to human activity. That variability shows how sensitive any single measurement is to when and where the sample is taken.⁵

The uncertainties

Not every reef scientist weighs this evidence the same way. When the Hanauma Bay study was published, the Honolulu Star-Advertiser reported that the Hawaiʻi Institute of Marine Biology said there was no strong evidence that sunscreens threaten coral reefs.⁶ The disagreement is mostly about magnitude: whether concentrations on most reefs, most of the time, are high enough to cause the damage seen in laboratories.

Scale matters too. The National Academies report treats UV filters as one stressor among several, to be assessed together with warming oceans and local pollution.² Choosing a different sunscreen addresses one local, controllable pressure. It does not address ocean temperature, and no credible source claims that it does.

The public-health side matters as well. The same National Academies report affirmed that consistent use of broad-spectrum sunscreen with SPF 30 or higher reduces the risk of skin cancer, and cautioned that reduced availability of the sunscreens people prefer could lower use.² The aim is to protect skin and reefs together, not to trade one for the other.

What "reef safe" does and does not mean

"Reef safe" and "reef friendly" are marketing terms. No U.S. federal standard defines them, no agency certifies them, and no required test stands behind them. A manufacturer can print the phrase on a product without showing that the formula has been tested on any marine organism. In practice it usually means "contains no oxybenzone or octinoxate," which says nothing about the other ingredients.

The law is more specific than the label, but narrower. Oxybenzone and octinoxate remain permitted in U.S. sunscreens. Hawaii has prohibited the sale and distribution of sunscreens containing either one without a prescription since January 1, 2021.⁷ Maui County went further in October 2022, allowing only mineral sunscreens without a prescription.⁸ Separately, the FDA proposed in 2019 that only zinc oxide and titanium dioxide be classed as generally recognized as safe and effective for people, and asked for more data on twelve chemical filters, including oxybenzone. That proposal concerns human safety, not reefs, but it points toward the same two ingredients.⁹

What to do with this

NOAA's advice is practical and stops short of promising certainty:¹

• Read the active ingredients, not the front of the bottle, and choose a sunscreen without the chemicals that can harm marine life.
• Consider a mineral sunscreen. NOAA describes it as a better option because there are fewer effects on aquatic organisms. Because nano-sized minerals appear on NOAA's list, non-nano formulations are the more cautious choice.
• Use less sunscreen by covering more skin: seek shade between 10 a.m. and 2 p.m., and wear UV-protective clothing such as UPF sun shirts.

Skipping sun protection is not on that list. The evidence that UV radiation damages human skin is far more settled than the evidence about any single sunscreen ingredient and reefs.

Where Sunspi fits

Sunspi is a booth, developed by Embry-Riddle engineers and UGA premedical and medical students, that applies a full, even coat of sunscreen from the neck down in seconds, for a few dollars, with the face and scalp left for you to do by hand. It dispenses only reef-safe sunscreen. Since that phrase has no legal definition, we think any such claim, ours included, should be judged by the ingredient list behind it. A demo is coming soon, and our aim is to make it easier to protect skin without adding to what reefs already face.