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Dermatologists, beauty gurus, and parents consistently offer one seemingly uncontroversial, well-intentioned piece of advice: Wear sunscreen. A majority of chemical-based sunscreen products contain the active ingredients oxybenzone and octinoxate. These are chemical ultraviolet (UV) filters that absorb harmful rays and help prevent skin cancer.

But over the last few years, a movement has emerged to ban such sunscreen ingredients based on evidence that they are toxic to coral reefs. And it's making significant progress politically: In February, Key West, Florida, banned the sale of sunscreens containing oxybenzone and octinoxate in an effort to protect coral reefs from bleaching and death. Similar bans were set in the state of Hawaii and the Republic of Palau last year.

The main catalyst for these bans was a 2015 study published in the Archives of Environmental Contamination and Toxicology titled "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." In a nutshell, the study demonstrated how exposure to oxybenzone can cause damage, including bleaching and DNA and endocrine disruption in corals, thus posing a threat to coral reef conservation and the resiliency of these ecosystems to climate change.

The day after this study was published online, government officials began contacting its authors to ask questions about the science and generate ideas for what to do about the damage. Within months, Hawaii state legislators began to propose bills to ban oxybenzone, and, in 2018, one of these bills passed into law.

Government agencies like the National Park Service and the National Oceanic and Atmospheric Administration have taken this research to heart, warning people to avoid using sunscreen whose ingredients include chemicals like oxybenzone. Yet some marine scientists think more research is needed to justify passing sunscreen bans, while others point out a group to whom oxybenzone potentially poses another health risk: humans.

Scientists Debate Oxybenzone's Environmental Impacts

Craig Downs, executive director of the Haereticus Environmental Laboratory, was the principal author of the 2015 study. His previous research on the subject was published in the journal Ecotoxicology in 2014, but, prior to that, only one major study had looked at the potentially harmful impacts of oxybenzone on coral reefs. That 2008 study, titled "Sunscreens Cause Coral Bleaching by Promoting Viral Infections," by Roberto Danovaro at the Marche Polytechnic University in Ancona, Italy, and others, found that chemical UV filters including oxybenzone cause "rapid and complete bleaching of hard corals" even at extremely low concentrations, and thus potentially play a large role in coral bleaching in areas popular for human recreational use.

For his 2015 study, Downs' lab placed coral larvae, which are just a couple of millimeters long, into the tiny wells of microplates filled with artificially created seawater containing oxybenzone at "environmentally relevant" concentrations measured in parts per million, parts per billion, and parts per trillion. The larvae reacted by losing their cucumber-like shape and turning white, indicative of bleaching. When corals are stressed by changes in conditions, they expel all their algae (their major source of food), turn white, and become more vulnerable to disease and death. Downs' lab found that oxybenzone was damaging to seven species of coral. Downs notes that other scientists have repeated versions of his experiment with similar results, allowing him to draw significant conclusions about the accuracy of his own findings.

A school of gray snapper, with some in the dark reddish brown phase, during a coral bleaching event, at Flower Garden Banks National Marine Sanctuary in the Gulf of Mexico.

A school of gray snapper, with some in the dark reddish brown phase, during a coral bleaching event, at Flower Garden Banks National Marine Sanctuary in the Gulf of Mexico.

Downs' lab also measured the levels of oxybenzone found at certain areas in the U.S. Virgin Islands and Hawaii. One place where the researchers found particularly high levels of oxybenzone was Trunk Bay on the Island of St. John, a popular tourist destination where up to 2,000 people can swim and snorkel in close proximity to an offshore coral reef every day, Downs says. Downs describes the extreme bleaching and damage evident in the Caribbean, and his study suggests that sunscreen-wearing tourists can make it more difficult for the reefs to recover from this damage. The National Park Service even advises visitors to this destination to avoid sunscreens with oxybenzone, among other chemicals.

Yet for a study that has been so widely embraced by conservationists, Downs' work has been received with skepticism by some members of the marine science community.

Eric Hochberg, an ecologist who studies reefs at the Bermuda Institute of Ocean Sciences, says he believes Downs' research offers weak evidence that oxybenzone is actually damaging corals because the experiment's laboratory conditions "completely lack realism." Hochberg is working on his own study that he hopes will yield what he believes will be more realistic results by placing corals in tanks with flowing water containing concentrations of oxybenzone that match those found in water samples around Bermuda.

Hochberg also points out that Downs' research team found barely detectable levels of oxybenzone in some waters tested in Hawaii. Hochberg, who got his Ph.D. in oceanography at the University of Hawaii and lived and researched in the state for 14 years, says that Hawaii's reefs were threatened by a mass bleaching event a few years ago as a result of El Niño, and he thinks such events pose a greater threat to the area than sunscreen does.

"If these chemicals were dangerous, we would see an impact already," Hochberg says. "These chemicals have been in the environment for a long time, for a decade at least, so why are there still lots of corals?"

Terry Hughes, director of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University in Australia, is also skeptical of Downs' findings. He says that laboratory experiments can be insightful when paired with field research that corroborates their conclusions. And to date, no studies have shown a harmful impact of sunscreen chemicals on corals in their natural environment, according to Hughes.

"The behavior of the sunscreen in the water when it washes off a tourist's back is a blank slate—no one has studied that," he says.

Hughes has won international awards for his work studying the impacts of climate change on coral reefs. His team used satellite data from NOAA to map the extent of bleaching measured in aerial and underwater surveys to the pattern of heat in the oceans during a global-scale bleaching event in 2015 and 2016. In each of Australia's four major bleaching events, the geological footprint of the bleaching perfectly matched where the water was hottest, Hughes says. To him, this indicates that global warming trumps all other factors: Even in areas of the ocean untouched by fishermen or farming, reefs experienced the same level of bleaching, he found.

Hughes says the three major threats to coral reefs are climate change, overfishing, and pollution from sediment runoff from land-based pollution. Sunscreen, on the other hand, is a local and relatively inconsequential issue, he says, and the main thing that needs to be done to mitigate damage to coral reefs is to reduce carbon emissions.

"I've seen online people wearing T-shirts [that say] 'Save the reef, ban sunscreens.' Those people could well be tourists who have flown to Miami for a holiday," Hughes says. "The irony there is the personal fossil fuel footprint of people is part of the problem. You can't deal with one tiny issue and tick a box to say 'problem solved.' But I think that's exactly what's happening in some places."

Downs, on the other hand, argues that reef pollution is a local issue. He claims he doesn't believe that oxybenzone is the biggest threat to the vitality of all coral reefs, as news outlets like the New York Times have reported; rather, each reef faces its own unique issues. In some, it may be warm water; in others, it may be tourism and sunscreen, he says.

The Science of Oxybenzone's Hormonal Impacts

It isn't just the environmental implications that make oxybenzone so controversial.

Beyond corals, some scientific evidence has shown that oxybenzone may produce harmful effects in mammals and fish by mimicking estrogen in the body and disrupting the endocrine system, which produces and regulates hormones. Downs' 2015 study touches on the estrogenic effects in mammals, citing five different studies. Downs and Joseph DiNardo, a retired toxicologist, also published a short paper earlier this month presenting data about the possible endocrine-disruptive effects of oxybenzone in children and young adults. The text of Hawaii's ban touches on oxybenzone's potential endocrine-disrupting effects as well.

A reef scene off of St. Croix, U.S. Virgin Islands.

A reef scene off of St. Croix, U.S. Virgin Islands.

The non-profit Environmental Working Group has published an annual Sunscreen Guide since 2007 in an effort to improve sunscreen safety and highlight concerns about oxybenzone's hormonal impacts. Nneka Leiba, the director of EWG's healthy living science program, says she is unsure why recent studies concerning the environmental impacts of oxybenzone were so quick to prompt change, while the chemical's potential hormonal effects haven’t been discussed as much.

"Obviously for Key West and Hawaii, reef health and safety is paramount to their tourism and their economy, but whether or not you live in a beach state or you're going on vacation, you should be concerned about these chemicals because of their impact on human health," Leiba says.

In a study published last year in the Journal of the Endocrine Society, endocrinologist Laura Vandenberg found that, in pregnant and lactating mice, exposure to similar concentrations of oxybenzone to those found in sunscreens caused disruption of the mammary gland in ways that could be risk factors for breast cancer. This doesn't necessarily mean that the mice will get breast cancer (Vandenberg is actually studying that now), but changes like cell proliferation in the mammary gland do suggest cause for concern, she says. (Vandenberg also notes that, while mice are a good predictor for humans, we don’t know for sure whether humans will experience the same effects.)

Estrogen promotes breast cell growth, so exposure to estrogen over long periods of time is a risk factor for breast cancer. For a woman, Vandenberg says, even just getting your period earlier in life and going through menopause later in life can raise your risk for breast cancer, since you are exposed to your own estrogen for a longer-than-average amount of time under those scenarios.

"People get worried when we start exposing them to other chemicals in the environment that mimic estrogen because if estrogen can promote breast cancer—and even under the best circumstances your own estrogen can do that—the worry is that, if a chemical can also act like estrogen, it can do the same thing," Vandenberg explains.

At the same time, the American Academy of Dermatology notes that "no data shows that oxybenzone causes any significant health problems" in humans, and that oxybenzone serves an incredibly important function as a chemical UV absorber to protect people from harmful rays that cause skin cancer in up to one in five Americans, regardless of age, gender, or race.

And thanks to the effectiveness of oxybenzone's sun protection, it's used in more than just sunscreen: In addition to a range of cosmetics claiming an SPF (sun protection factor) level, manufacturers have started adding oxybenzone to plastic bottles to protect their contents from sun damage. It can even be found on furniture and fabric to prevent discoloration, Vandenberg says.

While the first commercial sunscreens came on the market in the 1940s, they weren't regulated by the Food and Drug Administration as a drug until 1978. Oxybenzone was developed as a sunscreen ingredient in the 1950s, and it was first approved by the FDA in 1980s. Today, it can be found in 65 percent of non-mineral sunscreens, according to the EWG.

In February, the FDA announced a proposed rule to re-evaluate and improve safety regulations for sunscreen. The agency found that, of the 16 active ingredients currently used in over-the-counter sunscreens, only two are "generally recognized as safe and effective" (a classification known as GRASE). Another two, PABA and trolamine salicylate, are not GRASE owing to safety issues (no sunscreens currently sold in the U.S. contain these ingredients). The FDA called for the sunscreen industry to submit data on the remaining 12 marketed active ingredients, including oxybenzone, to determine whether these chemicals are GRASE.

Under the proposed rule, sunscreens that are considered GRASE would be able to enter the U.S. market without going through the FDA's New Drug Application approval process, while any sunscreens containing non-GRASE active ingredients would be required to go through the NDA approval process in order to be legally marketed. The announcement acknowledged that limited information is currently available about the safety of the chemicals, but, for the time being, the FDA encourages people to continue using sunscreens with these ingredients.

This move comes as part of the Sunscreen Innovation Act, passed by Congress in 2015 to ease the process of approving new active ingredients in sunscreen, such as the 27 active ingredients currently on the market in Europe. Despite the passage of this act, no additional ingredients have been approved for use in the U.S. since the 1990s. But according to FDA Press Officer Sandy Walsh, eight of the ingredients currently marketed in Europe have been proposed and are being considered for approval in the U.S.

"The FDA has identified the missing data for the eight ingredients in feedback letters to the manufacturers and looks forward to working with them to bring these active ingredients to the U.S. market," Walsh writes in an email.

Still, given that Hawaii's ban on oxybenzone and octinoxate went into effect in less than two years, the clock is ticking. The EWG is concerned that removing the vast majority of chemical sunscreens in Key West and Hawaii from the market without enough safe replacements will create a significant public-health issue. Even Vandenberg, who studies the harmful impacts of oxybenzone, echoed those concerns.

"This chemical, unlike some other chemicals, really needs to be considered very carefully because it is doing an important job, and that job is health-related," Vandenberg says. "So we have to make decisions that take into account that removing this chemical from the market will affect people's potential risk for a disease."

Since Downs' study came out, many sunscreen companies have created "reef-safe" formulas made with the mineral active ingredients zinc oxide and titanium dioxide. These minerals work as physical blockers of UV rays to prevent sunburn, as opposed to oxybenzone and octinoxate, which protect the skin by absorbing UV rays. Formulas with zinc oxide and titanium dioxide tend to leave a white cast on the skin (think Larry the Lobster from SpongeBob SquarePants with the white stuff on his nose), which Leiba is concerned can be a deterrent for some sunbathers. Recently, some companies have tried to formulate around that white cast by tinting their products to match different skin tones.

Downs adds that he believes it's dangerous to market products as "reef-safe" because it's not an official designation, and companies could be marketing their products as such without doing any toxicological tests to prove it. One study released last year in the journal Science of the Total Environment even showed that zinc oxide can cause coral bleaching.

To prove that a sunscreen product is actually safe for coral reefs, "you would have to do an ecological risk assessment on that product, and you would have to show that, at a reasonable, rational concentration, at a specific reef, that it wasn't harmful to the ecosystem as a whole," Downs says. "'Reef-safe' is a marketing claim. I don't know of a single government that demands a 'reef-safe' product. They do demand the absence or the exclusion of particular chemicals, but that's more pollution mitigation."

Johnson & Johnson, the parent company of sunscreen-carrying brands Neutrogena and Aveeno, maintains that there isn't enough research to prove the harmful impacts of oxybenzone, and continues to use the chemical in its products.

"With decades of science showing that sunscreens help prevent and reduce the risk of skin cancer, being able to choose a high-SPF sunscreen is essential to protecting public health and especially important to those who have survived melanoma or are at greater risk for developing skin cancer," Johnson & Johnson Media Relations writes in an emailed statement. "Oxybenzone is integral to formulating high-SPF, broad-spectrum sunscreens in the United States, which many consumers choose when working or playing outside."

What Do These Bans Mean for the Future of Sunscreen?

On the surface, Hawaii and Key West's laws to ban sunscreens containing oxybenzone might seem like helpful, easy ways for local governments to protect marine life. But the degree of debate and uncertainty within the scientific community, the implications for human health that come with both using chemical sunscreens and avoiding them, and the lack of clarity surrounding the safety of allegedly "reef-safe" sunscreen options all complicate the situation. These questions are difficult to answer, but they demonstrate a need for greater consideration and collaboration among scientists, governments, and industry leaders as these bans go into effect on January 1st, 2021, in Hawaii, and July 1st, 2021, in Key West.

"Hopefully the state or the entity that is banning ingredients will go that extra step to realize that it takes a little bit more than just banning an ingredient," Leiba says. "They have to work with the FDA, they have to work with manufacturers, and they have to make sure that people are educated that they still need sunscreens."