Some of the sunscreen you slather on this summer will end up in lakes, streams or the ocean, even if you don’t go swimming. And a growing body of evidence suggests that ultraviolet filters, the active ingredients in sunscreens, can harm creatures that live in the water.
Some products are marketed as “reef safe” or friendly to aquatic life. But has that been proved? We talked to a dermatologist, several ecologists and toxicologists, and a chemical engineer to find out the best way to protect your skin and the environment, too.
Your sunscreen options
There are two kinds of UV filters in sunscreens on the market today.
Mineral sunscreens create a physical barrier on your skin that reflects UV rays like a mirror, while chemical sunscreens are absorbed into the skin and convert the UV radiation into harmless heat. (Chemical sunscreens are also sometimes labeled “organic,” but that’s a chemistry term, not a claim of environmental friendliness.)
Any sunscreen you apply will eventually end up in water. Researchers estimate that between 25 and 50 percent of sunscreen comes off during a dip. The rest goes down the drain when you shower or enters the wastewater system through the laundry when you wash your beach towels.
Most standard treatment plants aren’t effective at removing trace levels of UV filters from wastewater, said Dunia Santiago, a chemical engineer at the University of Las Palmas de Gran Canaria in Spain who studies how treatment plants process contaminants. That means the chemicals are still in the water that flows out of the plant and into the world.
And, since many UV filters don’t biodegrade well, levels can build up over time in the environment, floating around, settling into sediment and being eaten by animals, especially in shallow areas popular with swimmers.
What we know and don’t know
There’s a growing body of evidence that both chemical and mineral UV filters have the potential to harm wildlife, including coral reefs, at high concentrations. A 2016 study on the potential for a chemical UV filter called oxybenzone to make coral more vulnerable to bleaching made a particularly big splash in the public consciousness, increasing demand for gentler alternatives and leading some places to ban the sale of some chemical sunscreens.
In response, some manufacturers started marketing mineral sunscreens as “reef safe.” But researchers generally agree you shouldn’t put too much stock in these labels, which aren’t regulated.
Calling one UV filter safer than another “implies that we have information to make a comparison, which we do not have,” said Sandy Raimondo, an ecologist at the Environmental Protection Agency who studies chemical contaminants.
The science on UV-filter toxicity isn’t rock-solid because the laboratory methods used to test them haven’t been standardized, according to ecologists and toxicologists we interviewed.
One important issue is the “stickiness” of chemical UV filters. They cling to the surface of the water, the sides of tanks and the inside of tools designed to measure their concentrations. When researchers can’t be certain of the concentration of a chemical in water, Dr. Raimondo said, the resulting data isn’t reliable.
While the data on mineral UV filters is more reliable, new formulations designed to minimize that ghostly white cast on the skin cause their own problems. Some manufacturers use so-called nano versions of zinc oxide and titanium dioxide. These even-tinier particles can get embedded in the tissues of plants and animals in ways that scientists are only beginning to understand, Dr. Raimondo said.
Trying to fill in the blanks
The E.P.A. is currently funding studies to fill the gaps in our understanding of UV-filter toxicity. Top priorities include resolving measurement issues and developing standardized methods to make comparisons easier. But President Trump’s plans for deep cuts at the agency have put the future of many environmental studies in doubt.
Even if those studies continue, they will probably take years to complete, and the agency could take several more years to conduct an official ecological risk assessment for any particular UV filter.
Some researchers say that, even with our incomplete knowledge of the impacts of UV filters, the existing evidence on certain chemical UV filters is damning enough for us to switch to alternatives that use non-nano mineral UV filters. Indeed, the stickiness of chemical UV filters may mean that existing research underestimates their environmental toxicity.
What you can do right now
Thankfully, you don’t have to broil to help the environment. Dermatologists and toxicologists agree on the best form of sun protection. But it’s not mineral or chemical sunscreen. It’s clothing.
Sunscreen is an important component of protection, “but it’s not the only component,” said Dr. Henry Lim, a dermatologist at Henry Ford Health in Detroit and a former president of the American Academy of Dermatology. “Staying in the shade, wearing photoprotective clothing, a wide-brimmed hat and sunglasses are very, very important.”
Cover as much real estate as you can with UPF rated clothing (that’s the SPF equivalent for fabric). “Sunscreen should be applied only in the areas that cannot be covered,” Dr. Lim said.
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