You’ve just had a lovely swim and it’s time to get back to your beach reading. But first, reapply your sunscreen. Where did the old sunscreen go, anyway? According to one study, at least 25% of it washes off within the first 20 minutes of a swim. The result? Thousands of tons of sunscreen floating in the ocean, wreaking havoc on the marine biosphere.

Sunscreen chemicals can kill coral, impair photosynthesis in algae, and deform marine life. But if you pick the right one, you can reduce the environmental footprint of your next beach day. In this Decider, we’ll investigate which sunscreens do the least harm while remaining effective.

Care for the Coral

Most sunscreens contain chemical barriers, which absorb UV rays to prevent your skin from doing so. Sunscreens with physical barriers — sometimes called mineral sunscreens — sit on the surface of the skin and reflect or scatter UV rays.

According to the US National Oceanic and Atmospheric Administration (NOAA), many common chemical sunscreen ingredients are potentially harmful to marine life, and can cause allergic reactions. They remain popular because they’re cheaper to make and blend into skin.

Oxybenzone (benzophenone-3) — one of the most common sunscreen ingredients — is a proven hormone disruptor that can decrease fertility and cause developmental defects in humans and marine life, such as mussels, coral, fish, and sea urchins. Other chemicals NOAA recommends avoiding include benzophenone-1, benzophenone-8, OD-PABA, 4-methylbenzylidene camphor, and 3-benzylidene camphor.

There’s a movement among coastal states and island nations to ban sunscreens containing oxybenzone and octinoxate, both of which bleach and kill coral.

There’s a growing movement among coastal states and island nations (from Hawai’i to Pulau) to ban sunscreens containing oxybenzone and octinoxate, both of which bleach and kill coral. Other chemicals may harm reefs too, but these two are singled out because they’ve been researched most extensively, explains Dr. Cheryl Woodley, a scientist and project manager at NOAA.

The problem is, sunscreen doesn’t stay put. Even if you never set foot in the ocean, sunscreen washed off in the shower or the sink will end up in your waste water and, ultimately, the environment. Swimmers and bathers release 4,000 to 6,000 tons of sunscreen near reefs every year, according to a 2008 estimate in the academic journal Environmental Health Perspectives.

“These chemicals are not stopped by wastewater treatment plants, and so there are also issues with where those waters end up,” explains Dr. Woodley. “It’s not just [in] our oceans, but in rivers and streams and other water bodies.”

Nano is a No-No

Physical sunscreens are generally safer, but users often complain they leave a bluish-white sheen on the skin. That’s because mineral barriers — which are made with zinc and/or titanium dioxide — need to be reflective to work.

Micronization technology can reduce their opacity and, therefore, their whiteness, by breaking them down into particles less than 100 nanometres in size. To give you an idea of how small that is, consider that a single sheet of paper is 100,000 nanometres thick. However, Dr. Woodley says, there’s growing evidence that zinc nanoparticles have a negative impact on marine life, and could harm humans if ingested.

“When you get particles that small, they’re able to transfer [through] cell membranes,” she explains. “Once they’re internalized, depending on the nature of those nanoparticles, they can do a lot of intracellular damage.”

There’s growing evidence that zinc nanoparticles have a negative impact on marine life, and could harm humans if ingested.

This isn’t so much an issue for people applying mineral sunscreen cream, as nano-sized zinc and titanium are unlikely to be absorbed through the skin. But once sunscreen washes off it enters the water system. And there simply isn’t a way for wastewater treatment plants to filter all nanoparticles out. Once released into rivers and streams, they bioaccumulate in the fish we eat. So, they do end up inside our bodies, after all. How much ends up in our drinking water and the extent to which these concentrations are toxic to humans is still unclear, so researchers are calling for additional study.

Mineral sunscreens are the clear winner when it comes to environmental and human safety, but the key is to buy non-nano versions. Advertising on labels can be misleading. Many sunscreens claiming to be “reef safe” contain zinc nanoparticles that could harm marine life. Make sure to check ingredient lists and avoid products with “nano-titanium dioxide” and “nano-zinc dioxide.”

Mineral sunscreens are the clear winner when it comes to environmental and human safety, but the key is to buy non-nano versions.

You can also check the Consumer Products Inventory’s online database of sunscreens containing nanoparticles. The Environmental Working Group (EWG) — a non-profit dedicated to protecting human health and the environment — considers sunscreens that use non-nano zinc, such as ThinkSport, Goddess Garden, and Badger, to be safest. These are also referred to as biodegradable sunscreens. If you’re concerned about whiteness, look for products with “clear zinc.”

For recommendations of specific sunscreens that are good choices for the environment, visit Haereticus Environmental Lab’s website or check out EWG’s online Sunscreen Guide. When in doubt? Cover up. Clothing, hats, and sunglasses are remarkably effective at blocking UV rays.

Stay Away from Spray

One potentially dangerous trend in both chemical and physical sunscreens is the proliferation of sprays. It’s one thing to spread sunscreen on your skin. It’s another to inhale it, which EWG says is dramatically more likely once it’s aerated for easier dispersion.

Zinc and titanium nanoparticles in aerated mineral sunscreen can end up in our bloodstream, because our lungs aren’t able to clear them out on their own. “Insoluble nanoparticles that penetrate skin or lung tissue can cause extensive organ damage,” according to EWG, which points out that workers producing these sunscreens may be at particular risk.

Can’t I just DIY?

Many body care products are easy enough to replicate at home with the right ingredients, but sunscreen isn’t one of them. Misinformation about sun protection factor (SPF) ratings abounds on health and homesteading blogs. Amanda Foxon-Hill, a professional cosmetic chemist, blogged about her failed attempts at creating sunscreen with a high enough SPF. Despite her best efforts, her sunscreens had dismal SPF rankings when tested at a lab — like a rating of SPF 12 when she was expecting 30, and SPF 8 when she was expecting 35.

Plenty of people have claimed to crack the code of effective homemade sunscreen simply because they haven’t received any sunburns while using it.

Plenty of people have claimed to crack the code of effective homemade sunscreen simply because they haven’t received any sunburns while using it. But sunburns are only one side effect of unprotected sun exposure. It’s impossible to know the extent to which homemade sunscreen protects your skin from UVA rays, which don’t burn the skin, but cause skin to age and develop melanoma.

Ultimately, negotiating the world of environmentally and personally safe sunscreens is more complex than heading to the nearest drugstore and buying what’s on sale. Don’t get burned by false assurances, the Decider’s got your back.