Over the past few years, there's been trouble in paradise when it comes to sunscreen. From Palau to Hawaii, tourist hot spots have been cracking down on two UV filters — oxybenzone and octinoxate — to save their coral reefs.

Craig Downs, a leading ecotoxicologist and the executive director of the nonprofit Haereticus Environmental Laboratory in Virginia, has been a driving force behind these bans. He says a growing body of data shows how disruptive these chemicals are to marine ecosystems. 

At the same time, misinformation on social media is fueling concerns about how safe sunscreen is for human health. Dermatologists also worry this debate could undermine skin cancer prevention efforts. So is sunscreen really bad for the environment and human health?

How sunscreen impacts coral reefs  

There are two types of sunscreens: chemical, which absorbs harmful UV radiation, and mineral, which forms a physical barrier that reflects and scatters the sun's rays. 

The UV filters in chemical sunscreens protect human skill cells from DNA damage, but oxybenzone, the organic compound used in such sunscreens, disrupts hormone function in young corals.

Baby corals float "at the surface of the ocean, which is where the sunscreen is at," says Downs, whose landmark study in 2016 linked coral death to oxybenzone.

The UV filter causes baby corals "to encase themselves in their own skeletons, essentially creating a stone coffin of their own making." This prevents them from traveling back to the reefs, which stifles reproduction and turns colonies into coral zombies.

Does sunscreen impact the environment in other ways?

Coral reefs, which as storm buffers help make coastlines resilient, have also been found to start bleaching after two weeks' exposure to 30.5 degree Celsius (86 degree Fahrenheit) heat. Downs's research, however, has shown oxybenzone can even cause bleaching at as low as 27 degrees Celsius, though it is not yet clear why that is.  

And UV filters have been affecting more than just coral development.

Octocrylene, a sunscreen compound that breaks down into benzophenone, a chemical also found in herbicides, has been detected in dolphins off the coast of Brazil.

It has been found to alter the development of embryos from zebrafish and shrimp, potentially weakening and ultimately killing them off prematurely. It also raises the risk of entering the food chain and making its way to humans. 

Meanwhile, sunscreen swirling around in the "swash zone," the area where the waves crash onto the shore, can become airborne and be carried inland to sand dunes. Downs points to growing data on the toxicity of these chemicals to other forms of plant life. He suspects they're also destroying dune vegetation.

"So, if you're killing the plants on the sand dunes, this is what we see the world over, you're going to lose those sand dunes.  And next time the storm comes, you're going to lose the coastline," he explains.

If sunscreen is bad for the planet, is it safe for humans?

Sunscreen chemicals are regulated either as pharmaceuticals or cosmetics, depending on the country, but ethical testing on humans is a challenge.

Miko Yamada, a senior research fellow at the Skin Research Centre at the University of York in the UK, says thanks to gains in artificial intelligence, her team has been able to see what happens when sunscreen is reapplied to the same skin area over a longer period of time.  

Based on that research, she says chemical sunscreens generally don't go beyond the middle layer of skin and enter the blood vessels. But the longer sunscreen stays on the skin, the greater the likelihood it might be absorbed more deeply into the body. 

Yamada says the limit is around two hours, at which point the sunscreen should ideally be washed off and reapplied. 

Does wearing sunscreen in the city pose a threat?

Yes. Sunscreen ingredients end up in the wastewater, either through the shower drain or human urine, if it has entered the body. And treatment facilities generally can't completely remove UV filters.

These chemicals have been found to have entered Alpine lakes through wastewater. Recent studies in Spain and Brazil have also detected them in tap water, albeit at low levels.

Are there any sunscreens that are better for people and the planet?

Yes. Mineral or physical sunscreens containing zinc oxide and titanium dioxide without nanoparticles are considered safe for the environment. Incidentally, these are the only two sunscreen ingredients the US Food and Drug Administration lists as both safe and effective.

These are better known as the types of sunscreens that stay on the skin's surface and turn it white. Yamada admits they're gloopy but says they're the best bet for people concerned about chemicals being absorbed into their bodies.

However, even when using a mineral sunscreen, Down says it's important to read the back of the bottle carefully. Some sunscreens contain additional chemicals that can be harmful to the environment, like ethylhexyl methoxycrylene, a UV filter he says also breaks down into benzophenone.

Is it wise to keep using sunscreen?

Sun protection is more vital than ever in this era of record-breaking heat. Dermatologists like Judith Sirokay at University Hospital Bonn, Germany says skin cancer is on the rise. More cases are being documented thanks to greater availability of screenings, but the cases themselves go back to years of inadequate sun protection, especially among older men who worked outdoors. 

Untreated skin cancer can have devastating consequences for patients. "If you have a wound that does not heal in your face and it gets larger, or even the tumor, for example, in your face, it may destroy bones, the nose, the lips," says Sirokay. "It may even kill you."

Dermatologists recommend wearing a hat and clothing that covers as much skin as possible. For the areas that can't be covered, sunscreen is still recommended.

"So, if someone would ask me, 'Do you apply sunscreen?' I would say, yes, I do," Sirokay said, adding that "we don't have a real threshold" for when sunscreen becomes dangerous to humans.

While more research is needed to understand the impacts better, dermatologists advise avoiding UV rays at peak hours between late morning and late afternoon. UPF sunwear, highly effective sun-protective clothing made with tightly woven fabrics and special coatings, can also protect beachgoers.

"Wearing a UPF sun shirt reduces the amount of sunscreen a person will use at the beach by 50% more. That is a big, big deal," says Downs.

Edited by: Tamsin Walker

This article was adapted from a DW environment's Living Planet podcast, which can be heard here: