The popularity of mineral UV filter-based sunscreens is increasing because they offer protection across a broad UV spectrum. And there is a belief among the general public that sunscreens with mineral UV filters are less harmful to coral reefs than sunscreens that contain chemical UV filters.

But is this really true?

The Science

Common chemical UV filters used in sunscreens are organic compounds such as oxybenzone (BP-3), octocrylene (OC), octinoxate or ethylhexyl methoxycinnamate (EHMC), which protect the skin by absorbing harmful UV rays. Mineral UV filters, on the other hand, are made up of inorganic compounds such as zinc oxide and titanium dioxide, which work primarily by scattering and absorbing UV rays. As zinc oxide (ZnO) and titanium dioxide (TiO₂) do not absorb readily into the skin and tend to leave a white trace behind, they are often broken down into nanoparticles (smaller than 100 nm) to reduce the white trace.

When released into the marine environment, these both organic and inorganic compounds interact with marine biota either through the water body or through the sediment and which can affect the whole ecosystem. The impact of these compounds in the marine environment has increasingly come under scrutiny in the last few years, as we have become more aware of the dangers surrounding the world’s coral reefs ecosystems. Given the high productivity and value of coral reef systems, there has been a significant increase in coral reef research, which has allowed us to identify the main threats to the ecosystem. However, the threat of sunscreens to coral reefs has only recently been confirmed, and there is still a lot of research to be done in this area.

The Findings So Far

A number of laboratory experiments have been conducted in which coral species are exposed to varying concentrations of organic and inorganic UV filters, similar to levels found within the natural environment, and the health of the coral is then assessed using a number of biological indicators, such as color change in the corals due to bleaching, the concentration of dinoflagellates and the maximal photosynthetic efficiency. Coral bleaching is obviously very harmful to the coral, as they lose their little symbionts and the nutrients they provide, and photosynthetic efficiency is indicative of photosystem stress, which is a precursor to coral bleaching.

The effects of organic UV filters on corals are more widely studied than that of inorganic filters. Organic UV filters, such as oxybenzone, have been shown to cause bleaching, increased viral infection, and ultimately accelerated death in corals. Other organic filters, such as ethylhexyl methoxycinnamate, octinoxate or other benzophenones, have also been linked to coral bleaching and increased mortality.

Whereas studies in the Acropora coral family have shown that inorganic uncoated ZnO nanoparticles induces rapid and severe coral bleaching due to the alteration of the symbiosis between coral and their zooxanthellae, but two modified forms of TiO₂ (Eusolex®-T2000 and Optisol™) caused minimal impact on corals and their symbionts suggesting that they are more eco-compatible than zinc oxide and possibly, other chemical UV filters. Interestingly, Optisol™ does not have an immediate effect, and the corals are only affected after about 24-48 hours after exposure. While the modified titanium dioxide compounds have performed well in this study, they have also been shown to be toxic and bio-accumulate in marine life – though these may be the unmodified compounds only.

In another study which looked at the effect of UV filters on photosystem two (PSII), ZnO also performed badly, and the maximum photosynthetic efficiency of the symbionts of the scleractinian coral, Stylophora pistillia, was reduced by 38% compared to the controls, after an exposure level of 90 mg/L for 35 days. Other UV filters tested in this study were organic filters such as terephthalylidene dicamphor sulfonic acid, drometrizole trisiloxane, ethylhexyltriazone, butylmethoxydibenzoylmethane and 2-ethylhexyl 2-cyano-3,3-diphenylacrylate, none of which showed an adverse effect on the coral or their symbionts at the concentrations to which they were exposed.

All in all, it’s clear that there is still a lot of research that needs to be done on the effects of UV filters from sunscreens on coral reefs, and which ones truly are reef-safe. It will also be important for researchers to investigate the effects of these UV filters in situ or in the environment, as there are many other factors that may influence the effects of these compounds on the local wildlife, such as rising sea water temperatures and ocean acidification.

While it looks like there is no perfect solution to the sunscreen debate, some sunscreens are definitely better than others, and it’s worthwhile doing some research and trying to find the best one that’s readily available to you. Just bear in mind that many cosmetic companies are there to make money, and you should always double check the ingredient list on the back of the product, or Google it if it’s not there, of any sunscreens labeled as “reef-safe”, since there is still a lot of research that needs to be done. However, there are still things that you can do to help protect the reef from sunscreen associated toxins:

What You Can Do To Limit The Impact Of Sunscreens On Corals?

1.     If possible, try avoiding direct sun exposure, especially during the midday hours, as the sun is strongest during these hours. If you are going to go for a snorkel, maybe plan to go in the early morning

2.     Use alternative forms of sun protection – such as clothing with a UV protection factor, rash vests, wetsuits, hats and beach umbrellas if you are going to be in the sun for a long time. This will limit the area of skin exposed to the sun, thereby reducing the quantity of sunscreen you need to use.

3.     When purchasing a sunscreen, avoid products that include organic filters and mineral UV filters that contain nanoparticles.

4.     Give the sunscreen time to dry on your skin before hitting the waves, as this will give it time to absorb and reduce the volume released into the ocean.

5.     Stay informed of the regulations and recommendations regarding sunscreen use in the area that you are going to visit.