The happy summer days are at the door, the sea is getting warmer and the sun is starting to heat us with all its strength. This time of the year, our skin, the biggest organ of our body, is more exposed than ever to ultraviolet (UV) light. In order to protect it, we are recommended to put some sunscreen on and we will be ready for the summer adventures to come. So the decision of which sunscreen to buy seems like an important factor to get that nice tan instead of getting sunburned and facing its harmful consequences.

Although this decision might appear straightforward and easy, it brings up many questions, like: which sunscreen should we buy, what SPF should it have, what bottle labels should we pay attention to, is it worth spending a lot of money on buying a good cream or is the most common and cheap one going to do its work? Truth is that nowadays, it is difficult to find the right sunscreen that will protect us and the ocean both at the same time.

Such personal care products (PCPs) are being detected mainly in the surface waters and have raised questions of their negative effects on marine life. As it turns out, since sunscreens contain chemical and physical filters that either convert light into heat or scatter and deflect it affecting marine organisms, they have been classified as contaminants of emerging concerns (CECs). Chemical filters are petroleum-based aromatic compounds, represented by benzophenone-3 (BP3) or else known as oxybenzone, ethylhexyl methoxycinnamate (EHMC) or octinoxate and ethylhexyl salicylate, to list a few. Physical filters, on the other hand, are nano-dimensional metal oxides, such as zinc oxide (ZnO) and titanium dioxide (TiO₂).

All these chemicals are being released into the water environment. Despite poor quantitative data, a French study estimated that 3000 beachgoers used approximately 52,5 kg of sunscreen per day, of which 15,7 kg were washed off. While the numbers might seem worry-free, it means that 15,7 kg of the abovementioned organic and inorganic compounds are interacting with seawater and organisms living in close proximity. Such concerns are not only related to the water environment but to human health, too. Even though the advancement of science and research is moving forward, knowledge of how these contaminants affect the human body and marine ecosystems is insufficient.

The interaction between ingredients found in some sunscreens and the water medium also plays a key role. At higher pH values, phenylbenzimidazole sulfonic acid (PBSA), for example, tends to predominate in a nonionized form which passes easily through cellular membranes affecting its processes. However, the threshold of PBSA negative impact, based on the pH value, is yet to be studied in detail.

If the organic filters have such effects, the inorganic filters, especially titanium dioxide, which is the most common nano-compound found in sustainable sunscreens, brings to the surface some controversy. For cosmetic purposes, this molecule is usually coated with alumina or silica to prevent TiO₂ from reacting under UV exposure. But such preventive layers quickly dissolve in the water medium, causing the production of reactive oxygen species (ROS), like hydroperoxide (H₂O₂). As observed in marine diatoms that belong to the phytoplankton, the increased level of hydroperoxide concentration causes a 4-fold decrease in the chlorophyll a concentration.

Fish change their behaviour, including how they feed, when exposed to oxybenzone. At 100 mg/L exposure, they move either slower or awkwardly faster after 48 hours, until they stop eating. As for humans, this same molecule disrupts the endocrine system, influencing the production of thyroid and reproductive hormones, which in turn affect kidney functioning and the immune system. But these abnormalities have been observed only in the short term.

We are far away from understanding the effects of sunscreens, their bioavailability and accumulation in the marine environment. The reasons to avoid massive application, however, are quite obvious. In turn, we can make a huge difference already by avoiding visits of the beach at peak UV radiation hours of the day and covering our skin with light colored clothes. It is easy, preventive and at the end, essential.