Understanding Sunscreens in Aquatic Environments

OVERVIEW

Exposure to the sun’s rays can harm human skin by causing sunburn, photo-aging, and increasing the risk of developing skin cancer. Sunscreen is one critical tool to help mitigate these harms.

But when people wear sunscreens, some of that sunscreen can end up in the water. As a result of their use in sunscreens and other consumer products, these ingredients have been detected in the water, sediments, and living things along coasts, in rivers, and in lakes.

Although the presence of sunscreen chemicals in the environment doesn’t necessarily indicate that they are causing harm, it has led to a rapid increase in research on their potential environmental impacts, and to a ban on certain sunscreen ingredients in Hawaii, the U.S. Virgin Islands, and several other locations.

But the science on sunscreens is still evolving. Environmental exposure and hazard data on sunscreen ingredients are limited, and there is not widespread agreement about whether the available research sufficiently supports conclusions that individual ingredients have, or do not have, negative effects on aquatic organisms.

How can scientists and regulators understand potential risks to aquatic environments to allow consumers to make informed choices about using sunscreen to protect their health while also protecting the health of the environment?

What is in sunscreen?

The active ingredients in sunscreens are ultraviolet (UV) filters, which are compounds that protect the skin by absorbing, reflecting, and/or scattering the sun’s ultraviolet rays.

Seventeen different UV filters can be used in products sold in the United States, although a handful are not currently in use. They are usually found in combination so that your sunscreen can protect against the full ultraviolet spectrum.

UV filters are not all the same. Two are inorganic particles (also referred to as “physical” or “mineral” filters) : titanium dioxide and zinc oxide. The remainder are organic (carbon-based) chemicals. The UV filters display a wide range of physical and chemical properties, such as their solubility, their stability under UV radiation, and their ability to biodegrade. These properties influence their variable fates in the environment and toxicity to aquatic organisms.

Key Sunscreen Terms
Sunscreen:
Topically applied products indicated to help prevent sunburn; some are also indicated to decrease the risk of skin cancer and early skin aging caused by exposure to the sun's UV radiation (when used as directed with other sun protection measures).
Ultraviolet (UV) Radiation:
The spectrum of electromagnetic radiation consisting of UVA (315-400nm), UVB (280-315nm), and UVC (100-280nm). Solar radiation is the most common source of UV light and the Earth’s atmosphere filters all UVC and some UVB radiation. UVA and UVB can cause damage to the skin.
UV filter:
A compound that impedes the passage of specific ranges of ultraviolet radiation, either through absorption, reflection, or scattering; the active ingredient in sunscreen. There are currently 16 UV filters allowed by the U.S. Food and Drug Administration for use in any sunscreen sold in the United States; plus an additional, proprietary UV filter, ecamsule, approved for use in limited products. Most sunscreens contain a mixture of UV filters to provide broad spectrum protection.
Organic UV filters:
Compounds that absorb UV radiation. Also referred to as chemical UV filters.
Inorganic UV filters:
Metal oxide particulates that absorb and also reflect or scatter UV radiation. Also referred to as physical or mineral UV filters.
Sunscreen Formulation:
The mixture of ingredients used in a sunscreen product including the inactive ingredients and one or more active ingredients (UV filters).

Using Ecological Risk Assessment to Understand Sunscreen Impacts in Aquatic Environments

Sunscreen active ingredients have been detected in water bodies, aquatic sediment, and aquatic organisms. The physical characteristics of the water body into which sunscreens are released is one influence on the amount of UV filters in a given area and therefore how they impact aquatic life in that water. The amount of water exchange is one factor as shown in the figure and images below.

There is a pattern emerging to their occurrence, where they are more likely to be found near recreational areas in water bodies with minimal water exchange, such as coastal bays or coves with sheltered beaches. However, there is more to learn about the spatial and temporal variability of their occurrence, and some UV filters have been monitored very little.

Physical Characteristics of Water Bodies Can Affect Accumulation of UV Filters
Physical Characteristics of Water Bodies Can Affect Accumulation of UV Filters

UV filter contaminants are more likely to be found near recreational areas in water bodies with minimal water exchange, such as the coral reef system in Hanauma Bay in Hawaii (left) where the beach is about 2,000 feet long. Contaminants are less likely to accumulate in the barrier reef system in the Florida Keys (right), which extends about 100 miles into the open ocean. SOURCE: Eric Tessmer via Flickr, (CC BY 2.0) (left), Jeff Schmaltz, MODIS Rapid Response Team, NASA/GSFC (right)

Laboratory observations show that, in high enough concentrations, some UV filters can be toxic to algal, invertebrate, and fish species. Impacts to corals have been measured and are of particular concern due to their sensitivity to stress, their high value to humans from provision of food to cultural traditions, and their proximity to recreational activity. However, it is challenging to study corals, in part due to their unique biological characteristics, such as the relationship between the coral host and their symbiotic algae and microbiome. Ideally, information would be available across a range of species and life stages for all UV filters to identify thresholds of concern for the most sensitive organisms.

The challenge for understanding the effects of sunscreens on aquatic environments is determining whether and under what conditions individual or mixtures of UV filters are a risk to organisms and ecosystems—and how UV filters may act either alone, or in combination with other environmental stressors. Then, scientists need to understand where these particular conditions might occur.

An ecological risk assessment (ERA) is a process to identify particular exposure settings in which a stressor—in this case, UV filters—could be the cause of environmental concern. To carry out the ERA, scientists will integrate information about exposures in the environment with information about adverse affects. The more information that is available, the more robust the ERA will be, and thus continuing to conduct research in this area is critical.

What Is Still Left To Learn: Understanding Sunscreen’s Real World Effects

Most of the research carried out to date has taken place in laboratories, but the characteristics of aquatic environments will undoubtedly influence the fate and effect sunscreen ingredients have on the environment. This makes it challenging to generalize about UV filters and their potential risks.

For example:

  • Some toxicity studies exposed organisms to levels of UV filters that are higher than have been found in aquatic environments to date or for durations that may or may not occur in the environment.
  • The environmental characteristics of the receiving waters—such as salinity, the physical mixing of the water, and light intensity—can influence whether UV filters persist in the environment, disperse, or break down.
  • Cumulative and interacting effects are possible from combinations of multiple UV filters, and from UV filters together with other environmental stressors including climate change and other contaminants.
  • Studies on the effects on ecosystem processes are largely absent; instead, ecosystem effects are mostly presumed based on effects on the species involved in key community and ecosystem functions.

The existing bans on certain UV filters may be considered precautionary in principle, in that they protect the environment against a potential threat now, rather than wait for more data. This approach has raised questions, though, about the potential human health implications resulting from reduced availability of some widely-used UV filter ingredients.

Balancing Environmental Health and Human Health