Titanium dioxide (TiO2) is a common mineral ultraviolet (UV) filter used in many sunscreens as an alternative to chemical UV absorbers. Mineral-based sunscreens have gained popularity as concerns rise over the environmental impact of certain ingredients on fragile marine ecosystems, particularly coral reefs. Pollution from sunscreen ingredients is recognized as a contributing factor to the widespread decline and stress of coral reefs globally. Since TiO2 is one of the two primary mineral UV filters, determining if it is truly safe for reefs requires examining its physical and chemical properties.
Particle Size and Reef Safety
The environmental safety of titanium dioxide in sunscreen is determined primarily by the particle size of the ingredient. Particles larger than 100 nanometers (nm) are classified as non-nano and are generally considered safer for the marine environment.
Non-nano titanium dioxide particles are bulky and tend to aggregate in the water, meaning they do not easily dissolve or become bioavailable to marine organisms. Their larger size limits their ability to penetrate the cell walls of coral and aquatic life, allowing them to pass through the ecosystem with minimal interaction. These larger particles cause the visible white film when non-nano sunscreens are applied.
In contrast, nano-sized particles are smaller than the 100 nm threshold. Manufacturers reduce the particle size to make the sunscreen transparent and less greasy, improving its cosmetic feel. This smaller size increases the surface area and reactivity of the titanium dioxide, allowing the particles to interact more readily with the surrounding environment and potentially penetrate tissues.
How Titanium Dioxide Harms Coral
The harm caused by nano-sized titanium dioxide is linked to a specific photo-chemical reaction that occurs when the particles are exposed to sunlight in water. When nano-TiO2 interacts with UV radiation, it generates highly reactive oxygen species (ROS), which are unstable molecules that act as powerful oxidizing agents. This process, known as photocatalysis, leads to oxidative stress in the surrounding biological environment.
Oxidative stress is destructive to the symbiotic algae, called zooxanthellae, that live within coral tissue. These algae provide the coral with most of its nutrients and vibrant color. Damage to the zooxanthellae causes the coral to expel them, a process known as coral bleaching, which leaves the coral white and vulnerable to disease and death.
Unmodified nano-titanium dioxide has been shown to cause the expulsion of zooxanthellae in laboratory settings, indicating acute stress in coral. The nano-form of the particle facilitates this destructive chemical reaction, posing a significant threat to the reef’s health.
Choosing Truly Reef-Friendly Sunscreens
Consumers can choose sunscreens that explicitly use non-nano titanium dioxide to protect both their skin and the oceans. The term “reef safe” is not currently regulated by government bodies, meaning manufacturers can use it as a marketing term even if the product contains potentially harmful ingredients. Reading the full ingredient list and looking for specific language is necessary to make an informed choice.
To ensure a product is reef-friendly, look for labels that explicitly state “non-nano” or “micro-sized” next to the titanium dioxide ingredient. If a mineral sunscreen is advertised as applying completely clear and rubbing in effortlessly, it is highly likely to contain nano-sized particles. This is because the larger, safer non-nano particles cause the visible white residue.
Specific regulations, such as those in Hawaii, target common chemical UV filters like oxybenzone and octinoxate. For consumers seeking true environmental safety, the particle size of mineral filters remains the primary concern.