4-MBC in Australian sunscreens as an unmonitored discharge pathway into Great Barrier Reef coastal waters

Abstract 4-Methylbenzylidene camphor (4-MBC, also known as enzacamene), a chemical UV filter prohibited or never permitted across the EU, UK, China, ASEAN, US, and Japan, remains legal in Australian therapeutic sunscreens at concentrations up to 4% w/w. The European Scientific Committee on Consumer Safety concluded in 2022 that no safe concentration could be established, and the UK Scientific Advisory Group on Chemical Safety confirmed this in 2025. In January 2026 the Australian Therapeutic Goods Administration also concluded that a safe dose cannot be established, yet declined to remove the compound from the Permissible Ingredients Determination, citing supply-shortage grounds. This article examines a narrower question: whether 4-MBC is entering Great Barrier Reef catchment coastal waters through a direct, unmonitored wastewater pathway, and whether the concentrations reaching reef-adjacent waters are sufficient to compound bleaching in already-stressed coral systems. Two Queensland focus zones and one downstream transport corridor are examined where documented bleaching overlaps with confirmed wastewater treatment plant discharge points: the inshore reefs north of Cairns, Magnetic Island and Cleveland Bay, and the transport corridor from Townsville to the Palm Islands. Peer-reviewed evidence establishes 4-MBC as a coral toxicant in single-compound laboratory exposure, though at concentrations well above those measured in coastal waters. A twelve-compound UV filter mixture containing 4-MBC at 200 ng/L per compound has been shown to amplify thermal bleaching in reef corals. Whether 4-MBC occurs in Queensland reef-adjacent waters at concentrations relevant to either finding is unknown, because the discharge pathway has never been monitored for this class of contaminant. The case for action does not rest on causation of the 2024 bleaching event. It rests on the continuous discharge of a coral toxicant, banned by Australia’s regulatory peers, through a pathway that has never been measured. This Perspective specifies the monitoring that would resolve the scientific question and identifies the existing Australian regulatory instrument through which alignment with the international scientific consensus could be achieved.