Re-Examining Cyanuric Acid: An Overlooked Non-Halogenated Cyclic Disinfection Byproduct in Swimming Pool Water

Cyanuric acid, as a by-product of chloroisocyanuric acids, is frequently detected in swimming pool water. However, its cytotoxicity, persistence, mobility, and bioaccumulation potential remain poorly understood. This study evaluates the cytotoxicity of cyanuric acid and investigates its occurrence and toxicological impact in water samples. Additionally, data on its biodegradation half-life, octanol-water partition coefficient (Kow), and bioconcentration factor (BCF) were collected. The average concentrations of cyanuric acid in 20 swimming pools were 26,600 µg/L, which were 3–5 orders of magnitude higher than other known organic disinfection byproducts (DBPs). The median lethal concentration (LC50) of cyanuric acid for Chinese hamster ovary (CHO) cells was 13.7 mM. Given its widespread occurrence and high concentration, cyanuric acid is likely a significant contributor to swimming pool cytotoxicity. The calculated cytotoxicity of cyanuric acid (15,054) exceeded that of all reported cyclic DBPs in swimming pools, including halonitrobenzoic acids (0.04), halonitrophenols (0.1), and halobenzoquinones (4.4). Furthermore, its calculated cytotoxicity was also much higher than most aliphatic DBPs. Overall, cyanuric acid contributed approximately 21.4% of pool water cytotoxicity, identifying it as an overlooked cyclic DBP. In terms of environmental persistence, cyanuric acid had a biodegradation half-life of 6.17 days, which is approximately comparable to the median level of aliphatic DBPs. The LogKow and BCF of cyanuric acid was −0.72 and 0.501 L/kg, suggesting a high mobility and minor bioaccumulation potential. Moreover, due to its extremely high concentration in swimming pool water (up to 79,000 µg/L), these values do not eliminate concerns about its environmental risks. Given that chloroisocyanuric acids have been identified as ideal oxidants in advanced oxidation processes (AOPs) for micropollutant degradation in water in recent years, this study may reveal the potential risk of cyanuric acid in swimming pool water and UV/chloroisocyanuric acid-treated water.