Overlooked risks of non-oxidizing antimicrobials (NOAMs) in water environments

Non-oxidizing antimicrobials (NOAMs) provide long-lasting microbial control without reacting with other components or causing equipment corrosion, yet their environmental and health risks are often overlooked. Common NOAMs include quaternary ammonium compounds (QACs), aldehydes, isothiazolinones, azoles, and biguanides. They are widely used in households, healthcare, industry, water treatment, and agriculture, entering the environment through wastewater, hospital/industrial sources, and urban/agricultural runoff. NOAM concentrations can reach mg/L in hospital/industrial wastewater and reverse osmosis (RO) concentrate. NOAMs have been detected globally in surface waters, sediments, and sewage sludge, with median concentrations of 0.01–0.1 μg/L, 3.2–12 μg/kg, and 5–7562 μg/kg, respectively. Risks associated with NOAMs include increased antibiotic resistance, ecotoxicity to aquatic organisms, and potential health hazards. Exposure to sub-inhibitory concentrations of QACs, cetrimide, or chlorhexidine can enhance resistance to other NOAMs and antibiotics by 1.3 to over 100 times. NOAMs exhibit comparable or higher ecotoxicity to luminescent bacteria, algae, daphnids, and fish compared to personal care products (PPCPs) and disinfection by-products (DBPs). NOAMs like QACs, isothiazolinones, and carbendazim can cause skin allergies, liver inflammation, fibrosis, or neuronal damage via multiple exposure routes. Most NOAMs require several weeks or more for complete biodegradation. NOAMs and PPCPs show similar biodegradability, both being less biodegradable than DBPs. Ozone reacts with QACs, carbendazim, and chloromethylisothiazolinone (CMIT) at rates below 10 M⁻¹s⁻¹. Hydroxyl radicals react rapidly with NOAMs (>10⁹ M⁻¹s⁻¹), while sulfate radical reactions with NOAMs are poorly understood. Future research requires expanded environmental monitoring, multi-endpoint toxicity assessments, resistance mechanisms under high NOAM pressure, and advanced disposal strategies.