Quantitative microbial risk assessment of inhalation exposure in reclaimed water reuse systems: scenarios, models, and applications

ABSTRACT Graphical abstract summarizing quantitative microbial risk assessment of inhalation exposure in reclaimed water reuse systems. The left panel lists seven aerosol-generating scenarios: landscape fountains, greenfield irrigation, road spraying, toilet flushing, cooling towers, car washing, and agricultural irrigation. The middle panel presents four inhalation exposure modelling approaches, representative bacterial, viral, and protozoan pathogens, and occupational and non-occupational population activity patterns. The right panel shows the QMRA framework from hazard identification and exposure assessment to dose–response modelling, risk characterization, and risk management, with outputs including infection probability and disability-adjusted life years. The bottom panel highlights challenges: incomplete pathogen databases, parameter uncertainty, and the need for multidisciplinary collaboration. Reclaimed water reuse is an important strategy for addressing global water scarcity; however, microbial risks from aerosol inhalation in non-potable applications remain insufficiently synthesized. This review critically assesses inhalation exposure pathways in reclaimed water reuse systems (RWRS), focusing on seven aerosol-generating scenarios: landscape fountains, greenfield irrigation, road spraying, toilet flushing, cooling towers, car washing, and agricultural irrigation. It compares four inhalation exposure models – the Gaussian plume model, volume estimation method, partition coefficient method, and computational fluid dynamics – by their assumptions, data requirements, applicability, strengths, limitations, and uncertainties. Evidence from rainwater, greywater, stormwater, wastewater, and related aerosol environments is included only when it provides transferable methodological insights and is distinguished from direct RWRS evidence. Studies show that pathogens commonly detected in RWRS, including Escherichia coli, Legionella, norovirus, adenovirus, Cryptosporidium, and Giardia, may cause gastrointestinal, respiratory, and systemic infections via aerosol inhalation. QMRA provides a framework for evaluating hazard, exposure, dose–response, and risk management across scenarios. Major uncertainties remain, including limited pathogen occurrence and viability data, incomplete water-to-air transfer parameters, incompatibility between molecular detection and infectious dose metrics, and lack of inhalation-specific health benchmarks for non-potable reuse. The review identifies key gaps and research priorities to improve risk assessment and management.