Characterising the Emission and Dispersion of Aerosols from the Toilet Plume and Quantification of the Associated Infection Risk

Toilet flushing is known to generate aerosol plumes that can contain pathogenic microorganisms, and shared toilet facilities have been implicated in the transmission of infectious diseases. However, few studies have quantified the infection risks posed by these aerosols or investigated in detail how they disperse after flushing.

This thesis presents an integrated analysis of airborne infection risks associated with toilet plume aerosols, combining controlled experimental studies, quantitative microbial risk assessment (QMRA), and computational fluid dynamics (CFD) simulations. The aim is to investigate aerosol generation, dispersion, and mitigation strategies in shared toilet settings.

Controlled chamber experiments were conducted using a gravity-flow, close-coupled toilet, with and without a cubicle enclosure over a range of ventilation rates. The generation of aerosols was characterised by flushing a salt solution, revealing a large proportion of smaller particles (< 5 μm). Particle concentrations peaked in the first 1 min after flushing and returned to background levels within 10 min. Bioaerosol sampling using Escherichia coli showed that most bacteria were released in the first 5 min after flushing, with ventilation rates having a modest influence on airborne concentrations. Continuous but low-level bacterial deposition suggested cumulative risks from surface contamination.

A QMRA framework was developed to assess the infection risks associated with single flushing events, accounting for transient aerosol dynamics and realistic occupancy durations. Two models were developed: one combining particle concentrations with estimated viral loads, and another using measured bioaerosol data. When applied to the experimental results, these models suggested non-negligible infection risks, particularly for pathogens present at high faecal concentrations, such as norovirus. The introduction of short delays (> 1 min) between toilet users significantly reduced the estimated infection risks.

CFD simulations further examined aerosol dispersion and exposure risks, demonstrating how room layout, ventilation rate, and outlet positioning influence aerosol removal. Optimised ventilation strategies, including increased airflow and repositioned outlets, substantially reduced airborne particle concentrations.

This research provides new insights and robust methods to evaluate and mitigate airborne infection risks from toilet plume aerosols, with implications for public health guidance and the design of safer shared toilet environments.