Aerosol and bioaerosol generation from toilet flushing: A systematic review and risk factor analysis

Toilet flushing is a daily activity that can generate bioaerosols in indoor spaces. This exposure pathway is increasingly important in the context of emerging pathogens, changing water systems, and high indoor occupancy. Stronger empirical evidence is needed to support risk-based decision-making to protect human health. This systematic literature review examined factors influencing aerosol and bioaerosol concentrations generated through toilet flushing, with a focus on toilet design, target microorganisms, and air sampling methods relevant to exposure and risk assessment. Twenty-two studies met the inclusion criteria. Across studies, higher seeding concentrations and larger flush volumes were generally associated with higher aerosol and bioaerosol concentrations, indicating that source loading and flushing energy are the primary drivers of exposure-relevant aerosol generation. In contrast, evidence for independent effects of flushing pressure, lid position, and ventilation was limited and inconsistent. Probability density functions of log₁₀-transformed aerosol and bioaerosol concentrations were developed for seeded and unseeded conditions to strengthen quantitative microbial risk assessment (QMRA) inputs where direct air sampling is not feasible. Overall, the findings highlight the central role of toilet design and source microbial loading in shaping bioaerosol exposure, with important implications for risk assessment, building design, and infection control in an era of water security pressures and emerging infectious diseases.