Impacts of dynamic aerosol and pathogen concentrations on risks of Legionella pneumophila for public showers in Switzerland based on a quantitative microbial risk assessment framework

L. pneumophila is a waterborne respiratory pathogen that causes Pontiac Fever and Legionnaires' disease, two clinically significant diseases with increasing incidence in Europe. In this study, we develop a Quantitative Microbial Risk Assessment (QMRA) framework on the risks of infection from showering in L. pneumophila-contaminated water supplies to inform health-based concentration targets and water quality monitoring programs. The developed QMRA model extends on previous work investigating the relationships between concentrations of L. pneumophila in water sources and infection, illness, and disease burden by incorporating dynamic pathogen concentrations in water and aerosol concentrations, extending the prior reliance on assumptions of constant, average concentrations over the exposure duration. When applying this approach to data collected from within a building in Switzerland at risk for legionellosis cases, we show that initial high concentrations of L. pneumophila in water and aerosols from hot showers contribute to risks above a commonly used benchmark for the acceptable infection risk (10^-4 infections per person per year) within the first 1-2 min of showers. Extending the model to estimate critical concentrations of L. pneumophila suggests concentrations at or above 2.5 × 10³ CFU/L to 1.6 × 10⁶ CFU/L for first draw samples and 2.5 × 10¹ CFU/L to 1.0 × 10³ CFU/L for samples obtained after flushing would increase infection risks above the benchmark, dependent on site-specific conditions including water temperature and shower head type. These critical values align with, but are less stringent than, values reported by previous studies for showers due to our consideration of dynamic aerosol concentrations. Sensitivity analysis suggests that controlling L. pneumophila concentrations in water is the most effective risk mitigation strategy. Ventilation to reduce risks is dependent on shower conditions but may be less effective. The QMRA model finds that consideration of dynamic L. pneumophila concentrations in water improves exposure estimates and therefore improve the risk assessment, informing the benefits of sampling strategies that assess both first draw and flush samples in routine water monitoring programs.