Quantitative microbial risk assessment of Legionella pneumophila in a drinking water supply system in Israel

Highlights: • Three years seasonal Legionella survey was performed in a drinking water system. • Quantitative Microbial Risk Assessment (QMRA) was calculated for Legionella. • The QMRA remained stable and did not vary significantly between the years. • Annual risk levels exceeded the acceptable thresholds regarding Legionella infection. • Risk levels varied significantly between seasons and peaked during summers. -- Abstract: Legionella pneumophila cause human infections via inhalation of contaminated water aerosols, resulting in severe pneumonia. Legionella spp. prevalence was monitored in a drinking-water distribution system (DWDS) in Northern Israel. Five points (toilet faucets and showers) were sampled seasonally along a three years period. Toilet faucets and shower use, both generating aerosols, are known transmission routes for this pathogen and thus, present a potential health risk. Quantitative Microbial Risk Assessment (QMRA) was applied in order to assess the health risks posed by Legionella for these two exposure scenarios, while considering Legionella seasonality. The obtained results were compared with estimated tolerable risk levels of infection and of disease set by the USEPA and WHO. Both limits were expressed as Disability-Adjusted Life Years index (DALY) being 1 × 10⁻⁴ and 1 × 10⁻⁶, respectively. The QMRA revealed that the annual risk levels for both faucets and showers use exceeded the acceptable risk of infection with an average of 5.52 × 10⁻⁴ and 2.37 × 10⁻³ DALY'S per person per year, respectively. Annual risk levels were stable with no significant differences between the three years. Risk levels varied significantly between seasons by up to three orders of magnitude. Risk levels were highest during summer, autumn, and lowest during winter. The highest seasonal infection risk values were found in summer for both faucets and showers, which corresponded to 8.09 × 10⁻⁴ and 2.75 × 10⁻³ DALY'S per person per year, respectively. In conclusion, during summer and autumn there is a significant increase of the infection risk associated with exposure to Legionella-contaminated aerosols, in the studied water system. Public health assessment and prevention measures should focus on these seasons.