Decision Support Tool to Assess Legionella and Mycobacterium avium Complex Risks from Common Indoor Water Uses

Opportunistic pathogens such as Legionella spp. (including Legionella pneumophila) and Mycobacterium avium complex (MAC) are common within building plumbing systems. These opportunistic pathogens can be amplified by conditions of premise plumbing, including diminished disinfectant residual, high surface area to volume ratios resulting from smaller-diameter pipes, and elevated water temperatures compared to distribution mains. Understanding the risk of infection from these pathogens through common exposure pathways is critical to addressing and mitigating human health risks. This study presents a quantitative microbial risk assessment (QMRA) framework for inhalation exposures to aerosolized Legionella spp. and MAC during three common residential water uses, showering, toilet flushing, and handwashing. Inhalation risks were quantified by Monte Carlo simulation of bulk water pathogen concentrations, size-specific aerosol generation, respiratory deposition, and dose–response assessment. Because co-located paired measurements for Legionella spp. and MAC were limited, organism-specific results reflect modeled concentration distributions and dose–response assumptions rather than direct field co-occurrence comparisons. Across modeled scenarios, Legionella spp. presents a higher risk of infection than MAC. Risks were greatest for shower exposures and lowest for handwashing. Correlation analysis shows that risks are most sensitive to exposure dose (ρ > 0.99) and the concentration of pathogen in bulk water (ρ > 0.96).