A quantitative microbial risk assessment (QMRA) approach to determine water quality guidelines for Mycobacterium avium complex (MAC) in building plumbing.

Species within the Mycobacterium avium complex (MAC), a subset of nontuberculous mycobacteria (NTM), are of major human health concern and contribute to a significant drinking water burden in the United States (US) and worldwide. The risk increases when MAC levels in environmental media are high, the hosts are susceptible, and exposures to aerosols containing MAC are frequent. MAC is typically not monitored in drinking water; currently, there are no numeric guidelines for NTM or MAC concentrations in distribution networks or building plumbing systems. To address this gap, the primary objective of this study was to identify risk-based critical MAC concentrations for identifying problematic conditions in building water. Critical concentrations were based on an annual risk benchmark of 1 in 10,000 commonly referenced in the US for drinking water-related infections using quantitative microbial risk assessment (QMRA). Critical concentrations of MAC were calculated for eight potential scenarios including inhalation and ingestion exposures for children, immune-compromised individuals, and the general population. Median critical concentrations corresponding to an annual infection risk of 10-4 are lower for the general population via the inhalation route (∼10-2 to 103 CFU/ L) compared to immunocompromised populations and children via the ingestion route (∼100 to ∼102 CFU/ L). Shower exposures were the main driver of the inhalation risk from building plumbing exposures. This study provides a risk framework for MAC exposure in the building plumbing environment for developing mitigation strategies.