Legionella spp.
is a key contributor to the United
States waterborne disease burden. Despite potentially widespread exposure,
human disease is relatively uncommon, except under circumstances where
pathogen concentrations are high, host immunity is low, or exposure
to small-diameter aerosols occurs. Water quality guidance values for Legionella are available for building managers but are generally
not based on technical criteria. To address this gap, a quantitative
microbial risk assessment (QMRA) was conducted using target risk values
in order to calculate corresponding critical concentrations on a per-fixture
and aggregate (multiple fixture exposure) basis. Showers were the
driving indoor exposure risk compared to sinks and toilets. Critical
concentrations depended on the dose response model (infection vs clinical
severity infection, CSI), risk target used (infection risk vs disability
adjusted life years [DALY] on a per-exposure or annual basis), and
fixture type (conventional vs water efficient or “green”).
Median critical concentrations based on exposure to a combination
of toilet, faucet, and shower aerosols ranged from ∼10–2 to ∼100 CFU per L and ∼101 to ∼103 CFU per L for infection and CSI
dose response models, respectively. As infection model results for
critical L. pneumophila concentrations were often
below a feasible detection limit for culture-based assays, the use
of CSI model results for nonhealthcare water systems with a 10–6 DALY pppy target (the more conservative target) would
result in an estimate of 12.3 CFU per L (arithmetic mean of samples
across multiple fixtures and/or over time). Single sample critical
concentrations with a per-exposure-corrected DALY target at each conventional
fixture would be 1.06 × 103 CFU per L (faucets), 8.84
× 103 CFU per L (toilets), and 14.4 CFU per L (showers).
Using a 10−4 annual infection risk target would
give a 1.20 × 103 CFU per L mean for multiple fixtures
and single sample critical concentrations of 1.02 × 105, 8.59 × 105, and 1.40 × 103 CFU
per L for faucets, toilets, and showers, respectively. Annual infection
risk-based target estimates are in line with most current guidance
documents of less than 1000 CFU per L, while DALY-based guidance suggests
lower critical concentrations might be warranted in some cases. Furthermore,
approximately <10 CFU per mL L. pneumophila may
be appropriate for healthcare or susceptible population settings.
This analysis underscores the importance of the choice of risk target
as well as sampling program considerations when choosing the most
appropriate critical concentration for use in public health guidance.