Quantitative Microbial Risk Assessment Model for Legionnaires' Disease: Assessment of Human Exposures for Selected Spa Outbreaks

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Abstract
Evaluation of a quantitative microbial risk assessment (QMRA) model for Legionnaires' disease (LD) required Legionella exposure estimates for several well-documented LD outbreaks. Reports for a whirlpool spa and two natural spring spa outbreaks provided data for the exposure assessment, as well as rates of infection and mortality. Exposure estimates for the whirlpool spa outbreak employed aerosol generation, water composition, exposure duration data, and building ventilation parameters with a two-zone model. Estimates for the natural hot springs outbreaks used bacterial water to air partitioning coefficients and exposure duration information. The air concentration and dose calculations used input parameter distributions with Monte Carlo simulations to estimate exposures as probability distributions. The assessment considered two sets of assumptions about the transfer of Legionella from the water phase to the aerosol emitted from the whirlpool spa. The estimated air concentration near the whirlpool spa was 5 to 18 colony forming units per cubic meter (CFU/m3) and 50 to 180 CFU/m3 for each of the alternate assumptions. The estimated 95th percentile ranges of Legionella dose for workers within 15 m of the whirlpool spa were 0.13–3.4 CFU and 1.3–34.5 CFU, respectively. The modeling for hot springs Spas 1 and 2 resulted in estimated arithmetic mean air concentrations of 360 and 17 CFU/m3, respectively, and 95 percentile ranges for Legionella dose of 28 to 67 CFU and 1.1 to 3.7 CFU, respectively. The Legionella air concentration estimates fall in the range of limited reports on air concentrations of Legionella (0.33 to 190 CFU/m3) near showers, aerated faucets, and baths during filling with Legionella-contaminated water. These measurements may provide some indication that the estimates are of a reasonable magnitude, but they do not clarify the exposure estimates accuracy, since they were not obtained during LD outbreaks. Further research to improve the data used for the Legionella exposure assessment would strengthen the results. Several of the primary additional data needs include improved data for bacterial water to air partitioning coefficients, better accounting of time-activity-distance patterns and exposure potential in outbreak reports, and data for Legionella-containing aerosol viability decay instead of loss of capability for growth in culture.