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.

Even extremely low dosages of ultraviolet light can be highly effective for inactivating Cryptosporidium oocysts.Recent studies have shown that Cryptosporidium parvum oocysts demonstrate high susceptibility to low dosages of medium‐pressure ultraviolet (UV) light. These investigations have raised several questions, which include determination of minimum medium‐pressure UV dosages necessary to inactivate C. parvum oocysts, elucidation of differences (if any) between medium‐ and low‐pressure UV light for inactivating C. parvum oocysts, and evaluation of medium‐pressure UV effectiveness in inactivating oocysts suspended in poorer quality water. To compare low‐ and medium‐pressure UV, the authors exposed oocysts suspended in deionized water to UV delivered by either medium‐ or low‐pressure UV lamps at bench scale using a collimated beam apparatus. The applied UV dosages ranged from 3 to 33 mJ/cm2, and oocyst inactivation was assessed using the neonatal mouse infectivity assay. At 3 mJ/cm2, medium‐pressure UV showed a 3.4‐log inactivation of oocysts, and low‐pressure UV showed a 3.0‐log inactivation, demonstrating medium‐ and low‐pressure UV did not differ significantly in inactivating C. parvum oocysts.

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In vivo studies indicate the infectivity of treated Cryptosporidium parvum oocysts more reliably than do in vitro assays. Inactivation of Cryptosporidium parvum oocysts in finished drinking water by medium‐pressure ultraviolet (UV) light was investigated at bench scale using a collimated beam apparatus and at demonstration scale using a UV reactor. Oocyst viability was assessed in vitro (using 4',6‐diamidino‐2‐phenylindole with propidium iodide and maximized in vitro excystation) and in vivo (using neonatal mouse infectivity assays). In vivo bench‐scale studies showed > 4‐log inactivation at UV dosages as low as 41 mJ cm–2, although in vitro surrogate assays showed little or no inactivation at this or higher UV dosages. The in vitro assays, which indicate oocyst viability, grossly overestimated the UV dosages required to prevent oocyst infection in susceptible hosts. Results of demonstration studies, carried out under the Environmental Technology Verification program of the National Sanitation Foundation and the US Environmental Protection Agency, agreed with the bench‐scale results and showed that a UV dosage as low as 19 mJ cm–2 provided 3.9‐log inactivation of Cryptosporidium oocysts.

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