Quantitative microbial risk assessment of Legionella pneumophila in a drinking water supply system in Israel

Occurrence of pathogenic Mycobacteria avium and Pseudomonas aeruginosa in outdoor decorative fountain water and the associated microbial community

Outdoor decorative fountains usually attract residents to visit. However, opportunistic pathogens (OPs) can proliferate and grow in the stagnant fountain water, posing potential health risks to visitors due to the inhalation of spaying aerosols. In this study, the abundance of selected OPs and associated microbial communities in three large outdoor decorative fountain waters were investigated using quantitative PCR and 16S rRNA sequencing. The results indicated that Mycobacteria avium and Pseudomonas aeruginosa were consistently detected in all decorative fountain waters throughout the year. Redundancy analysis showed that OPs abundance was negatively correlated with water temperature but positively correlated with nutrient concentrations. The gene copy numbers of M. avium varied between 2.4 and 3.9 log10 (gene copies/mL), which were significantly lower than P. aeruginosa by several orders of magnitude, reaching 6.5-7.1 log10 (gene copies/mL) during winter. The analysis of taxonomic composition and prediction of functional potential also revealed pathogenic microorganisms and infectious disease metabolic pathways associated with microbial communities in different decorative fountain waters. This study provided a deeper understanding of the pathogenic conditions of the outdoor decorative fountain water, and future works should focus on accurately assessing the health risks posed by OPs in aerosols.

Applications of Quantitative Microbial Risk Assessment to Respiratory Pathogens and Implications for Uptake in Policy: A State-of-the-Science Review

BACKGROUND

Respiratory tract infections are major contributors to the global disease burden. Quantitative microbial risk assessment (QMRA) holds potential as a rapidly deployable framework to understand respiratory pathogen transmission and inform policy on infection control.

OBJECTIVES

The goal of this paper was to evaluate, motivate, and inform further development of the use of QMRA as a rapid tool to understand the transmission of respiratory pathogens and improve the evidence base for infection control policies.

METHODS

We conducted a literature review to identify peer-reviewed studies of complete QMRA frameworks on aerosol inhalation or contact transmission of respiratory pathogens. From each of the identified studies, we extracted and summarized information on the applied exposure model approaches, dose-response models, and parameter values, including risk characterization. Finally, we reviewed linkages between model outcomes and policy.

RESULTS

We identified 93 studies conducted in 16 different countries with complete QMRA frameworks for diverse respiratory pathogens, including SARS-CoV-2, Legionella spp., Staphylococcus aureus, influenza, and Bacillus anthracis. Six distinct exposure models were identified across diverse and complex transmission pathways. In 57 studies, exposure model frameworks were informed by their ability to model the efficacy of potential interventions. Among interventions, masking, ventilation, social distancing, and other environmental source controls were commonly assessed. Pathogen concentration, aerosol concentration, and partitioning coefficient were influential exposure parameters as identified by sensitivity analysis. Most (84%, n = 78 ) studies presented policy-relevant content including a) determining disease burden to call for policy intervention, b) determining risk-based threshold values for regulations, c) informing intervention and control strategies, and d) making recommendations and suggestions for QMRA application in policy.

CONCLUSIONS

We identified needs to further the development of QMRA frameworks for respiratory pathogens that prioritize appropriate aerosol exposure modeling approaches, consider trade-offs between model validity and complexity, and incorporate research that strengthens confidence in QMRA results. https://doi.org/10.1289/EHP12695.

Quantitative Microbial Risk Assessment Framework Incorporating Water Ages with Legionella pneumophila Growth Rates

Water age in drinking water systems is often used as a proxy for water quality but is rarely used as a direct input in assessing microbial risk. This study directly linked water ages in a premise plumbing system to concentrations of Legionella pneumophila via a growth model. In turn, the L. pneumophila concentrations were used for a quantitative microbial risk assessment to calculate the associated probabilities of infection (Pinf) and clinically severe illness (Pcsi) due to showering. Risk reductions achieved by purging devices, which reduce water age, were also quantified. The median annual Pinf exceeded the commonly used 1 in 10,000 (10-4) risk benchmark in all scenarios, but the median annual Pcsi was always 1-3 orders of magnitude below 10-4. The median annual Pcsi was lower in homes with two occupants (4.7 × 10-7) than with one occupant (7.5 × 10-7) due to more frequent use of water fixtures, which reduced water ages. The median annual Pcsi for homes with one occupant was reduced by 39-43% with scheduled purging 1-2 times per day. Smart purging devices, which purge only after a certain period of nonuse, maintained these lower annual Pcsi values while reducing additional water consumption by 45-62%.

Legionella pneumophila Presence in Dental Unit Waterlines: A Cultural and Molecular Investigation in the West Bank, Palestine

A Legionella pneumophila bacterium is ubiquitous in water distribution systems, including dental unit waterlines (DUWLs). Legionellosis is atypical pneumonia, including Legionnaires' disease (LD) and the less acute form of Pontiac fever. Legionellosis occurs as a result of inhalation/aspiration of aerosolized Legionella-contaminated water by susceptible patients, health workers, and dentists. In this study, we undertook to determine the prevalence of Legionella in water and biofilm samples from Tap and DUWLs collected from five sites of dental clinics and faculties across the West Bank. Water samples were tested for physical and chemical parameters. The study samples included 185 samples, 89 (48%) water samples, and 96 (52%) biofilm swabs, which were analyzed by cultivation-dependent analysis (CDA) and by the cultivation-independent technique (CIA). Also, partial sequencing of the 16S rRNA gene for fifteen L. pneumophila isolates was performed for quality assurance and identification. L. pneumophila was isolated from 28 (15%) of 185 samples using CDA and was detected in 142 (77%) of 185 samples using CIA. The abundance of culturable L. pneumophila was low in DUWL of the sampling sites (range: 27-115 CFU/Liter). PCR was 5× more sensitive than the culture technique. L. pneumophila Sg 1 was detected in (75%) of the isolates, while (25%) isolates were L. pneumophila Sg 2-14. All fifteen sequenced Legionella isolates were identified as L. pneumophila ≥ 94.5%. The analysis of phylogenetic tree showed that L. pneumophila branch clearly identified and distinguished from other branches. These results show that DUWLs of the examined dental clinics and faculties are contaminated with L. pneumophila. This finding reveals a serious potential health risk for infection of immunocompromised patients and dentists' post-exposure.

Impact of prolonged hotel closures during the COVID-19 pandemic on Legionella infection risks

In general, it is accepted that water stagnation and lack or poor maintenance in buildings are risk factors for Legionella growth. Then, in theory, the prolonged hotel closures due to the COVID-19 pandemic may have increased the risk of Legionella infections. However, there are very few field studies comparing the level of Legionella colonization in buildings before the pandemic and the new situation created after the lockdown. The objective of this study was to analyze these differences in a group of hotels that experienced prolonged closures in 2020 due to the COVID-19 pandemic. We have studied the Legionella spp. results, analyzed by standard culture, from the domestic water distribution systems of 73 hotels that experienced closures (from 1 to >4 months) during 2020, immediately after the reopening. The results were compared with those obtained in similar samplings of 2019. For the comparative analysis, we divided the hotels in two groups: Group A that have suffered closures for ≤3 months and Group B that remained closed for more than 3 months, both in relation to the opening period of 2019. In the Group B (36 sites), the frequency of positive samples in the hot water system increased from 6.7% in 2019 to 14.0% in 2020 (p < 0.05). In the Group A (37 sites), no significant differences were observed. No statistically significant differences were observed in terms of positive sites (defined as hotels with at least 1 positive sample), Legionella spp. concentrations and prevalence of Legionella pneumophila sg1 between the samplings of the two periods studied. The results suggest that hotels that suffered the longest prolonged closures (> 3 months) could have carried a higher risk of exposure to Legionella in the domestic hot water system. These findings highlight the importance of adequate preopening cleaning and disinfection procedures for hotel water systems, and the convenience of considering the most effective disinfection methods especially for hot water systems and after prolonged closure periods.

Surveillance of Legionella spp. in Open Fountains: Does It Pose a Risk?

Clusters of outbreaks or cases of legionellosis have been linked to fountains. The function of fountains, along with their inadequate design and poor sanitation, in combination with the warm Mediterranean climate, can favor the proliferation of Legionella in water systems. Public fountains in Mediterranean cities may pose a significant risk for public health due to the aerosolization of water. Nevertheless, few studies have been conducted on Legionella and the risk of infection in humans through fountains. In our study, the presence and quantity of Legionella spp. in fifteen external public fountains were investigated. Two samplings were performed in two different periods (dry and wet). Sixty samples were collected, quantified and analyzed with a culture ISO method. The operation of all fountains was evaluated twice using a standardized checklist. In accordance with their operation, a ranking factor (R factor) was suggested. Finally, based on these results, a quantitative microbial risk assessment was performed. Thirty water samples taken from the fountains (100%) during the dry sampling period were positive for Legionella (mean log concentration: 3.64 ± 0.45 cfu/L), whereas 24 water samples taken from the fountains during the wet period were Legionella-positive (mean log concentration: 2.36 ± 1.23 cfu/L). All fountains were classified as unsatisfactory according to the checklist for the evaluation of their function. A statistically significant correlation was found between Legionella concentration and the assessment score. The risk of Legionella infection was estimated in both periods, with higher risk in the dry period. The surveillance and risk assessment of Legionella spp. in the fountains of Patras confirmed a high prevalence and a high risk to public health.

Assessment of the risk of Legionella pneumophila in water distribution systems in hospitals of Tehran city

When a sensitive host inhales aerosols containing these bacteria, Legionella infection occurs. Therefore, monitoring and assessing Legionella in the environment and water distribution systems of such places are critical due to the prone population in hospitals. However, the health risks of Legionella bacteria in the environment are not adequately evaluated. In this study, for hospitalized patients, we performed a quantitative health risk assessment of Legionella in selected hospitals in Tehran city using two scenarios of shower and toilet faucet exposure. This study identified Legionella in 38 cases (38%) out of 100 samples collected from toilet faucets and showers in 8 hospitals. The information gathered was used for quantitative microbial risk assessment (QMRA). The microbial load transmitted by inhalation was calculated using the concentration of Legionella in water. Other exposure parameters (inhalation rate and exposure time) were obtained using information from other studies and the median length of hospital stay (3.6 days). The exponential model was used to estimate the risk of infection (γ = 0.06) due to Legionella pneumophila (L. pneumophila) inhalation for each exposure event. For the mean concentration obtained for Legionella (10³ CFU/L), the risk of infection for toilet faucets and showers was in the range of 0.23-2.3 and 3.5-21.9, respectively, per 10,000 hospitalized patients. The results were compared with the tolerable risk level of infection determined by the US EPA and WHO. The risk values exceeded the WHO values for waterborne pathogens in hospitals in both exposure scenarios. As a result, our QMRA results based on monitoring data showed that despite using treated water (from distribution networks in the urban areas) by hospitals, 38% of the samples were contaminated with Legionella, and faucets and showers can be sources of Legionella transmission. Hence, to protect the health of hospitalized patients, the risk of Legionella infection should be considered.

Risk assessment and disease burden of legionella presence in cooling towers of Iran's central hospitals

Regular monitoring and measurement of Legionella in tower water and preventive measures against contamination are particularly important in hospitals. This study aimed at risk assessment and disease burden because of legionella presence in cooling towers of Iran's central hospitals. Then its correlation with temperature, pH, turbidity, residual chlorine, and EC was investigated by the Pearson test. The health risk and burden of diseases caused by Legionella exposure were determined using QMRA and DALY models. Statistical analysis and modeling were performed in MATLAB₂₀₁₈. Of the total samples, 30-43% was infected with Legionella. The mean concentrations in hospital A and B were 5-102.5 ± 10 and 5-89.7 ± 0.7 CFU/L, respectively. Among environmental factors, turbidity and pH were the most effective factors in increasing and decreasing Legionella concentration, respectively. According to the QMRA model, the risks of Legionella infections and annual mortality in both hospitals were 0.2-0.3, 0-0.19, 2-2.9 × 10^-5, and 0-0.7 × 10^-5, respectively, which was higher than the acceptable risk range for Legionella (10-4-10-7). However, the trend of its change was negatively correlated with time (R_B = - 0.77). According to the results, the concentration of Legionella and the exposure risk in both hospitals were higher than the permissible range, which is necessary to decrease to 0.1 current concentrations.

Legionella and other opportunistic pathogens in full-scale chloraminated municipal drinking water distribution systems

Water-based opportunistic pathogens (OPs) are a leading cause of drinking-water-related disease outbreaks, especially in developed countries such as the United States (US). Physicochemical water quality parameters, especially disinfectant residuals, control the (re)growth, presence, colonization, and concentrations of OPs in drinking water distribution systems (DWDSs), while the relationship between OPs and those parameters remain unclear. This study aimed to quantify how physicochemical parameters, mainly monochloramine residual concentration, hydraulic residence time (HRT), and seasonality, affected the occurrence and concentrations of four common OPs (Legionella, Mycobacterium, Pseudomonas, and Vermamoeba vermiformis) in four full-scale DWDSs in the US. Legionella as a dominant OP occurred in 93.8% of the 64 sampling events and had a mean density of 4.27 × 105 genome copies per liter. Legionella positively correlated with Mycobacterium, Pseudomonas, and total bacteria. Multiple regression with data from the four DWDSs showed that Legionella had significant correlations with total chlorine residual level, free ammonia concentration, and trihalomethane concentration. Therefore, Legionella is a promising indicator of water-based OPs, reflecting microbial water quality in chloraminated DWDSs. The OP concentrations had strong seasonal variations and peaked in winter and/or spring possibly because of reduced water usage (i.e., increased water stagnation or HRT) during cold seasons. The OP concentrations generally increased with HRT presumably because of disinfectant residual decay, indicating the importance of well-maintaining disinfectant residuals in DWDSs for OP control. The concentrations of Mycobacterium, Pseudomonas, and V. vermiformis were significantly associated with total chlorine residual concentration, free ammonia concentration, and pH and trihalomethane concentration, respectively. Overall, this study demonstrates how the significant spatiotemporal variations of OP concentrations in chloraminated DWDSs correlated with critical physicochemical water quality parameters such as disinfectant residual levels. This work also indicates that Legionella is a promising indicator of OPs and microbial water quality in chloraminated DWDSs.

Cytotoxicity, Intracellular Replication, and Contact-Dependent Pore Formation of Genotyped Environmental Legionella pneumophila Isolates from Hospital Water Systems in the West Bank, Palestine

Legionella pneumophila is the causative agent of Legionnaires’ disease. Due to the hot climate and intermittent water supply, the West Bank, Palestine, can be considered a high-risk area for this often fatal atypical pneumonia. L. pneumophila occurs in biofilms of natural and man-made freshwater environments, where it infects and replicates intracellularly within protozoa. To correlate the genetic diversity of the bacteria in the environment with their virulence properties for protozoan and mammalian host cells, 60 genotyped isolates from hospital water systems in the West Bank were analyzed. The L. pneumophila isolates were previously genotyped by high resolution Multi Locus Variable Number of Tandem Repeat Analysis (MLVA-8(12)) and sorted according to their relationship in clonal complexes (VACC). Strains of relevant genotypes and VACCs were compared according to their capacity to infect Acanthamoeba castellanii and THP-1 macrophages, and to mediate pore-forming cytotoxicity in sheep red blood cells (sRBCs). Based on a previous detailed analysis of the biogeographic distribution and abundance of the MLVA-8(12)-genotypes, the focus of the study was on the most abundant L. pneumophila- genotypes Gt4(17), Gt6 (18) and Gt10(93) and the four relevant clonal complexes [VACC1, VACC2, VACC5 and VACC11]. The highly abundant genotypes Gt4(17) and Gt6(18) are affiliated with VACC1 and sequence type (ST)1 (comprising L. pneumophila str. Paris), and displayed seroroup (Sg)1. Isolates of these two genotypes exhibited significantly higher virulence potentials compared to other genotypes and clonal complexes in the West Bank. Endemic for the West Bank was the clonal complex VACC11 (affiliated with ST461) represented by three relevant genotypes that all displayed Sg6. These genotypes unique for the West Bank showed a lower infectivity and cytotoxicity compared to all other clonal complexes and their affiliated genotypes. Interestingly, the L. pneumophila serotypes ST1 and ST461 were previously identified by in situ-sequence based typing (SBT) as main causative agents of Legionnaires’ disease (LD) in the West Bank at a comparable level. Overall, this study demonstrates the site-specific regional diversity of L. pneumophila genotypes in the West Bank and suggests that a combination of MLVA, cellular infection assays and hierarchical agglomerative cluster analysis allows an improved genotype-based risk assessment.

Quantifying the risk of indoor drainage system in multi-unit apartment building as a transmission route of SARS-CoV-2

The COVID-19 pandemic has had a profound impact on human society. The isolation of SARS-CoV-2 from patients' feces on human cell line raised concerns of possible transmission through human feces including exposure to aerosols generated by toilet flushing and through the indoor drainage system. Currently, routes of transmission, other than the close contact droplet transmission, are still not well understood. A quantitative microbial risk assessment was conducted to estimate the health risks associated with two aerosol exposure scenarios: 1) toilet flushing, and 2) faulty connection of a floor drain with the building's main sewer pipe. SARS-CoV-2 data were collected from the emerging literature. The infectivity of the virus in feces was estimated based on a range of assumption between viral genome equivalence and infectious unit. The human exposure dose was calculated using Monte Carlo simulation of viral concentrations in aerosols under each scenario and human breathing rates. The probability of COVID-19 illness was generated using the dose-response model for SARS-CoV-1, a close relative of SARS-CoV-2, that was responsible for the SARS outbreak in 2003. The results indicate the median risks of developing COVID-19 for a single day exposure is 1.11 × 10-10 and 3.52 × 10-11 for toilet flushing and faulty drain scenario, respectively. The worst case scenario predicted the high end of COVID-19 risk for the toilet flushing scenario was 5.78 × 10-4 (at 95th percentile). The infectious viral loads in human feces are the most sensitive input parameter and contribute significantly to model uncertainty.

Flushing of Stagnant Premise Water Systems after the COVID-19 Shutdown Can Reduce Infection Risk by Legionella and Mycobacterium spp

There is concern about potential exposure to opportunistic pathogens when reopening buildings closed due to the COVID-19 pandemic. In this study, water samples were collected before, during, and after flushing showers in five unoccupied (i.e., for ∼2 months) university buildings with quantification of opportunists via a cultivation-based assay (Legionella pneumophila only) and quantitative PCR. L. pneumophila were not detected by either method; Legionella spp., nontuberculous mycobacteria (NTM), and Mycobacterium avium complex (MAC), however, were widespread. Using quantitative microbial risk assessment (QMRA), the estimated risks of illness from exposure to L. pneumophila and MAC via showering were generally low (i.e., less than a 10^-7 daily risk threshold), with the exception of systemic infection risk from MAC exposure in some buildings. Flushing rapidly restored the total chlorine (as chloramine) residual and decreased bacterial gene targets to building inlet concentrations within 30 min. During the postflush stagnation period, the residual chlorine dissipated within a few days and bacteria rebounded, approaching preflush concentrations after 6-7 days. These results suggest that flushing can quickly improve water quality in unoccupied buildings, but the improvement may only last a few days.

Characterization of Legionella pneumophila Populations by Multilocus Variable Number of Tandem Repeats (MLVA) Genotyping from Drinking Water and Biofilm in Hospitals from Different Regions of the West Bank

The West Bank can be considered a high-risk area for Legionnaires' disease (LD) due to its hot climate, intermittent water supply and roof storage of drinking water. Legionella, mostly L. pneumophila, are responsible for LD, a severe, community-acquired and nosocomial pneumonia. To date, no extensive assessment of Legionella spp and L. pneumophila using cultivation in combination with molecular approaches in the West Bank has been published. Two years of environmental surveillance of Legionella in water and biofilms in the drinking water distribution systems (DWDS) of eight hospitals was carried out; 180 L. pneumophila strains were isolated, mostly from biofilms in DWDS. Most of the isolates were identified as serogroup (Sg) 1 (60%) and 6 (30%), while a minor fraction comprised Sg 8 and 10. Multilocus Variable number of tandem repeats Analysis using 13 loci (MLVA-8(12)) was applied as a high-resolution genotyping method and compared to the standard Sequence Based Typing (SBT). The isolates were genotyped in 27 MLVA-8(12) genotypes (Gt), comprising four MLVA clonal complexes (VACC 1; 2; 5; 11). The major fraction of isolates constituted Sequence Type (ST)1 and ST461. Most of the MLVA-genotypes were highly diverse and often unique. The MLVA-genotype composition showed substantial regional variability. In general, the applied MLVA-method made it possible to reproducibly genotype the isolates, and was consistent with SBT but showed a higher resolution. The advantage of the higher resolution was most evident for the subdivision of the large strain sets of ST1 and ST461; these STs were shown to be highly pneumonia-relevant in a former study. This shows that the resolution by MLVA is advantageous for back-tracking risk sites and for the avoidance of outbreaks of L. pneumophila. Overall, our results provide important insights into the detailed population structure of L. pneumophila, allowing for better risk assessment for DWDS.

Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MRSA) in drinking water fountains in urban parks

The presence of Staphylococcus aureus in drinking water is a concern because of its potential to cause human infection and also because of its multiple antimicrobial resistance. This study evaluated the water quality of drinking water fountains and mist makers in four municipal parks of São Paulo for 13 months. Although all samples met bacteriological water quality criteria according to Brazilian regulations, the absence of residual chlorine (<0.1 mg/L) was observed. These data were significantly correlated with the frequency of S. aureus that was found in 25.2% of the samples. The mecA gene was detected in 36.7% of the isolates demonstrating its potential for resistance to several antimicrobials. Furthermore, 27.3% isolates carrying the mecA gene had methicillin-resistant Staphylococcus aureus (MRSA) phenotypic potential. The presence of S. aureus with characteristics of microbial resistance in water for human consumption is an unprecedented finding. Hence, conducting surveillance for opportunistic bacteria, such as staphylococci in drinking water, is reasonable to take control measures and to protect human health, especially in public places with high attendance.

Implementation of quantitative microbial risk assessment (QMRA) for public drinking water supplies: Systematic review

In the more than 15 years since its introduction, quantitative microbial risk assessment (QMRA) has become a widely used technique for assessing population health risk posed by waterborne pathogens. However, the variation in approaches taken for QMRA in relation to drinking water supply is not well understood. This systematic review identifies, categorises, and critically synthesises peer-reviewed and academic case studies of QMRA implementation for existing distributed public drinking water supplies. Thirty-nine English-language, peer-reviewed and academic studies published from 2003 to 2019 were identified. Key findings were synthesised in narrative form. The overall designs of the included studies varied widely, as did the assumptions used in risk calculation, especially in relation to pathogen dose. There was also substantial variation in the degree to which the use of location-specific data weighed with the use of assumptions when performing risk calculation. In general, the included studies' complexity did not appear to be associated with greater result certainty. Factors relating to pathogen dose were commonly influential on risk estimates whereas dose-response parameters tended to be of low relative influence. In two of the included studies, use of the 'susceptible fraction' factor was inconsistent with recognised guidance and potentially led to the underestimation of risk. While approaches and assumptions used in QMRA need not be standardised, improvement in the reporting of QMRA results and uncertainties would be beneficial. It is recommended that future authors consider the water supply QMRA reporting checklist developed for the current review. Consideration of the broad types of uncertainty relevant to QMRA is also recommended. Policy-makers should consider emergent discussion on acute microbial health-based targets when setting normative guidelines. The continued representation of QMRA case studies within peer-reviewed and academic literature would also enhance future implementation. Further research is needed on the optimisation of QMRA resourcing given the application context.

A valuable experimental setup to model exposure to Legionella's aerosols generated by shower-like systems

The mechanism underlying Legionella aerosolization and entry into the respiratory tract remains poorly documented. In previous studies, we characterized the aerodynamic behaviour of Legionella aerosols and assessed their regional deposition within the respiratory tract using a human-like anatomical model. The aim of this study was to assess whether this experimental setup could mimic the exposure to bioaerosols generated by showers. To achieve this objective we performed experiments to measure the mass median aerodynamic diameter (MMAD) as well as the emitted dose and the physiological state of the airborne bacteria generated by a shower and two nebulizers (vibrating-mesh and jet nebulizers). The MMADs of the dispersed bioaerosols were characterized using a 12-stage cascade low-pressure impactor. The amount of dispersed airborne bacteria from a shower was quantified using a Coriolis® Delta air sampler and compared to the airborne bacteria reaching the thoracic region in the experimental setup. The physiological state and concentration of airborne Legionella were assessed by qPCR for total cells, culture for viable and cultivable Legionella (VC), and flow cytometry for viable but non-cultivable Legionella (VBNC). In summary, the experimental setup developed appears to mimic the bioaerosol emission of a shower in terms of aerodynamic size distribution. Compared to the specific case of a shower used as a reference in this study, the experimental setup developed underestimates by 2 times (when the jet nebulizer is used) or overestimates by 43 times (when the vibrating-mesh nebulizer is used) the total emitted dose of airborne bacteria. To our knowledge, this report is the first showing that an experimental model mimics so closely an exposure to Legionella aerosols produced by showers to assess human lung deposition and infection in well-controlled and safe conditions.

Risk assessment of hybrid rain harvesting system and other small drinking water supply systems by game theory and fuzzy logic modeling

The complexity and uncertainties affecting drinking water supply systems and threatening hazards require a comprehensive and effective risk assessment to increase the reliability of drinking water safety, especially for small or household systems. This study presents a hierarchical structure risk assessment model based on fuzzy logic and game theory to assess the quantity, treatment technology, distribution systems, and storage at household level risks, for small systems in remote areas. Game theory combined with an analytical hierarchy process with entropy weight method is employed. The efficiency tradeoffs in risk from use of this model are examined in a case study which includes three types of small systems (i.e., rainwater harvesting, surface water combined with rainwater and groundwater) in Gansu Province, China. Fifteen risk factors are employed, with evaluation results showing that "Source water" is the most important factor. The hybrid (surface & rainwater) system in the driest year has "Medium" risk with the highest aggregate risk value as a result of source water availability and the distribution system is the most susceptible to failure. The groundwater system consistently has the lowest risk in the case study area. The utility of the model provides scientific support to decision-makers to plan for the most effective risk mitigation measures for water supply systems in remote areas. A cloud-based online-platform employing this methodology has been developed to facilitate the adoption of the methodology in remote areas with mobile or internet access.