Research Article
5 January 2015

Aerosolization of Respirable Droplets from a Domestic Spa Pool and the Use of MS-2 Coliphage and Pseudomonas aeruginosa as Markers for Legionella pneumophila

ABSTRACT

Legionnaires' disease can result when droplets or aerosols containing legionella bacteria are inhaled and deposited in the lungs. A number of outbreaks have been associated with the use of a spa pool where aeration, a high water temperature, and a large and variable organic load make disinfectant levels difficult to maintain. Spa pool ownership is increasing, and the aim of this study, using two surrogate organisms (MS-2 coliphage and Pseudomonas aeruginosa [a natural contaminant]), was to assess the potential risk to domestic users when disinfection fails. A representative “entry level” domestic spa pool was installed in an outdoor courtyard. The manufacturer's instructions for spa pool maintenance were not followed. A cyclone sampler was used to sample the aerosols released from the spa pool with and without activation of the air injection system. Samples were taken at increasing heights and distances from the pool. An aerodynamic particle sizer was used to measure the water droplet size distribution at each sample point. When the air injection system was inactivated, neither surrogate organism was recovered from the air. On activation of the air injection system, the mean mass of droplets within the respirable range (10 cm above the water line) was 36.8 μg cm−3. This corresponded to a mean air concentration of P. aeruginosa of 350 CFU m−3. From extrapolation from animal data, the estimated risk of infection from aerosols contaminated with similar concentrations of Legionella pneumophila was 0.76 (males) and 0.65 (females). At 1 m above and/or beyond the pool, the mean aerosol mass decreased to 0.04 μg cm−3 and corresponded to a 100-fold reduction in mean microbial air concentration. The estimated risk of infection at this distance was negligible.

INTRODUCTION

Legionellae are Gram-negative bacteria found naturally in freshwater environments such as rivers, lakes, and reservoirs and, as such, may also inhabit the municipal water supply and be found in industrial, commercial, and domestic hot and cold water systems (13). Legionellosis, primarily Legionnaires' disease (a serious form of pneumonia) but also including nonpneumonic legionellosis and Pontiac fever (a self-limiting flu-like illness), can result when droplets or aerosols containing legionella bacteria are inhaled and deposited in the lungs.
Legionnaires' disease is most commonly, but not exclusively, caused by Legionella pneumophila. It is a rare but serious disease. In 2009, there were 344 confirmed cases in England and Wales (4), while in the United States, 3,522 cases were reported to the Centers for Disease Control and Prevention (5). It is likely, however, that many more cases are unreported and/or undiagnosed, and it is estimated that Legionnaires' disease accounts for between 3% and 6% of the community-acquired pneumonias that occur in the United Kingdom and United States each year (4, 6). Even with treatment, the case fatality rate of community-acquired Legionnaires' disease ranges from 10% to 15% (4). It is, therefore, important to investigate and assess any potential risk.
A number of legionellosis outbreaks have been associated with the use of a spa pool (717). Transmission is facilitated by a relatively high water temperature (30°C to 40°C), which favors the growth of Legionella spp. (and other human pathogens) and an air injection system which, when operated, causes increased turbulence and the generation of water droplets and aerosols from the surface. The water droplets generated include those of appropriate size (between 1 μm and 8 μm) to transport legionellae deep into the respiratory tract (7, 18). Outbreak investigations have found an association between time spent in a spa pool and likelihood of illness, suggesting a dose-response effect (7, 9). However, aerosols released from the pool can be dispersed by air currents or ventilation systems, meaning that people outside the spa pool may also be at risk of acquiring a Legionella infection (9, 11). One of the world's largest outbreaks of Legionnaires' disease was linked to a spa pool being exhibited at a Dutch flower show (12). Simply pausing at the pool was deemed the most important risk factor for infection, confirming that a contaminated spa pool, even if not used for bathing, can cause illness in susceptible people.
Spa pool ownership is increasing (19), and domestic style spa pools, while on display in retail outlets, have been implicated in a number of Legionella outbreaks (10, 14, 17). In these cases, as in the majority of spa-associated outbreaks, the pools were considered the most likely source of infection in view of poor maintenance and/or inadequate disinfection. Domestic users may be more vigilant than retail staff in changing and treating spa water (14). However, aeration, the high water temperature, and the large and variable organic load mean that disinfectant levels are difficult to maintain (7).
The aim of this study was to investigate the aerosols released from a domestic spa pool and, using two surrogate organisms (MS-2 coliphage and Pseudomonas aeruginosa), to assess the potential risk to those in and around the pool should there be failures in disinfection practice.

MATERIALS AND METHODS

Spa pool.

In July 2012, an outbreak of Legionnaires' disease was linked to a portable, inflatable spa pool that was on display in a retail outlet in Stoke-on-Trent, United Kingdom (17). A spa pool identical to that linked to this outbreak was purchased from a United Kingdom catalogue merchant (retail price, approximately £550 [approximately $920]) and installed in an outdoor courtyard. The pool was placed on a wooden stage (10 cm in height) that was smooth, flat, level, and capable of uniformly supporting the combined weight of the spa pool and water (1,100 kg). The pool was inflated, as per the manufacturer's instructions (inflated size [diameter by height]: 196 cm by 61 cm) and, using a rubber hose connected to the main water supply, filled to the minimum water level printed on the spa wall (approximately 848 liters). The protective cover was secured, and the heating and filtration systems were activated. Once assembled, the automated control system circulated the water, heated it to 40°C, and, when the air injection system was operated, forced air through a series of 80 air holes located on a skirt that extended around the inner circumference of the pool.

Preparation of MS-2 coliphage.

Escherichia coli NCIMB 9481 was subcultured onto Trypticase soy agar (TSA; bioMérieux, Basingstoke, United Kingdom) and incubated at 37°C for 20 h. A 10-μl loopful of the overnight culture was aseptically transferred to a 500-ml conical flask containing 60 ml of sterile Trypticase soy broth (TSB; bioMérieux) and incubated in a shaking water bath (120 rpm) for 2.5 h at 37°C. A 1-ml aliquot of phage stock (MS-2 coliphage; NCIMB 10108) was added to the E. coli suspension and aerated in a shaking water bath (120 rpm) at 37°C for a further 4 h. Turbidity was compared to that of an E. coli suspension prepared in the absence of MS-2 coliphage. A reduction in turbidity confirmed that bacterial lysis had occurred. Cell debris was removed via centrifugation (2,000 × g for 20 min), and the supernatant was transferred to a clean flask and stored at 4°C.
To prepare the pool inoculum, an aliquot of the supernatant was passed through a syringe filter (0.2 μm) and diluted 10-fold in sterile distilled water. One milliliter of the resulting suspension was transferred to 1 liter of sterile distilled water and, prior to the inoculation of the pool, incubated at 37°C for 2 h. This incubation step avoided subjecting the phage particles to a sudden change in temperature (4°C to 40°C) and potentially inactivating the MS-2 coliphage.

Inoculation of spa pool water.

This investigation was carried out between 17 October and 29 November 2013. The outside temperature, relative humidity, and dew point were measured using a thermohygrometer (model 7425; TSI Inc., Shoreview, MN) and ranged from 10.5°C to 18.5°C, from 49% to 74%, and from 1.4°C to 11.2°C, respectively. Wind speed ranged from 0.2 m s−1 to 1.4 m s−1. The temperature of the pool water was monitored using a Tinytag Aquatic 2 logger (Gemini Data Loggers UK Ltd., Chichester, United Kingdom). The average water temperature (although displayed on the control panel as being 40°C) was 35.2°C ± 3.1°C.
The suspension of MS-2 coliphage (approximately 4 × 1010 PFU) was added to the pool and mixed by activating the air injection system for 5 s. The pool water was assayed before and after each experimental run to confirm MS-2 concentration (see procedure below). To provide assurance that Legionella spp. were absent from the pool, routine water samples were taken and processed as previously described (20) and cultured on glycine-vancomycin-polymyxin-cycloheximide (GVPC) agar (Oxoid Ltd., Basingstoke, United Kingdom). Legionella spp. were not recovered from any of the water samples taken. However, Pseudomonas aeruginosa, although not detected in the main water supply, had naturally contaminated the pool (potentially from rainwater that had collected on the pool cover entering the pool when the cover was removed) and was subsequently used as a second surrogate organism.
Aliquots of the collected rainwater were cultured on TSA. Colonies were identified using biochemical test strips (API 20NE; bioMérieux) and/or via matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) mass spectrometry (Bruker Daltonik MALDI biotyper; Bruker, Bremen, Germany).

Air samples.

An all-glass cyclone sampler (21) operating at 650 liter min−1 was used to sample the aerosols released from the spa pool with and without activation of the air injection system. Airborne particles were deposited on the cyclone wall and washed off by the collecting fluid (phosphate buffer containing Manucol and antifoam). Triplicate samples were taken at 10 cm, 20 cm, and 1 m above the water line and at increasing distances downwind from the pool (0 cm, 25 cm, 50 cm, and 1 m). On each occasion, the sampler was operated for 2 min before the collecting fluid was withdrawn, the volume measured, and the fluid transferred aseptically to a plastic container. Duplicate aliquots (100 μl) were removed and cultured on Pseudomonas agar (Oxoid) supplemented with cetrimide (200 mg liter−1) and sodium nalidixate (15 mg liter−1), followed by incubation at 37°C for 48 h. The remaining fluid was passed through a syringe filter (0.2 μm) and the filtered fluid assayed to determine MS-2 coliphage recovery.
An aerodynamic particle sizer (APS; model 3321; TSI Inc.) was used to measure the water droplet size distribution above and beyond the pool with and without activation of the air injection system. The APS was positioned at the same height and distance as the cyclone air sampler, and aerodynamic particle sizes from 0.5 μm to 20 μm were measured.

MS-2 coliphage assay.

E. coli 9481 was subcultured onto TSA and incubated at 37°C for 20 h. A 10-μl loopful of the overnight culture was aseptically transferred to 10 ml of TSB and incubated at 37°C for 260 min. Glass bottles containing 3 ml of soft phage agar (comprising agar at 4 g liter−1 and nutrient broth at 25 g liter−1) were heated at 90°C for 90 min and then maintained at 60°C until required. The bottles were then cooled to 45°C.
Three drops of the E. coli 9481 suspension were added to 3 ml of molten soft phage agar using a Pasteur pipette, followed immediately by 100 μl of the collection fluid (or water sample) to be assayed. The contents of the bottle were gently mixed and poured immediately onto a TSA plate. Each collection fluid was sampled in triplicate. All plates were incubated at 37°C for 18 h and the numbers of PFU determined.

Analysis of results.

The concentration of MS-2 coliphage or P. aeruginosa in each air sample was calculated by multiplying the number of PFU or CFU on each agar plate by the volume of collecting fluid and dividing by the volume of air sampled. The minimum detection limit of the MS-2 and P. aeruginosa assays was 2 PFU m−3 and 3 CFU m−3, respectively. Spray factor—the proportion of a suspension aerosolized (22)—was calculated by dividing the air concentration (PFU or CFU m−3) by the concentration of surrogate organism in the pool (PFU or CFU ml−1). Data analysis was performed using Microsoft Excel 2010. Statistical significance was at a P value of <0.05 and was determined by use of t tests. Particle size and microbiological data were incorporated in quantitative risk assessment models suggested by Armstrong and Haas (23) and Bouwknegt et al. (24).

RESULTS

The size distributions of the water droplets sampled using the APS at increasing heights and distances from the pool are shown in Fig. 1 and 2, respectively. The results are expressed as mean mass concentrations (i.e., the mean mass of the particles per unit volume of air sampled). When the air injection system was inactivated, the mean mass of droplets between 2 and 10 μm that were detected 10 cm above the water line was 0.46 μg cm−3 (n = 9). When the air injection system was switched on, the mean mass concentration at this height increased almost 100-fold (Fig. 1). However, as sample height increased, mean mass concentration decreased. The mean mass of droplets between 2 and 10 μm that were detected 1 m above the water line was 0.04 μg cm−3 (n = 9). At this height, mean mass concentration did not significantly change with increasing distance from the pool (Fig. 2). Closer to the water surface, the mean mass of droplets between 2 and 10 μm that were detected 50 cm from the pool was 1,000-fold lower than that detected at the pool edge (38.6 μg cm−3). Increasing the sample distance to 1 m increased the mean mass concentration from 0.01 μg cm−3 to 0.06 μg cm−3.
FIG 1
FIG 1 Effect of sample height and activation of the air injection system upon the mass distribution of particles above a domestic spa pool. *, the mean mass of droplets detected 1 m above the water line was similar to that generated when the air injection system was inactivated, hence an overlapping distribution.
FIG 2
FIG 2 Mass distribution of particles at increasing heights and distances from a domestic spa pool with the air injection system activated.
The mean numbers of MS-2 coliphage and Pseudomonas aeruginosa organisms recovered from the pool water were 7.1 × 107 PFU liter−1 and 3.2 × 105 CFU liter−1, respectively (Table 1). Neither organism was recovered from air samples taken when the air injection system was inactivated (i.e., the mean air concentrations of MS-2 and P. aeruginosa were <2 PFU m−3 and <3 CFU m−3, respectively). When the air injection system was activated, the highest numbers of the contaminating organism (regardless of surrogate) were recovered close to the water surface (i.e., from air samples taken 10 cm above the water line at the edge of the pool). The mean air concentration of MS-2 coliphage was significantly greater than that of P. aeruginosa at 20 cm and 1 m above the water line (P < 0.05). However, the comparatively high concentration of MS-2 in the pool meant that the proportion of the suspension aerosolized (i.e., the spray factor) was higher for P. aeruginosa (Fig. 3). For both surrogate organisms, as sample height increased, air concentration and, as a consequence, spray factor decreased (Table 1; Fig. 3).
TABLE 1
TABLE 1 Mean concentrations of MS-2 coliphage and Pseudomonas aeruginosa in the pool water and in the air above the pool when the air-injection system was activateda
Experimental runEnvironmental conditionsSurrogate organismConcn in pool water (PFU or CFU liter−1)Mean (n = 9) concn in air sample (PFU or CFU m−3) at indicated ht above water line
Air temp (°C)Relative humidity (%)10 cm20 cm1 m
118.558–63MS-2 coliphage5.2 × 107115NG43
   P. aeruginosa5.1 × 105512413
212.561–67MS-2 coliphage6.9 × 1077491759
   P. aeruginosa2.2 × 10544593NG
31367–74MS-2 coliphage9.3 × 10718314017
   P. aeruginosa2.2 × 1059944NG
Mean (n = 27) ± SD  MS-2 coliphage7.1 × 107 ± 2.1 × 107349 ± 348105 ± 9323 ± 18
   P. aeruginosa3.2 × 105 ± 1.6 × 105352 ± 22247 ± 454 ± 8
a
Air samples were taken at the edge of the pool at 10 cm, 20 cm, and 1 m above the water line. NG, no growth (i.e., below the detection limit of the assay [<2 PFU m−3 or <3 CFU m−3]).
FIG 3
FIG 3 Effect of sample height on mean (n = 27) spray factor.
The mean air concentrations of MS-2 coliphage and P. aeruginosa in samples taken 10 cm above the water line but 50 cm away from the pool were 4.6 PFU m−3 and 3.1 CFU m−3, respectively, 100-fold lower than that recovered from samples taken at the same height but at the pool edge (Table 2). Mean air concentration increased with increasing distance from the pool. However, this increase was not significant (P > 0.05), and at 1 m from the pool, the spray factors were just 3.98 × 10−4 ml m−3 and 3.8 × 10−2 ml m−3 for MS-2 and P. aeruginosa, respectively. At this distance, neither organism was recovered from air samples taken 1 m above the water line (Table 2).
TABLE 2
TABLE 2 Mean concentrations of MS-2 coliphage and Pseudomonas aeruginosa in the pool water and in the air above the pool when the air-injection system was activateda
Environmental conditionsSurrogate organismConcn in pool water (PFU or CFU liter−1)Distance from poolMean (n = 9) concn in air sample (PFU or CFU m−3) at indicated ht above water line
Air temp (°C)Relative humidity (%)10 cm1 m
11.549–65MS-2 coliphage2.7 × 107Pool side5288
    25 cm2NS
    50 cm5NS
    75 cm6NS
    1 m11NG
  P. aeruginosa1.8 × 105Pool side415NG
    25 cm19NS
    50 cm3NS
    75 cm6NS
    1 m7NG
a
Air samples were taken 10 cm and 1 m above the water line at increasing distances (25 cm to 1 m) from the pool edge. NS, not sampled; NG, no growth (i.e., below the detection limit of the assay [<2 PFU m−3 or <3 CFU m−3]).

DISCUSSION

During 2007 and 2008, 18 of the 116 outbreaks associated with treated recreational water that were reported to the CDC were caused by bacteria. Ten (55.5%) of these outbreaks were caused by Legionella spp. All were linked epidemiologically to spa pools and resulted in 122 persons developing legionellosis (25). Key to reducing the microbiological contamination of a spa pool is an appropriate and effective management system. Control measures should include a sufficient concentration of disinfectant (≥2 ppm of free chlorine) and frequent emptying and cleaning of the pool (26). However, Legionella spp. have been recovered from public spa pools that have been treated and/or emptied appropriately (11, 26, 27), implying that the concentration of disinfectant necessary to achieve microbial control may be higher than that normally recommended. Thus, domestic spa pool users may be at risk from Legionella despite complying with manufacturers' maintenance instructions.
During this investigation, the manufacturer's instructions for spa pool maintenance were not followed. At no point during the 2-month study was the spa pool water changed or disinfected, nor were the surfaces of the pool cleaned. This lack of a control regimen was primarily to ensure the viability of the MS-2 coliphage but, in terms of the pool hygiene compliance of the domestic user, represented the worst-case scenario. Bacterial colonization of the spa pool was rapid. P. aeruginosa has very simple nutritional requirements, and trace elements naturally present in water are sufficient to enable the organism to survive and to multiply (28). Although not detected in the main water supply, high numbers of P. aeruginosa (105 CFU liter−1) were recovered from the pool water within 3 days of filling. P. aeruginosa may have been present in the source water at levels of <1 CFU 100 ml−1 (i.e., lower than the detection limit of the membrane filtration assay). Alternatively, bacteria may have entered the pool in rainwater. During the current study, P. aeruginosa together with other pseudomonads (e.g., P. fluorescens, P. antarctica, and Brevundimonas vesicularis [identified using MALDI-TOF]) were recovered from rainwater that had collected on the pool cover. L. pneumophila, although previously recovered from rainwater (29), was not detected. Regardless of the source of contamination, once the organism was transferred to the spa pool, the elevated temperatures facilitated the survival and growth of P. aeruginosa.
Outbreaks of P. aeruginosa folliculitis and otitis externa are frequently associated with spa pools (30). Unlike legionellosis, these types of Pseudomonas infection result from contact with contaminated water rather than the inhalation of aerosols. However, the virulence of L. pneumophila depends upon the size of the particles in which the cells are contained and their site of deposition in the respiratory tract (31). P. aeruginosa is similar in size to L. pneumophila (approximately 2 μm in length and 0.3 to 0.9 μm in width) (18); therefore, when investigating the aerosolization of water droplets, it may be a more appropriate surrogate for L. pneumophila than MS-2 coliphage (ca. 25 nm in diameter) (32).
L. pneumophila induces a lethal infection in guinea pigs when administered as a small-particle aerosol (<5 μm in diameter) (31). Droplets of this size are large enough to contain the bacteria (i.e., >2 μm) but small enough to deposit in the respiratory tract. The mass of the droplets relates to the number of bacteria contained within (18). Aerosolization of droplets from the spa pool when the air injection system was inactivated was minimal. However, when the air injection system was turned on, the mean mass of droplets within the respirable range generated close to the water surface increased significantly (Fig. 1), as did the mean air concentration of P. aeruginosa (Table 1).
Published quantitative microbial risk assessment models (23, 24) have used aerosol mass measurements to estimate the microbiological concentration in the air above a whirlpool spa. The results of the current study, which reports corresponding aerosol mass and microbial concentration data, suggest these models may overestimate bacterial aerosol emission.
Armstrong and Haas (23) used particle size data generated by Baron and Willeke (18) to estimate the aqueous aerosol load 35 cm above a turbulent 40°C spa pool (total mass of aerosolized particles [5 mg m−3] × water density = 5 × 10−3 ml m−3). By multiplying the aqueous aerosol load by the Legionella concentration in the bulk water (assumed in this model to be 3.6 × 103 CFU ml−1), they estimated the concentration of Legionella organisms in the aerosol released by a whirlpool spa to be 18 CFU m−3. Subjecting the particle size data generated during the current study to this model results in an estimated aqueous aerosol load 20 cm above the turbulent spa pool of 350 ml m−3 (350 μg cm−3 [mass of 2 to 10 μm aerosol] [Fig. 1] × water density). The concentration of P. aeruginosa in the water was 3.2 × 102 CFU ml−1, leading to an estimated concentration of P. aeruginosa in the air of 1.12 × 105 CFU m−3, 10,000-fold higher than the actual mean air concentration (47 CFU m−3) (Table 1).
An alternative model uses a bacterial water to air partitioning coefficient to predict microbial air concentration. The value suggested by Armstrong and Haas (2.3 × 10−5 CFU of Legionella m−3 of air per CFU of Legionella liter−1 of water) was derived from endotoxin data and was considered a reasonable surrogate for Gram-negative bacteria in a hot spring environment (23). However, supporting data are limited, and the authors acknowledge that uncertainty regarding this input parameter contributes to variability in estimated dose. The results from the current study provide additional data regarding water-to-air partitioning coefficients (i.e., the proportion of a suspension aerosolized, i.e., the spray factor).
The use of spray factor in microbiological risk assessment has been demonstrated (22). The lower spray factor associated with MS-2 coliphage compared to P. aeruginosa (Fig. 3) may be due to their presence in particles with an aerodynamic diameter of <0.5 μm. These particles may not have been captured efficiently by the cyclone samplers used in these experiments (33), and for this reason, the values obtained for P. aeruginosa should be the most relevant for L. pneumophila risk assessment.
When the spa pool air injection system was deactivated (simulating a natural hot spring), the air concentration of P. aeruginosa 10 cm above the spa pool was <1 CFU m−3, a spray factor of < 3 × 10−3 ml m−3 (i.e., <3 × 10−6 liters m−3), 10-fold lower than the water-to-air partitioning coefficient used previously (23). Activating the air injection system significantly increased the spray factor (1.1 ml m−3 [i.e., 1.1 × 10−3 liters m−3]) and, thus, the air concentration of P. aeruginosa (3.5 × 102 CFU m−3) (Table 1) and the risk of aerosol transmission.
When bubbles rise through water, they can collect bacteria on their surface, resulting in bacterial enrichment. When the bubbles burst, they produce film droplets which can range from 1 μm to 10 μm in diameter (34). The number of bacteria in film droplets can be 20-fold higher than that in the bulk water (34). The presence of high numbers of L. pneumophila organisms in spa pool water does not always result in infection, and in previous investigations, the absence or low level of illness has been attributed to the absence of air compressors or failure to use the air injection system (7, 27). The results of the current investigation support these conclusions.
Dose-response models used to estimate exposure dose (23) and probability of infection (24) are based on the exposure of guinea pigs to L. pneumophila. Subjecting our microbiological data to the same inhalation rates (9 liters min−1 [male] and 6.7 liters min−1 [female]) (24), exposure time (15 min), retention rates (50% deposition of a 5-μm-range aerosol) (23), and infectivity per L. pneumophila CFU (0.06) (24) as assumed in these models results in estimated exposure doses 10 cm above a turbulent spa pool of 23.8 CFU and 17.6 CFU for males and females, respectively. At this height, the estimated probabilities of infection are 0.76 for males and 0.65 for females. When the air injection system is inactivated, the estimated risk of infection is negligible.
The recovery of L. pneumophila aerosolized from bath taps has been shown to decrease with increasing distance from source (35). Similarly, microbial air concentration and, thus, exposure dose decreased with increasing height and distance from the spa pool (Fig. 3; Table 2), implying that while activating the air injection system may increase the risk of infection for those either in or at the edge of the pool, there is no increased risk to those at any distance from the pool (>1 m). However, it is acknowledged that the spa pool used in the current investigation was in a very sheltered location; the cyclone sampler was generally positioned downwind from the pool, and the effect of wind speed and/or direction on the spread of the aerosolized particles was not investigated. When the air injection system was operated, air was forced through a series of air holes that extended around the inner circumference of the pool. How direction and/or force of the air injection can affect the aerosolization of particles is also not known.
Nonetheless, the results of this study confirm that if not properly maintained, a domestic spa pool rapidly becomes contaminated. The elevated temperature of the water facilitates the growth of human pathogens which become aerosolized during activation of the air injection system. This study provides direct data on the aerosolization of bacteria from a spa pool, including water-to-air partitioning coefficients and corresponding bacterial air concentrations and aerosol mass measurements. These data will help inform quantitative risk assessments, increase the robustness of the model(s) used (by improving estimates and assumptions), and, in doing so, increase confidence in the results. After subjecting our microbiological data to published dose-response models, it can be concluded (assuming that the level of aerosolization of P. aeruginosa is similar to that of L. pneumophila) that susceptible individuals either in or at the edge of the pool may be at risk of acquiring legionellosis. However, under the experimental conditions described here, the risk of infection is reduced with increasing height and distance from the pool.

ACKNOWLEDGMENTS

This study was funded by Public Health England.
The views expressed in this report are those of the authors and not necessarily those of PHE or any other government agency.
We thank the PHE Biodefence and PreClinical Evaluation Group for use of the aerodynamic particle sizer and Haroun Shah and Renata Culak of the PHE Proteomics Research Unit for use of the MALDI-TOF mass spectrometer.

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Information & Contributors

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Published In

cover image Applied and Environmental Microbiology
Applied and Environmental Microbiology
Volume 81Number 215 January 2015
Pages: 555 - 561
Editor: F. E. Löffler
PubMed: 25381233

History

Received: 5 September 2014
Accepted: 30 October 2014
Published online: 5 January 2015

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Authors

Ginny Moore
Public Health England, Porton Down, Salisbury, United Kingdom
Matthew Hewitt
Public Health England, Porton Down, Salisbury, United Kingdom
David Stevenson
Public Health England, Porton Down, Salisbury, United Kingdom
Jimmy T. Walker
Public Health England, Porton Down, Salisbury, United Kingdom
Allan M. Bennett
Public Health England, Porton Down, Salisbury, United Kingdom

Editor

F. E. Löffler
Editor

Notes

Address correspondence to Ginny Moore, [email protected].

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