Abstract
Background
The infectious disease public health exposure hazard from toilets may include flush-related aerosol generation, yet little is known about it. A necessary first step was to establish appropriate and effective methodology for evaluating flush-related aerosol generation. The goal of this study was to develop and test a methodology for evaluating toilet-flush-related aerosol generation. To accomplish this goal, an experimental apparatus was developed, and preliminary experiments conducted to contribute to establishing the methodology.
Material and Method
The apparatus consisted of a chamber, 4 feet tall and 18 inches wide, with two high-efficiency air particulate (HEPA) air intake filters mounted on opposite sides. On the top of the chamber, a HEPA exhaust/vacuum filter was located to evacuate the particles in the chamber and toilet adaptor at the bottom. The chamber contained a sampling port connected to a direct-reading TSI Aerodynamic Particle Sizer Spectrometer we used to sample and measure particle concentration in the chamber. Particle concentrations in the chamber were collected and measured with and without the toilet flush for comparison.
Results
The mean standard background concentrations of aerosolized particles before toilet flushing were much statistically lower than in the post- flush conditions. The results confirmed that toilet flushing did increase the concentrations of airborne particles in the bathroom air, and the relative changes in particle concentration were inversely related to their size.
Conclusion
With improved design of the chamber, this methodology may serve as an acceptable approach to evaluate flush-related aerosol generation.
Keywords: infection, bacteria, toilet flushing, aerosolization, particles.
Introduction and Purpose
Aerosolized pathogenic microorganisms are claimed to be a source of numerous human infections, especially involving the respiratory system. Aerosolization of pathogens can occur through a variety of modes from various sources, including spray irrigation, sprays from ocean surf, as well as in sewage treatment processes (Wallis, Melnick, Rao & Sox, 1985). 100 years ago, Horrocks discovered that sewage flows going through pipes could transport substantial amounts of airborne microbes, which remained viable across considerable distances (Anonymous, 2005; Johnson, Lynch, Marshall, Mead, & Hirst, 2013). From the perspective of indoor air quality in residences and other settings, flush-related aerosolization of microbes may be a source of human exposure. Indeed, public toilets may present a substantial source of exposure, as the waters repeatedly flushed through them exemplify a tangible vehicle for the transmission of viral and bacterial agents by air (Jackson, Aldred, Canady, Corsi & Siegel, 2010). Today, norovirus and gastroenteritis remain the most common facets of aerosol generation in toilets and other indoor spaces (Nazaroff, 2011).
Earlier researchers sought to implement different methods of research, but only recently the importance of quality methods in measuring bioaerosolization in public toilets has been widely recognized. Johnson and Lynch (2008) proposed a cost-effective method for measuring pathogenic bioaerosols in toilets, which involved fluorescent polystyrene latex microspheres and an isolation enclosure. Johnson et al. (2013) also used monodisperse fluorescent microspheres to estimate changes in bioaerosolization with the help of a high-energy flushometer. Unfortunately, none of the existing methods is universally effective, as the technical challenges of using traditional techniques to calculate bioaerosol generation remain significant (Johnson & Lynch, 2008). At the same time, the validity and reliability of the earlier findings raise questions as to the appropriateness of various methods in measuring particle counts at particularly low concentrations. All these difficulties justify the need to further develop approaches and methods for measuring flush-related bioaerosol generation in public toilets.
Unfortunately, the current knowledge of aerosol generation in toilets is still scarce. 100 years ago, Horrocks discovered that sewage flushing carried airborne microbes that remained viable over long periods of time and lengthy distances (Johnson et al., 2013). Later in the 21st century, the SARS outbreak was attributed to the transmission of bioaerosols through public toilets (Johnson et al., 2013). Still, the potential of public toilets to serve as a mode of transmission for airborne pathogens has not been officially recognized (Johnson et al., 2013). In the absence of valid research methods, this task becomes even more complicated. The importance of the study is justified by the need to reduce the scope of airborne infectious diseases in humans. However, at first, new methods to evaluate the risks of bioaerosolization through public toilets need to be developed. None of these objectives can be achieved without providing a firm empirical basis to validate popular assumptions about aerosolization of microorganisms in toilets.
The purpose of this study was to develop and test an apparatus and method for measuring aerosol generation associated with toilet flushing. A chamber connected to a direct reading particle sizing spectrometer was constructed to attempt to increase the collection efficiency of flush-related aerosols from toilets. Real time sampling was conducted with and without toilet flush with the overall goal to further refine the method to sample and measure toilet flush-related Bioaerosols that may be a hazardous source of human exposures to pathogenic microbes.
Literature Review
Aerosol Generation: The Hidden Facets of the Problem
Bioaerosol generation and release have long been a matter of serious public health concerns. According to Sanchez-Monedero, Stentiford and Urpilainen (2005), aerosol generation can have potentially devastating impacts on public health. Sanchez-Monedero et al. (2005) undertook a monitoring project that lasted one year and covered one composting plant. The monitoring parameters included mesophilic bacteria and aspergillus fumigatis spores (Sanchez-Monedero et al., 2005). The researchers discovered that pile turning, green wastes shredding, and mature compost screening were associated with the biggest changes in bioaerosolization (Sanchez-Monedero et al., 2005).
Any public toilet and similar bioaerosol generation sites can increase the risks of airborne infections. Such sites may include sewage pipes and composting plants, but the exact number and scope of such spaces cannot be identified (Horrocks, 1907; Johnson et al., 2013; Sanchez-Monedero et al., 2005). The current state of literature provides abundant information on airborne infections and the ways, in which they can be detected and treated. Nonetheless, viruses remain one of the most common factors leading to infectious diseases. Toilets, particularly public ones, play one of the central roles in facilitating the transmission of bacterial and viral diseases (Morawska, 2005). Morawska (2005) acknowledges that the direction of bioaerosolization and separate droplets depends considerably on the physical principles of speed and transport, but droplets size remains the most essential factor of deposition and dispersion (Morawska, 2005). Unfortunately, the current methods of research provide limited data as to the dynamics of virus-laden bioaerosols (Morawska, 2005). As a result, these methods do not allow controlling and preventing the spread of viruses. The degree to which each toilet is dangerous to public health depends mainly on the type of the virus found, followed by the levels of concentration and its potential effects on health.
Today, it is generally assumed that aerosol generation is rooted in toilet flushing; in other words, it is flushing that leads to the subsequent aerosolization of microorganisms (Morawska, 2005). It is a major exposure process, since it cannot be eliminated. Morawska (2005) simply reviewed the current knowledge of bioaerosolization and possible solutions to decrease the spread of airborne microbes and related infections. The assumption that bioaerosols are the chief sources of infections and bacterial diseases in public toilet chambers is simply taken for granted. Not surprisingly, Johnson et al. (2012) assert that “the potential risks associated with toilet plum aerosols produced by flushed toilets is a subject of continuing study.” It is flushing that is responsible for generating substantial amounts of aerosolized microorganisms, and this is also why toilet users are constantly exposed to various public health risks.
Johnson et al. (2012) recently reviewed the literature relating to flushing and its potential to generate aerosols. The review was justified by the need to explore the potential risks of bioaerosolization through toilet plums, as well as to verify the evidence of bioaerosol generation in public toilets (Johnson et al., 2012). The researchers used peer-reviewed journal articles to identify those, which examined the role played by toilets in infectious disease outbreaks (Johnson et al., 2012). The resuts of their review confirmed that toilet flushing was substantially related to aerosolization of infectious particles. At the same time, the researchers have failed to prove that toilet plums could be responsible for infectious disease transmission (Johnson et al., 2012). New methods of analysis are needed to improve the current knowledge of toilet-related transmission of airborne infections
Why the Problem of Aerosol Generation Is So Important
The current literature provides ample evidence to justify the severity of the bioaerosol generation problem. As mentioned earlier, aerosol generation is associated with airborne infections, which are easily transferred to new recipients through the aerosolization of microorganisms (Wallis et al., 1985). Johnson et al. (2013) referred to sewage-related bioaerosols as a potential source of SARS; more specifically, sewage-related mechanisms were proposed as the central mode of transmitting SARS in Hong Kong. The fact is that toilet flushing is associated with the production of major and minor droplets, whose nuclei contain or carry a variety of microorganisms (Johnson et al., 2013). The problem is further aggravated by the fact that “droplet nuclei are the residuals of larger droplets whose water content has largely or completely evaporated; they are of sufficiently small size that they will not settle on surfaces due to the force of gravity but rather will remain airborne and be carried on air currents” (Johnson et al., 2013, p.1047). At the same time, many medical professionals and researchers cannot acknowledge the potential of toilet flushing to generate bioaerosols leading to infectious disease outbreaks, mostly due to the absence of valid research and analytical methods. Consequently, new studies are needed to reinforce the public awareness of the problem and develop new guidelines to avoid the transmission of infectious diseases through public toilets. However, at first, new credible methods to measure bioaerosol generation in public toilets need to be developed.
Sources of Aerosolized Microorganisms in Toilets
Despite the controversies surrounding the topic, researchers provide sufficient information to inform the public of the main sources of airborne microorganisms in toilets. A common assumption is that toilet flushes may be responsible for the risks of infectious transmissions, but no compelling evidence to support this thesis has been produced so far. Leed (2011) describes toilet flushing as an event that results in an increase in the number of bacteria that are sent into the air and further down the airways. In other words, toilet flushing creates an outburst of microorganisms, including bacteria that are riding on water droplets, which appear when the toilet is flushed. The droplet itself quickly evaporates, but the microorganism may remain viable for hours or even days, posing a serious hazard to humans (Leed, 2011). Thousands of unsuspecting people inhale aerosolized droplets and have dermal (e.g. hand) contact without even suspecting it (Leed, 2011).
The problem is that the information provided by Leed (2011) was not tested empirically. Leed (2011) simply reviewed the existing literature and made conclusions based on the earlier findings. In the absence of credible research methods, the validity and reliability of the earlier findings can be readily questioned. Still, the author’s fears that toilet flushes can transmit infectious agents related to diseases are not totally unsubstantiated. Johnson et al. (2012) conducted a detailed review of literature to investigate the risks of contact transmission as a result of surface contamination with flush droplets. The results confirmed that the contamination of toilet seats and lids with flush aerosols was a reality, rather than theory (Johnson et al., 2012). The highest were the concentrations of the organisms that have the highest capacity to survive on the surface (Johnson et al., 2012). At the same time, Johnson et al. (2012) found toilet plume droplet nuclei to be an important mode of transmission for various infectious diseases.
Toilet flushing as a source of microorganisms and aerosolized bacteria was also confirmed by Gerba, Wallis, and Melnick (1975). The goal of their study was to gather more relevant information on the fate of bioaerosolization in household toilets (Gerba et al., 1975). The researchers used E. coli bacteriophage and poliovirus to perform total bacterial counts and see if infectious agents could be responsible for airborne transmission after toilet flushing (Gerba et al., 1975). The results confirmed that viruses and bacteria generated by toilet flushes remained airborne long enough to settle on all bathroom surfaces (Gerba et al., 1975). As a result, the researchers assumed that infectious transmission was possible through an aerosol produced by toilet flushing (Gerba et al., 1975). Apparently, the methods used by Gerba et al. (1975) were too “fresh” to be considered as reliable, but they can serve as a model for further methodological improvements in this field.
Diseases, Microorganisms, and Bacteria Transmitted through Toilet Flushing
Present-day literature provides information on the types and varieties of the microorganisms, such as bacteria and viruses that are easily transmitted through toilet flushing. It is interesting to note that even influenza can become a considerable threat to individual and public health. Researchers indicate that influenza can be transmitted directly and indirectly, and aerosol particles of various sizes exemplify one of the most convenient modes of transmission (Noti et al., 2012). Besides, respirable particles carrying influenza virus can be airborne for prolonged periods of time, and toilet environments can be particularly conducive to their quick penetration into the lung alveoli (Noti et al., 2012). Still, until present, only one resource has provided useful information on influenza and the risks of infection through toilet flushes (Noti et al., 2012). Much more frequent are the analyses of gastroenteritis, norovirus, and similar infectious diseases that are transmitted through public spaces.
Gastroenteritis and the Potential of Toilet Flush Transmissions
The current literature on indoor bioaerosol dynamics suggests that norovirus and gastroenteritis remain the most common example of agent and illness potentially associated with aerosol generation from toilets and other indoor sources. A distinctive feature of both norovirus and gastroenteritis is that the occupants of the same indoor environment get sick within a very short time period (Nazaroff, 2011). Despite the dramatic advancements in medicine, gastroenteritis remains quite prevalent in the developed world, which further supports the urgent need to explore the role of toilet aerosol generation in transmitting these airborne infections. Yet, even in the presence of considerable empirical evidence, few studies confirm that aerosol generation is related to the transmission of norovirus (Nazaroff, 2011). Furthermore, the relationship between aerosol generation and the risks of getting infected can be potentially moderated by the number and size of droplets contained in the fecal masses and vomiting of the sick individual (Nazaroff, 2011). Still, other research findings confirm the important role of toilet surfaces and flushes in transmitting the norovirus gastroenteritis disease.
Such results were shared by Jones, Kramer, Gaither, and Gerba (2007). The researchers investigated the patterns of norovirus gastroenteritis transmission in three groups of houseboaters living on a large recreational lake. Jones et al. (2007) analyzed interior board surfaces, as well as onboard toilet reservoirs. 83% bathroom samples were found to have fomite contamination, compared to only 40% of kitchen surface samples (Jones et al., 2007). It is bathroom surfaces that serve as a chief source of infectious contamination in indoor spaces. Moreover, gastroenteritis remains one of the most commonly spread infectious diseases through toilets. This topic was further expanded by Barker and Jones (2005), who referred to infectious diarrhea as an infection carried through bowl water of domestic toilets. Again, toilets were proved to be the major source of airborne microorganisms leading to the outbreak of infectious diarrhea within a short time period. Here, only Barker and Jones (2005) measured the presence of virus-laden aerosols and bacteria in the air within the toilet cubicle, whereas Jones et al. (2007) explored only the impacts of bioaerosolization on the interior surfaces. Due to the differences in methodology, these results can be difficult to compare.
Clostridium Difficile
Clostridium difficile often becomes a relevant object of analysis in the context of infectious diseases. However, until present, only Best, Sandoe and Wilcox (2012) explored the role of toilet flushing in airborne dissemination of C. difficile. Best et al. (2012) suggest that public toilets in hospitals and other medical facilities vary greatly in their nature and size, but in almost all cases, patient toilets operate as shared entities and do not carry any lids. However, the most common strategies to reduce the risks of airborne hospital infections rarely include the analysis of toilet flushing problems. More often than not, control measures to avoid the rapid dissemination of C. difficile include barrier methods, hand hygiene procedural compliance, isolation of infected patients, as well as adherence to disinfection and environmental cleaning policies (Best et al., 2012).
Best et al. (2012) found C. difficile to be present in the air samples collected up to 25 cm above the patient toilet. The researchers conducted an in-situ analysis with the help of bacteria-laden fecal suspensions to measure the degree of splashing as a result of flushing in toilet hospitals (Best et al., 2012).The experiments were organized to measure bioaerosol contamination both in the air and the environment after the toilet was flushed (Best et al., 2012). The method involved the use of an air sampler, and six settle plates were used for 24 hours before the experiment to identify the microbiological status of the environment (Best et al., 2012). The highest were the concentrations immediately after toilet flushing (Best et al., 2012). Best et al. (2012) concluded that shared toilets without lids increased the risks of infectious diseases caused by C. difficile. As a result, C. difficile turned into one of the major infectious agents spread through toilet flushing, while the latter, again, was confirmed to facilitate the dissemination of infections through aerosolization of microorganisms.
Other Infections, Including S. Epidermidis
Toilet flushing also increases the risks of many other infectious diseases, including S. epidermidis. Jackson et al. (2010) conducted an empirical study of toilets and their potential contribution to the rapid transmission of infectious diseases. The results showed that aerosolization of fecal matter in toilets could raise significantly the risks of infectious disease transmission (Jackson et al., 2010). Actually, aerosolization of fecal matter alone proved to be a sufficient driver of transmission (Jackson et al., 2010). Certainly, it is not always that airborne transmission creates a favorable environment for the development of infectious diseases, but it is a primary vector for bacteria and viruses that increase the risks of infectious disease outbreaks (Jackson et al., 2010). The study findings were supported by empirical results and can be regarded as valid. Here, one of the most interesting is the way, in which researchers explore the topic.
Exploring the Role of Toilet Flushes in Aerosol Generation: Methods
In their study, Wallis et al. (1985) reported a simple and effective method for detecting viruses in aerosols. The proposed model was developed specifically to recover human enteric viruses from aerosols (Wallis et al., 1985). The researchers used filterite filters and glycine buffer to absorb the aerosolized virus (Wallis et al., 1985). As a result, even with the air flow exceeding 100 liters per minute, not a single virus managed to pass the filter (Wallis et al., 1985). The results of the experiment provide useful hints as to how the level of aerosol generation after toilet flushing could be checked.
Much more advanced was the method proposed by Johnson and Lynch (2008): the researchers tested an analytical method to be used for particle counting. The method was proposed to evaluate airborne infectious and isolate them with the help of fluorescent microscopes (Johnson & Lynch, 2008). The importance of the study was justified by the need for developing an effective, simply, and safe technique to measure low concentrations of particles in bioaerosols (Johnson & Lynch, 2008). As a result, the technique proved to be effective in estimating the number of surrogate bioaerosol particles; the results confirmed that the proposed technique could be used to calculate the number of particles on the filter and their containment efficiency (Johnson & Lynch, 2008).
Unfortunately, even with abundant literature on the topic, the problem of measuring bioaerosols after toilet flushing continues to persist. More importantly, not all medical entities realize the scope of the infectious dangers presented by toilet flushing. Aerosol generation in toilets is an ever expanding area, and this methodology has the potential to expand the current knowledge of the field.
Materials and Methods
Key Variables
The purpose of this study was to develop and test an apparatus and method for measuring aerosol generation associated with toilet flushing. The instruments included: a clear cylindrical 4 feet tall, 18 inches wide plastic chamber with HEPA filtration system, an Aerodynamic Particle Sizer Spectrometer, a HEPA Allergen remover and Renegair® pump. The method consisted of installing a chamber on two residential toilets. The proposed method was developed to test the researchers’ ability to seal the chamber tight enough to keep particle counts low, as well as to be able to detect potentially small increases in concentration associated with flushing. The noise had to be minimized, in order to detect a small signal.
The experiment was conducted to estimate the relationship between the levels of particle concentration after flush and compare them to the levels of particle concentration without flushing the toilet. Therefore, the toilet flush served as an independent variable, with the dependent variable represented by particles concentration. The relationship between the two variables was measured in the second experiment, whereas the first experiment was designed simply to confirm that toilet flushes result in aerosol generation.
Instrumentation
- Holmes® True | HEPA Allergen Remover. The instrument was used in the bathrooms involved in the experiment to reduce the concentrations of particles in the background. The use of the instrument was justified by its proven effectiveness in removing 99.9% of airborne allergens from the air that is passed through the filter. The device has the capacity to keep the air cleaner during 16 hours in a row.
- Model 3321 Aerodynamic Particle Sizer® Spectrometer. This model of the spectrometer is well-known for its high performance in measuring aerosolization in various settings. According to TSI (n.d.), the APS allows measuring and calculating particles, whose size range from.5 to 20 micrometers. The light-scattering technique is used to detect the particles sized between.37 and 20 micrometers (TSI, n.d.). The choice of the instrument was justify by its value in detecting particles’ aerodynamic diameter in real time (TSI, n.d.).
Setting and Procedure
The experiment was conducted in two half and one full residential standard bathrooms. Both bathrooms were equipped with 1.6 GPF (6.0 LPF) toilets, which released the same amount of water during flushing. Figures 1.0 and 1.1 provide a detailed view of the toilets.
The process of the experiment was divided into two different stages. The first stage was primarily intended to confirm that toilet flushing resulted in aerosol generation. As a result, the first stage of the experiment was intended to validate that assumption. To meet the goal of the study, all bathroom floors and surfaces were cleaned thoroughly with the basic Lysol bathroom cleaner. A Holmes® True | HEPA Allergen Remover was used to reduce the amount of particles in the background. Figure 1.2 shows the location of the allergen remover in the bathroom.
A clear cylindrical air-tight plastic chamber was designed specifically for this study (Figure 1.4). Subsequent to evaluating the general room air, the air filter chamber was mounted on the rim top of the toilet bowl.
The chamber is 4 feet tall, 18 inches wide, and has HEPA air intake filters on opposite sides. On the top of the chamber, a HEPA exhaust/vacuum filter is located; its task is to evacuate the particles in the chamber and toilet adaptor at the bottom (figure 1.6). The chamber also contains a sampling port that is connected to the APS monitor with the help of a tube, 1 inch in diameter and 40 inches long. This tube is used to facilitate the collection of particles in the chamber. Another tube, 100 inches long, was connected to the Renegair® regenerative pump, Nodel A37A001 [J111KX] which, in turn, was connected to the exhaust port of the chamber to evacuate the air out of the chamber and, therefore, minimize the concentration of particles in the chamber (figures 1.7 ).
During the experiments, both bathrooms were sealed with duct tape. The Holmes® Allergen Remover was switched on to run for 30 minutes before flushing the toilet to establish low background levels of aerosols. Three toilet flushes were initiated, each with a one-minute interval. Aerosol generation following each flush was measured using direct reading TSI Aerodynamic Particle Sizer (APS), model 3321. The instrument was used to measure the number and size of the particles in the general bathroom air that were generated by the flush.
The Renegair® pump has an inlet and outlet mechanism, and its air flow was 141 liters per minute (figure 1.6).
In the experiment, the pump was used to filter the air out of the chamber, while the TSI Particle Sizer® was collecting and measuring particles. Both processes were run simultaneously. Medium duty white caulk gum was also used to serve as a gasket between the toilet bowl rim and the chamber and prevent any air leakages during the experiment.
Sample Collection
Three of the four experiments were conducted in the full bathroom (toilet one). The last, fourth experiment was conducted in the half bathroom (toilet two). The Holmes® True | HEPA allergen remover and the Renegair® pump were left functioning in the bathroom for two hours before the samples were collected. Both instruments were running during the entire experiment. The particle sizer (TSI APS) was set to collect particle data every minute, during 60 minutes (1 hour). Every ten minutes, the toilet was flushed manually. All experiments were intended to measure particle counts generated by the toilet flush and compare them to the particle counts generated without any flush. In each experiment, six flushes were used, and the rates of particle decay were measured manually after each flush. The graph below shows the chamber that was used during the experiment.
Toilet Seeding
Toilet seeding experiments were designed to imitate the environmental conditions associated with human excretion. Similar experiments were set up by Barker and Bloomfield (2000) in their analysis of Salmonella in bathrooms and household toilets following salmonellosis. The toilet was seeded with 0.05µm polystyrene beads, in order to compare the results of the experiment to the results without seeding.
Data Analysis Procedures
Once the data were collected, statistical analyses were used to determine the preliminary results. Basic Excel and Stata were used to analyze the variations in aerosol concentrations across the chamber before and after toilet flushing. The choice of Stata was justified by the fact that it is a “versatile program aimed at data management, statistical analysis, and graphics for research” (Juul, 2006, p.xv). In this study, two sample t-tests with unequal variances were used, and the mean differences between total counts of flush-generated particles and particles without flush were quantified. Individual particle sizes were also calculated and evaluated, in order to estimate variations in concentrations for each particle size below 0.523µm up to 1.98 µm.
Results
A total of three bathrooms were involved in the experiment, but only the data on two bathrooms were used for the analysis. The data obtained during the first stage of the experiment were not analyzed further, as no distinction between the concentration of particles with and without a toilet flush was detected. In the second stage of the experiment, the following analyses were performed.
Experiment 1, Toilet 1
A total of 60 samples were analyzed, and the data confirmed that toilet flushing generated aerosols. In this experiment, total particle size concentration after flushing was measured and compared to the same data, but without a flush. The results showed that, during toilet flushes, total particle concentration increased dramatically. More specifically, mean particles concentration in the 60 samples collected after toilet flushing was 133.4 particles/cm³ with standard deviation 72.2, compared to only 42.9 particles/cm³ with standard deviation of 7.8 without any flushing (figures 2.3, 2.4, Appendix 1).
In this experiment, a considerable increase in individual particle size was observed, as well as the growing number of particles sized less than 2.0µm. The greatest was the number of the particles less than 0.523µm. Mean concentrations of these particles reached 36.1 particles/cm³ with standard deviation of 1.5 after flushing the toilet, compared to only 21.6 particles/cm³ with standard deviation of 0.70 without flushing (figures 2.5 and 2.6, Appendix 1).
As the size of particles increased, their concentration decreased: the concentration of particles sized 1.98µm was only 1.9, compared to 36.1 for the particles less than 0.523µm in diameter (figures 2.1, 2.2, and 2.7-3.4, Appendix 1).
Experiment 2, Toilet 1
In the second experiment, the toilet was seeded with 0.05 µm polystyrene beads to simulate human excretion. During and after toilet flushing, a considerable increase in the number of particles was observed, compared to no flushes. Mean total particle count during and after flushes was 177.9 particles/cm³ with standard deviation of 162.3 compared to only 50.1 particles/cm³ with standard deviation of 14.9 without flushes. Again, it is the particles sized 0.523µm and less that showed the greatest quantity increases. The mean results for particle counts with seeding were much higher than without it. After flushing the seeded toilet, mean particle counts reached 177.9 particles/cm³ compared to 133.6 particles/cm³ after flushing the toilet, which was not seeded. The main differences were observed in the number of particles sized less than 2.0µm (figures 3.5-4.8, Appendix 2).
Experiment 3, Toilet 1
The third experiment was conducted in toilet one, and the results were similar to those obtained in Experiment 1 without seeding. Total particle concentration means in the third experiment were similar to those obtained in Experiment 1, although counts for individual particle sizes in Experiment 3 were slightly lower than in Experiment 1. The patterns of changes observed in the third experiment were similar to the first one (figures 4.9-6.2, Appendix 3).
Experiment 4, Toilet 2
The fourth experiment was conducted in a different bathroom, to ensure that the results of the experiment were generalizable to other settings. In Experiment 4, no seeding was used. The mean for particle concentration after flushing was 107.6 particles/cm³ compared to only 30.1 particles/cm³ without flushing. Without flushing, particle concentration was 3.5 times lower than after flushing the toilet. The greatest were the changes in concentrations for the smallest particles. For the particles less than 0.523µm in diameter the mean was 27.0 particles/cm³ compared to 0.85 particles/cm³ for the particles sized 1.98µm (figures 6.3-7.7, Appendix 4). These patterns were observed in all four experiments.
Discussion
Overall, the results of these experiments confirm the earlier results reported by Johnson et al. (2013). However, the study was the first of its kind to use TSI Aerodynamic Particle Sizer to monitor and evaluate changes in the number and sizes of particles with and without flushing. With the help of the APS monitor, the changes in behaviors across different-sized particles could be observed, depending on toilet flushing. All four experiments confirmed that toilet flushing led to an increase in particle counts, compared to when the toilet was not flushed. Similar results were reported by Johnson et al. (2013), who found that toilet flushing was directly related to aerosol generation. The greatest was the number of particles, when the toilet was seeded with 0.5 µm polystyrene beads (Table 1). The hypothesis that toilet flushing generates aerosols that may be home to viral or pathogenic bacteria was fully supported.
Table 1. Comparison of the total mean particle counts between experiments when the toilet is flushed versus when not flushed. (Experiment #2, toilet was seeded with 0.05µm polystyrene.)
One of the biggest questions raised during the study was in how changes in particle counts were related to their size. Statistically, more than 96% of the particles counted in this study were not more than 2µm in diameter. The highest concentrations were noted for the particles less than 0.523µm in diameter. Small particles were also noted to remain elevated for at least three minutes, before their concentrations returned to the pre-flush level (figure 7.7, Appendix 3). No notable increases in particle counts sized more than 2.0µm were observed. It is possible to assume that large particles are not produced significantly during flushes. It is also likely that they leave the chamber quicker than the particles of smaller sizes. Still, the most essential fact is that the size of the particles, whose concentrations were found to be the highest, is very similar to that of pathogenic bacteria and viruses. This finding alone suggests a high probability of raising infection risks through swallowing and inhalation during and immediately after toilet flushing.
The current state of research suggests that numerous bioaerosols generated during flush can remain in the air for much longer periods of time than earlier suspected. Consequently, individuals who use toilets are inadvertently exposed to higher risks of infections. For the particles sized 1µm and less, the risks of settling slowly or being airborne for a prolonged period of time are the highest. These assumptions are also confirmed by Nazaroff (2011), who found that aerosolized viral particles could be related to the outbreak of norovirus in school. Marks et al. (2003) also discovered that the time period between exposure to bacteria and the actual illness are consistent with being directly infected by aerosolized viral particles from the vomit masses in toilets. Toilet flushing can be readily responsible for generating other aerosolized pathogens, such as Shigella, E. coli, C. difficile, coronavirus, and SARS. Unfortunately, this study did not evaluate the changes in particle counts, depending on the intensity of flushing. Johnson et al. (2013) already reported that the mean numbers of aerosolized particles increased with flushing energy. Yet, these questions can readily become a good basis for future studies.
Limitations and Recommendations for Future Studies
One of the most considerable design limitations is in that the experiments were conducted in only three household toilets. For this reason, the generalizability of the experimental results to other toilet settings may be limited. Also, the chamber was not effective enough to reduce particle background concentration to a zero particle concentration. Another possible limitation is in the air that was pumped in and out of the chamber and, for that reason, could have contribute to changes in aerosolized particles counts during flushes. Finally, the use of polystyrene beads to imitate seeding could also be responsible for an increase in the number of particles in the chamber, since the latter had no separate port for seeding introduction.
It should be admitted that the mere finding that toilet flushes generates pathogenic aerosols does not provide any valuable information as to how these processes could be controlled. This study was not intended to reinforce the vision of dangers in public toilets; rather, it was designed to introduce and test the use of new equipment for assessing aerosolization following toilet flushes. This being said, future studies will have to use this methodology as a base to evaluate toilet aerosol generation. Another recommendation for the future study is to understand the role of droplets size as a moderating factor of person-to-person transmission risks. Nazaroff (2011) writes that “the number and size of droplets as well as the extent to which they contain NoV particles are crucially important factors that could affect person-to-person transmission risks” (p.355). Future studies have a huge potential to inform feasible real-life decisions to improve the state of hygiene in household and public toilets.
Conclusion
The purpose of the study was to develop and test a methodology for evaluating toilet-flush aerosol generation. The results confirm that toilet flushing leads to aerosolization of particles, such as microorganisms, which in turn can have potentially negative impacts on humans who are exposed. The results contribute and add to the current knowledge of bioaerosol generation and create a firm basis for developing new guidelines and protocols to minimize the risks of infectious disease transmissions through bioaerosols generated by toilet flushing.
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Appendix 1
Experiment #1, Toilet 1
Appendix 2
Experiment # 2, Toilet 1 Flushes were treated with Polystyrene beads of 0.5µm
Appendix 3
Experiment # 3, Toilet 1. Flushes not treated with Polystyrene beads
Appendix 4
Experiment # 4, Toilet 2. Toilet flushed every ten minutes without polystyrene beads
