3.1 Bioaerosols in Indoor Air
3.1.1 Initial Concentration of Bioaerosols
The initial concentrations of bacteria and fungi before the disinfection treatment were shown in Table 1. Average temperature, relative humidity, airflow velocity, carbon dioxide, PM1, PM2.5, PM7, and PM10 during the experimental periods were 18.9 to 24.9°C, 62.0% to 80.6%, 0.3 to 1.2 m/min, 398 to 1,298 ppm, 0 to 1.12 mg/m3, 0.001 to 1.170 mg/m3, 0.002 to 1.175 mg/m3, and 0.002 to 1.179 mg/m3, respectively. The magnitude of these environmental factors represents the typical conditions of the indoor air in the fitness center. It is necessary to note that the experiments were conducted at existing conditions and different times; hence, the initial concentrations of bacteria and fungi in each experiment were different. Therefore, the determination of whether an indoor environment is polluted or not is often based on a comparison of bioaerosol concentration in the indoor air with that of outdoors (Zhu et al. 2003). It can be seen that the indoor air tended to contain less bacteria and fungi than the outdoor air; nonetheless, there was no specific correlation. This is possibly due to the bioaerosol filtration effect via air conditioning system of the fitness center and the activities happened prior to the tested session.
Table 1. Initial concentration of bacteria and fungi (mean±SD)
|
Indoor
|
Outdoor
|
|
ClO2
|
WAHW
|
Water
|
ClO2
|
WAHW
|
Water
|
Initial concentration of bacteria (CFU/m3)
|
0 people
|
401±179
|
249±65
|
308±231
|
855±41
|
370±86
|
613±163
|
5 people
|
761±169
|
470±238
|
433±172
|
953±136
|
616±203
|
498±124
|
10 people
|
715±153
|
619±259
|
812±111
|
765±403
|
684±591
|
879±83
|
Initial concentration of fungi (CFU/m3)
|
0 people
|
226±39
|
228±53
|
618±499
|
1203±603
|
873±460
|
1064±139
|
5 people
|
837±838
|
275±212
|
307±16
|
1734±580
|
738±174
|
908±706
|
10 people
|
492±278
|
255±133
|
613±319
|
1220±156
|
465±108
|
764±677
|
3.1.2 Effect of Environmental Factors on Bioaerosols Distribution
Table 2 shows the correlations between the bacteria and fungi colony counts and the environmental factors after the disinfection treatment process. Significant positive correlations were found between the effectiveness of bacteria disinfection and the effectiveness of fungi disinfection (r = 0.610, p < 0.01). By contrast, significant negative correlations were obtained between the effectiveness of bacteria disinfection and the number of users (r = -0.584, p < 0.05), carbon dioxide concentration (r = -0.581, p < 0.05). Significant negative correlations also existed between the effectiveness of fungi disinfection and the number of users (r = -0.690, p < 0.01), temperature (r = -0.517, p < 0.05) and carbon dioxide concentration (r = -0.680, p < 0.01). In other words, the effectiveness of bacteria disinfection increased along with the effectiveness of fungi disinfection but decreased with an increasing number of users and carbon dioxide concentration. Similarly, the effectiveness of fungi disinfection decreased with an increasing number of users, temperature, and carbon dioxide concentration. Other factors including relative humidity, airflow velocity, and particulate matters had no statistically significant impact on both bacteria and fungi inactivation.
3.1.3 Effectiveness of Bioaerosols Disinfection
The effectiveness of bacteria and fungi disinfection by three different methods including ClO2, WAHW, and water were compared within treatment period of 15 and 60 minutes as shown in Figures 2 and 3, respectively. The results showed that all the methods had the ability to reduce bacteria and fungi present in the aerosols in the indoor air although water scrubbing which served as a control had the lowest capability in both bacteria and fungi removals. Hence, it implies that the removal of bacteria and fungi in the presence of ClO2 and WAHW was mainly derived from chemical interaction rather than physical phenomenon. The inactivation potential decreased as the number of users increased. It is interesting to observe that fungi can be inactivated in all treatment scenarios at 15 minutes of contact time but ClO2 had the highest efficiency. However, the number of fungi noticeably rebounded in the WAHW and water scrubbing treatments at 60 minutes. This regrowth behavior of fungi might be due to the ability of fungi to form spores to protect themselves from inactivation. Another factor might be due to an increase in relative humidity after prolonged aerosolization which could promote the enumeration of fungi (Rajasekar and Balasubramanian 2011). Figures 2 and 3 also illustrate that ClO2 seemed to be a better fungicide whereas WAHW seemed to be a slightly better bactericide than ClO2 for long time exposure.
According to Ishihara et al. (2017), the hypochlorous acid in aqueous solution is unstable against ultraviolet (UV) light, sunshine, air contact, and elevated temperature (≥ 25 ◦C); hence, ClO2 seems to be a better choice for bioaerosol disinfection. As shown in Table 1, bacteria and fungi disinfection effectiveness were not related to treatment time; hence, disinfecting the fitness room for 15 minutes after usage is a more appropriate option that can reduce both treatment time and cost. Within the ranges obtained during the experimental period, other compositions in the room including temperature, relative humidity, air flow velocity, PM10, PM7, PM2.5, and PM1 had no statistically significant effect on bacteria and fungi disinfection.
The results from this study showed that the effectiveness both of bacteria and fungi disinfection decreased with an increasing number of users, time, temperature, and carbon dioxide concentration. It was found that ClO2 seemed to be the most appropriate selection for bacteria and fungi control in the bioaerosols under the studied conditions. This is because ClO2 is a very powerful oxidizer that effectively eliminates pathogenic microorganisms including fungi, bacteria, and viruses (Lenntech B.V. 2020). Findings from this study are in agreement with several other studies using ClO2 to control the amount of bioaerosols in indoor air including student cafeterias (Hsu et al. 2014), library (Hsu et al. 2015); and pet shop (Lu et al. 2018). These studies recommended that treatment with ClO2 is an efficient way of compliance with the Taiwan EPA guidelines for indoor air quality. Nonetheless, this study found that airflow velocity had no significant effect on both bacteria and fungi removal which contrasts with the observations from Hsu, et al. (2014) and Lu, et al. (2018) who found that a higher air velocity was helpful for spreading the disinfectants through indoor space as well as to enhance disinfection efficiency accordingly. This might be because the minimum airflow rate of 0.3 m/min detected in this study was already sufficient enough to thoroughly spread the disinfectant solution in the room space. The results confirmed that ClO2 disinfection could sufficiently suppress the numbers of bacteria and fungi present in the bioaerosols to comply with the Taiwan EPA guidelines for the indoor air quality.
3.2 Bioaerosols on Sports Equipment
3.2.1 Initial Concentration of Microorganisms
Table 3 shows the initial concentrations of bacteria and E. coli before cleaning the sports equipment. The initial concentrations of bacteria and E. coli were different among each experiment because it is based on the number of people who exercised with the specific equipment namely bicycle handle, dumbbell, and sit-up bench in the fitness center prior to the testing period. It is necessary to note that the experiments were conducted at existing conditions and at different times.
Table 3. Initial concentration of bacteria and E. coli on sports equipment (mean±SD)
|
ClO2
|
WAHW
|
ZnO
|
Commercial disinfectant
|
Initial concentration of bacteria (CFU/cm2)
|
Bicycle handle
|
756±278
|
390±2
|
3720±736
|
1132±430
|
Dumbbell
|
684±232
|
1070±144
|
1596±418
|
860±260
|
Sit-up bench
|
820±28
|
816±210
|
840±566
|
1486±234
|
Initial concentration of E. coli (CFU/cm2)
|
Bicycle handle
|
990±146
|
1080±114
|
628±266
|
550±70
|
Dumbbell
|
786±50
|
700±28
|
732±118
|
1002±224
|
Sit-up bench
|
866±94
|
802±286
|
960±170
|
910±14
|
3.2.2 Factors Affecting Microbial Inactivation on Sports Equipment
Table 4 shows the correlations between the bacteria and E. coli disinfection and the related factors. Significant positive correlations were found between the effectiveness of bacteria disinfection and the effectiveness of E. coli disinfection (r = 0.437, p < 0.01) and types of equipment (r = 0.446, p < 0.01). Significant positive correlations also existed between the effectiveness of E. coli disinfection and types of equipment (r = 0.396, p < 0.01). In other words, the effectiveness of bacteria and E. coli disinfection depends on the surface characteristics of sports equipment. Sports equipment selected in this study had different surface characteristics: (1) the handles of bicycle is soft with foam-like surface, (2) the dumbbell is steel and has a rough surface and (3) the sit-up bench is covered with leather and has smooth surface. Other factors including time and types of the disinfectants had no statistically significant impact on both bacteria and E. coli inactivation.
Table 4. Correlations between inactivation effectiveness and environmental factors on sports equipment
|
% efficiency on bacteria disinfection
|
% efficiency on
E. coli disinfection
|
Types of Equipment
|
Disinfectant
|
% efficiency on E. coli disinfection
|
0.437**
|
|
|
|
Equipment
|
0.446**
|
0.396**
|
|
|
Disinfectant
|
0.092
|
0.009
|
0.000
|
|
Time
|
-0.097
|
0.040
|
0.000
|
0.000
|
** Correlation is significant at the 0.01 level (2-tailed).
|
3.2.3 Effectiveness of Bacteria and E. Coli Disinfection
Figures 4 and 5 show the efficiency of the four disinfectants in reducing bacteria and E. coli present on the sports equipment surface after applying for 2, 5, 10 and 30 minutes. The results show that all the disinfectants are highly effective in reducing bacteria and E. coli present on sports equipment (about 66.8-95.4%) even within a short period of time after application (2 minutes).
Figures 4(a) and 5(a) show that chlorine dioxide is highly effective in reducing bacteria and E. coli present on sports equipment (81.0-93.9%). It can be seen that ClO2 could remove bacteria and E. coli immediately after 2 minutes of application. Therefore, users can use this equipment straightaway after disinfection without a long-waiting period. According to Cho, et al. (2017), chlorine dioxide (ClO2) has emerged as an alternative because it has better antimicrobial effectiveness with a higher solubility, shorter response time, and wider pH range. Moreover, the inactivation effectiveness of ClO2 on both bacteria and E. coli was not significantly affected by surface characteristic variation among the sports equipment. Hence, it can be used as a common disinfectant for all equipment in the fitness center. WAHW appeared to be less slightly effective in bacteria and E. coli inactivation than ClO2 as illustrated in Figures 4(b) and 5(b). Required contact time for WAHW seemed to be 10 minutes which was much longer than those of ClO2. This observation is in agreement with the studies of Block and Rowan (2020) and Quan, et al. (2017) who stated that hypochlorous acid required at least 10 minutes of contact time to be effective for antimicrobials.
Zinc oxide is highly effective in reducing bacteria all of sports equipment as shown in Figures 4(c) and 5(c). However, it is more effective in reducing E. coli on bicycle handle and sit-up bench than dumbbell. Comparing to ClO2 and WAHW, it was found that ZnO has a slightly better performance on bacteria and E. coli disinfection.
Commercial disinfectant could inactivate bacteria and E. coli on sit-up bench better than bicycle handle and dumbbell as shown in Figures 4(d) and 5(d). The commercial disinfectant seems to be suitable for the sports equipment which has smooth surface like sit-up bench rather than rough surface as in the case of bicycle handle and dumbbell.
In summary, all disinfectants could remove bacteria and E. coli immediately after application. although the highest disinfection efficiency occurred at different contact times. Nonetheless, it implies that users can use these equipment straight-away after disinfection without a long waiting period. Chlorine dioxide and ZnO had comparable inactivation effectiveness regardless of sports equipment and surface roughness as shown in Figure 4(a) and 4(c), respectively. It is interesting to observe that WAHW and commercial disinfectant could disinfect total bacteria on the sit-up bench slightly better than handles of the bicycle and dumbbell which had rougher surface as shown in the Figure 4(b) and 4(d), respectively. The results also demonstrated that the effectiveness of disinfectants somehow depends on the surface characteristics of sports equipment as well.
The E. coli disinfection of all disinfectants are quite comparable although the commercial disinfectant showed noticeable less effectiveness on the handles of the bicycle as shown in Figure 5. Coupled with the findings from bioaerosol disinfection as reported earlier, ClO2 seems to be a better
choice for sanitizing the sports equipment since it was also found to be highly efficient for bioaerosol disinfection as well. Although ZnO and commercial disinfectant were effective for surface disinfection, they are not appropriate nor recommended to apply aerially for bioaerosol disinfection.
It is important to note that this study was carried out during the time of COVID-19 pandemic so the number of users in the fitness center had decreased drastically from normal period. According to Kalogerakis, et al. (2005), the most important source of airborne bacteria is the presence of people who come to exercise. Particular activities like talking, sneezing, coughing, walking, washing and toilet flushing can generate airborne biological particulate matter. Therefore, the initial concentrations of bacteria detected in this study were not as high as it should be under normal condition. Nonetheless, the effectiveness of each disinfectant on targeted microorganisms is still valid and can be further applied for normal condition with certain modifications.