The Risk of Exposure to Infectious Bacterial Bioaerosols in Different Hospital Wards: A Case Study

Background: Biological aerosol particles smaller than 10 microns in diameter are among the health concerns in hospitals since they remain in the air for a long time and are infectious and easily transported. We aimed to investigate the concentration of Escherichia coli and Staphylococcus aureus bioaerosols and evaluate their risk in the indoor environment of different wards of Khatam al-Anbia hospital, Jask, Iran, in 2020. Methods : This descriptive-analytical study was performed by collecting and analyzing 50 samples from seven different hospital wards. The active sampling of bioaerosols was performed according to the standard method of the National Organization for Occupational Health and Safety and by a pump with a flow rate of 28.3 L/min for 10 minutes. Blood agar and eosin methylene blue were used to detect bacteria. Then, the Monte Carlo simulation technique was used to assess the microbial risk. Results: The concentration of S. aureus in different wards of the hospital was 4.81 to 18.11 CFU/m 3 . The lowest and highest concentrations of S. aureus were in the operating room and general emergency wards, respectively, while the highest and lowest concentrations of E. coli were in the inpatient wards (0 CFU/m 3 ) and infectious emergency ward (21.22 CFU/m 3 ), respectively. The highest and lowest daily risk of S. aureus was observed in the neonatal and general emergency wards (8.03×10 -4 and 3.02×10 -4 ), respectively. Moreover, the lowest and highest daily risk of E. coli was found in the neonatal and male inpatient wards (zero and 7.21×10 -3 ), respectively. Conclusion : In some hospital wards, the concentration and infection risk of E. coli and S. aureus were found to be higher than the acceptable value. Since high concentrations of airborne bacteria can play an important role in producing nosocomial infections in patients and staff, it is necessary for hospital officials to take corrective measures in equipment control, use proper ventilation systems in the wards, and closely monitor the disinfection process.

Hormozgan Med J . Vol 27, No 2, 2023 74 hmj.hums.ac.ir http between 0.1-0.5 μm, that remain in the air for a long time, are easily transported, and cause disease because of their small size (9,10). It is estimated that bioaerosols, especially bacterial and fungal bioaerosols of biological origin, are responsible for about 5%-34% of air pollution in various environments such as hospitals, dental centers, shopping malls, subway stations, public libraries, and other workplaces (6,10). Bioaerosols enter the hospitals through various routes such as air conditioners, visitors, and patients. The density of bioaerosols varies from department to department and from hospital to hospital in a specific city or geographical area. Numerous physical and environmental factors can affect the number and type of bioaerosols in hospital settings (6,9,10). Inhalation is the main route for exposure to bioaerosols, respiratory infections, and their symptoms, and decreased lung function is the most important health complication of bioaerosol exposure (6).
Quantitative microbial risk assessment is a useful tool for estimating the health risks of human exposure to pathogens in various environmental settings (11). Probabilistic risk assessment is performed by Monte Carlo simulations in which sequential and random sampling is performed based on the cumulative distribution function of each input variable. The simulation results are expressed as the probability of infection, disease, or death (12).
Considering the importance of biological air quality in hospitals and given few studies conducted in the field of microbial risk assessment of hospitals (7), we aimed to evaluate the risk of exposure to Escherichia coli and Staphylococcus aureus in different parts of Khatam Al-Anbia hospital, Jask, Iran.

Sampling
This descriptive-analytical study was performed in Khatam Al-Anbia hospital in Jask affiliated to the Hormozgan University of Medical Sciences in 2020. This hospital covers a population of 58 884 individuals with 18 000 square meters of area and 64 beds. This study examined different wards of the hospital for the existence of E. coli and S. aureus bacterial aerosols and then calculated the risk of exposure to them. Sampling was performed in the general emergency, infectious emergency, dialysis, operating room, intensive care, inpatient, and internal wards, as well as the corridor, nursing station, and outside the hospital. In total, 50 samples were taken for identification using Anderson single-stage bioaerosol sampling pump (SKC, USA). For this purpose, a Biostage was installed on the inlet pipe of the pump, and a plate containing the culture medium was placed in the Biostage. The flow rate of the sampling pump was calibrated (Digital calibrators, Defender) before sampling. The Biostage was placed at the height of 150 cm above the ground and a distance of more than one meter from the walls and windows (13). Then, the air was pumped with a certain flow rate (28.3 L/min) and passed through the culture medium for 10 minutes (13).

Identification of Bacteria
This study used blood agar and eosin methylene blue culture (Merck Conda, Spain) medium for bacterial colony growth. To this end, 40 g of blood agar powder was dissolved in 1000 mL of distilled water and sterilized at 121°C and 15 Pa pressure for 45 minutes. Then, under completely sterile conditions, 50 mL of defibrinated blood was added, and the culture medium was kept upside down in the refrigerator until use. After sampling, the plates containing the culture medium were incubated at 37°C for 48 hours (13). The formed colonies were counted using the CW-HPC400 (B) colony counting machine (Laser Counter Device China Way), and the bacterial concentration was calculated in CFU/m 3 according to the flow rate and sampling time. For differential diagnosis of bacteria, hot staining methods and biochemical detection methods were used, including catalase, oxidase, coagulase, urease, citrate test, antibiotic resistance of novobiocin, and bacitracin (7).

Quantitative Microbial Risk Assessment
The quantitative microbial risk assessment consisted of two steps:

Exposure Assessment
This step measures the amount, frequency, and duration of exposure to the target organism and describes the number and characteristics of the exposed population (14). Daily exposure (d) to bacteria can be calculated by the following equation (15): where EC is the exposure concentration (CFU/m3), BR is the adult respiration rate (lognormal distribution: mean= 0.58 and SD= 0.22 m 3 /hour), T is the exposure time (8 hours), and AG is the aerosol swallowing rate (uniform distribution: min=10 and max=50%). The route of exposure to bioaerosols containing gastrointestinal pathogens is assumed to be a combination of inhalation and ingestion because inhaled pathogens can be accumulated in the upper respiratory tract and then ingested. A uniform distribution of 10%-50% was considered for AG due to the uncertainty and variability associated with this type of exposure (15).

Dose-Response Evaluation
Dose-response evaluation can be defined as the quantitative relationship between dose and response, which is generated after an exposure time and according to the level of exposure. The β Poisson model was used where P id denotes the daily risk of infection, N 50 is the average infectious dose, α refers to the infectious factor, and d is the exposure rate. The values of α and N 50 for E. coli are 0.155 and 2.11 (10 6 ), respectively (16).
Furthermore, the following exponential model was used to estimate the risk of S. aureus.
The calue of k for S. aureus is 7.64 (10 -8 ) (16). Furthermore, the annual infection risk (P id ) was calculated using the following equation (17): where d is the number of days a person is exposed to a microbial contaminant, and the P50 and P90 were applied to compare the infection risk with acceptable limit.

Monte-Carlo Simulation
The constant value of the parameters in the models causes uncertainty in the calculated risk. To overcome this problem, the Monte-Carlo simulation with 10 000 replications was used. The Monte-Carlo simulation technique selects the parameter value within the specified range and then calculates the response. These iterations eliminate the uncertainty and variability of the parameters (18). Therefore, the obtained results are more reliable and valuable than the results calculated by the point estimation method. The simulation was performed using Oracle Crystal Ball software.

Results
In this study, the concentration of bioaerosols is based on CFU/m 3 , and the daily and annual risks of E. coli and S. aureus are presented in Tables 1 to 5.

Discussion
According to Tables 1 and 2, bacterial concentration and the daily and annual risks of E. coli and S. aureus were not at the standard level of 10 -4 in the infectious emergency, intensive care, general emergency, operating room, and men's admission wards (16). However, E. coli and S. aureus and their subsequent risks were at the standard Note. SD: Standard deviation. The indoor air of medical centers contains a mixture of fungal, bacterial, viral, and allergenic bioaerosols that originate from a variety of sources. These sources include outside air, staff and patients (when talking, coughing, and sneezing), ventilation systems, toilet flushing, and cleaning activities. Hospital staff, visitors, and patients who are exposed to bioaerosols during their stay in hospitals and those with weaker immune systems are at higher risk of possible infections (19). Using the Monte-Carlo relationship, Adhikari et al found that the mean daily risk of exposure to S. aureus and E. coli was 1.33×10 -8 , 1.18×10 -8 , 6.36×10 -9 , and 2.73×10 -8 for nurses, healthcare workers (e.g., physicians), visitors, and other patients in public rooms, respectively (20).
In the general emergency ward, the highest concentration of S. aureus was 18.11 CFU/m 3 . The reason for this is the high rate of admission and the subsequent crowdedness of this ward. According to the WHO guidelines, the acceptable level of bacteria in the air of the general ward of the hospital is 100 CFU/m 3 (21). One study reported that the highest concentration of S. aureus was observed in the general ward (28.85%), which is consistent with our results (22).
In the operating room, the lowest concentration of S. aureus was 4.81 CFU/m 3 . This is related to the sensitivity of this unit and constant monitoring of accurate disinfection of surfaces and equipment in the operating room. According to the Environmental Protection Agency, an amount of 30 to 500 CFU/m 3 is permitted for operating rooms (23). In a study in Sri Lanka, the concentration of S. aureus bacterium in the operating room was 0.84 CFU/ m 3 , which is in the standard range and consistent with our study (24).
In the neonatal ward, the concentration of S. aureus was 17.82 CFU/m 3 , which is consistent with the recommended standard. This result was attributed to limited human traffic in the ward, the use of a proper ventilation system, and the low number of visitors. In another study, the highest concentration of bacteria was reported in the intensive care unit (ICU) because of poor environmental factors such as temperature, humidity, light, external factors related to all health workers (doctors, nurses, and other staff) and other patients and visitors, as well as controlling the conditioning equipment (heating, ventilation, and air conditioning) in the ICU, which is not consistent with our study (25).
Moreover, the highest daily risk of S. aureus was observed in the neonatal ward (8.03×10 -4 ) and the general emergency ward (3.02×10 -4 ). This is attributed to the presence of mothers, opening the windows, lack of proper disinfection, crowdedness, and the presence of critically ill patients in the wards. Mirzaei et al reported a bacterial risk of 1.03×10 2 in the general emergency departments (26). However, the concentration in the operating room was 6.33×10 2 CFU/m 3 (13). Hoseinzadeh et al reported moderate levels of bioaerosol concentration in hospital wards 1.6×10 2 CFU/m 3 (27). The results of a study in India showed that the bacterial bioaerosol concentration is in the range of 3.7×10 2 to 1.9×10 5 CFU/m 3 (28). Bielawska-Drózd et al reported that the concentration of bacterial bioaerosols in the health emergency department is 1.3×10 2 to 4.2×10 3 CFU/m 3 (29).

Conclusion
In this study, the concentration of S. aureus and E. coli bacteria in the air of different wards of a hospital in Jask was measured. In addition, the risk of infection due to contact with these bacteria through the air was determined. In some hospital wards, the concentration and infection risks of E. coli and S. aureus were found to be higher than the acceptable value. Since high concentrations of airborne bacteria can play an important role in creating nosocomial infections in patients and staff, it is necessary for hospital officials to take corrective measures to control equipment and use proper ventilation systems in the wards. Furthermore, it is essential to strictly monitor the disinfection process and control the movement of people, especially the patient's companions in different wards and wards with high susceptibility to nosocomial infections (e.g., operating rooms, ICUs, and pediatricians).