Exposure of health care workers and occupants to coughed airborne pathogens in a double-bed hospital patient room with overhead mixing ventilation.
Hospitals are the places where the sick and the weakened are accepted to be cured and to recover. The medical staff, as well as the visitors and the patients in hospitals are experiencing elevated risk from infection with airborne infectious disease from contagious patients. Ventilation is a major way to control and reduce the spread of pathogens via the airborne route in hospital premises (Streifel 1999; Kaushal et al. 2004; Beggs et al. 2008). However, ventilation in hospital facilities may fail to successfully and fully evacuate the pathogens from the air and may even further increase their spread within the building envelope, thus contaminating more people (Li et al. 2007) and leading to hospital acquired infections (HAIs) (van der Wel et al. 1996; Menzies et al. 2000; Qian et al. 2006). Therefore, good ventilation design plays an important role for controlling the spread of airborne diseases (Streifel 1999; Kaushal et al. 2004; Beggs et al. 2008).
Different guidelines and standards have been established for the ventilation of health care facilities (ASHRAE 2008 [Standard 170-2008]; CDC 2005; etc.) prescribing the lowest ventilation equivalent of 12 [h.sup.-1] air changes per hour (ACH) for isolation rooms. The dispersion of airborne contaminants and the subsequent risk from airborne cross-infections and emergence of HAIs via the airborne route strongly depend such factors as air change rates (Kao and Yang 2006), air distribution pattern (Qian et al. 2006; Noakes et al. 2009; Tung et al. 2009b), and pressure difference with the surroundings (Decker 1995; Tung et al. 2009a). The distance between the infected patient (source of pathogens) and the medical staff member (doctor/nurse), as well as the mechanism of atomization of airborne particles (through speaking, breathing or coughing), is also of great importance. Under certain conditions (room layout, positioning of doctor and infected patient, etc.), these may even be more important than the ventilation rate itself. In an occupied space, the airflow pattern is a result of the complex interaction of different flows: the free convection flow around occupants (Homma and Yakiyama 1988; Zukowska et al. 2008), transient flows as a result of pulmonary activities (breathing, coughing or talking), buoyancy flows generated by heated sources, the positioning of the ventilation diffusers, etc.
The initial peak velocity of cough flow varies from 6 m/s (1181.1 fpm) up to 30 m/s (5905.51 fpm) (Edwards et al. 2004; Zhu et al. 2006; Sun and Ji 2007). Therefore, the coughed flow produced by infected patient becomes one of the dominating transport mechanisms indoors for spreading contaminated airborne particles (Morawska 2006). Though the cough has been studied and effort has been made to describe the cough flow dynamics in detail (Mahajan et al. 1994; Singh et al. 1995; Gupta et al. 2009), not much has been done to study the complex airflow interaction in hospital rooms and its impact on the occupants' exposure. The exposure will also depend on such factors as proximity to the source patient (coughing patient), amount of air supplied to the space, posture of the doctor or patient, etc. Hence, the strategy of supplying extra amounts of outdoor air aiming to dilute the polluted room air may not always be effective in protecting from airborne cross-infection due to coughing.
The exposure of a doctor and a patient to the air coughed by a second infected patient was studied in a mock-up of a two-bed hospital isolation room with mixing ventilation and presented in the current article.
Experiments were designed and performed in a full-scale room with dimensions 4.65 m x 4.65 m x 2.60 m (15.26 ft x 15.26 ft x 8.53 ft) (W x L x H) furnished to simulate a hospital isolation room with two beds. The distance between the beds was set at 1.3 m (4.27 ft). Five ceiling-mounted light fixtures, 6 W (20.5 Btu) each, provided the background lighting. The room was located in a tall hall, where the temperature was kept constant and equal to the air temperature in the test room. A heated dummy, 60 W sensible heat (204.7 Btu), with simplified body geometry, equipped with a coughing machine was used to simulate a coughing sick patient lying in one of the beds. The characteristics of the cough were
volume peak flow: 10 L/s (21.2 cfm),
volume of the cough: 2.5 L (11.3 [ft.sup.3]),
cough time interval: 0.5 s, and
maximum cough velocity: 28.9 m/s (5689 fpm).
The mouth of the coughing patient was a circular opening (diameter of 0.021 m [0.069 ft]). A second heated dummy, giving same heat load as the first, was used to simulate a patient lying in a bed aligned with the coughing patient. A dressed breathing thermal manikin (1.02 Clo, cotton T-shirt, medical cotton apron, cotton trousers, underwear, socks, and thin soled shoes; ISO Standard 9920 [ISO 2006]) with realistic human body size, shape, and surface temperature distribution was used as a "doctor" (Melikov 2004). The thermal manikin maintained the same sensible heat as that released by a healthy average person in a state of thermal comfort; under the studied conditions, this corresponded to 60 W (204.7 Btu) sensible heat load. The layout of the setup is shown in Figure 1.
The mixing type of air distribution was used to condition the air in the room. The air supply diffuser (a four-way diffuser) and the two exhaust diffusers (perforated square diffusers mounted above the heads of the patients) were installed on the ceiling. The supply was also set with a plenum box unit. According to the manufacturer, the air supply diffuser can be used in the range 35-350 L/s (74.2-741.6 cfm). The supply flow rates tested were well within the range specified by the manufacturer, i.e., 47 L/s (99.3 cfm), corresponding to 3 [h.sup.-1], 94 L/s (198.5 cfm) for 6 [h.sup.-1] and 188 L/s (397.1 cfm) for 12 [h.sup.-1]. The supply and exhaust air diffusers were positioned after carefully considering the recommendations for hospital room air distribution to create sufficient mixing and achieve good dilution, as suggested in CDC and HICPAC (2003) and ANSI/ASHRAE Standard 170-2008 (ASHRAE 2008). The exhausted air was equally balanced between the two outlets. The supplied air was 100% outdoor air (no recirculation was used). A slight under-pressure of 1.6 [+ or -] 0.2 Pa (0.48 [+ or -] 0.06 in. of Hg) was kept during all experiments in order to avoid leaking of air from the test room into the tall hall. The supply air temperature and the supplied and exhaust air flow rates were continuously controlled to keep the set values defined for each of the tested conditions.
[FIGURE 1 OMITTED]
Experiments were performed at three air-change rates: 3, 6, and 12 [h.sup.-1]. Under all studied conditions, the room temperature was kept at 22[degrees]C (71.6 F), while the relative humidity was not controlled but was measured to be between 35% and 45% during all experiments. The coughing patient was either lying on one side and facing the doctor or on its back looking at the ceiling (Figure 1). Measurements were performed when the doctor was positioned downstream from the coughing patient at 0.55, 1.1, and 2.8 m (1.8, 3.61, and 9.19 ft) (points C, D, and E in Figure l a). When the doctor was either 1.1 m or 2.8 m (3.61 ft or 19.9 ft) away from the "sick" coughing patient, only the case with coughing sideways toward the doctor was studied. In the experiments studying the exposure of the second patient, under the three different ventilation levels, the thermal manikin used to mimic the doctor was placed lying in the neighboring bed with head turned sideways and facing the coughing dummy. In these sets of experiments, the doctor was assumed to be out of the room. This resulted in lower heat load generated in the room compared to the case when the doctor was inside. To keep the same room temperature at 22[degrees]C (71.6[degrees]F), the supply air temperature was elevated by maximum 1.5 K at the highest flow rate corresponding to12 [h.sup.-1]. In reality, the doctor and other health care staff members do not spend a long time in the room of the sick occupant, and only when on visitations or medication handling. Therefore, the absence of the doctor in the room would be the most common case when studying the exposure of the second patient in a two-bed hospital room. Hence, the total heat load in the room was either 150 W (511.8 Btu) with the doctor not in the room or 210 W (716.5 Btu) with the doctor present in the patients' room.
The coughed flow was 100% C[O.sub.2]. The time-dependent C[O.sub.2] concentration was measured at the mouth of the doctor with a specially developed instrument (PS331) with time constant of 0.8 s and a sampling rate of 4 Hz. The sampling tube of the PS331 was placed at the mouth 0.005 m (0.016 ft) away from the lips. The breathing function of the heated manikin was switched off. As reported in the literature, the C[O.sub.2] concentration measured in this way is equal to the C[O.sub.2] concentration in the air inhaled by the breathing thermal manikin (Melikov and Kaczmarczyk 2007; Rim and Novoselac 2009). The acquired data were analyzed by specially developed software. The software used second-order polynomial extrapolation to get the initial value of the C[O.sub.2] concentration up to 14,000 ppm by applying calibration equations. Frequency correction of the signal from the instrument and compensation for the time needed for the C[O.sub.2] sample to travel in the sampling tube from the measuring point to the instrument was applied. Indoor air quality monitors (IAQMs; PS32), with a recording frequency of 0.1 Hz, resolution of 1 ppm, and expanded uncertainty of 10 ppm + 3% of the measured value, were used to measure the C[O.sub.2] concentration at the air supply diffuser (position 4, Figure 1), at the exhaust diffuser (position 5 above the coughing patient; point l, Figure l), as well as in a point in the room located at 1.7 m (5.58 ft) above the floor close to bed of the infected patient (point A, Figure 1). For all studied cases reported here, 15 to 20 repeated measurements of simulated cough were collected and averaged. Only one cough at a time was generated. The uncertainty of the measurement for the maximum peak concentration registered was 150 ppm. Every following cough in each set of experiments was generated after the background C[O.sub.2] level reached the value before the cough. The time between the produced repeated coughs decreased with the increase of the background ventilation rate. The excess concentration of C[O.sub.2] over the background level was used as criteria for exposure assessment.
Two more parameters were analyzed, namely the peak concentration level (PCL) and the peak concentration time (PCT). PCL is defined as the maximum concentration measured at the mouth of the doctor after a cough is generated; PCT is defined as the time at which the PCL is reached after a cough is generated (Melikov et al. 2009).
The background velocities in the room were measured at five locations: in the middle of the room (point B, Figure 1), close to the feet of the coughing patient (point A, Figure 1), and at the three locations of the doctor: 0.55 m (1.8 ft) (point C, Figure 1), 1.10 m (3.61 ft) (point D, Figure 1), and 2.80 m (9.19 ft) (point E, Figure 1). The velocities were measured at seven heights: 0.1, 0.6, 1.1, 1.7, 2.0, 2.2, and 2.3 m (0.33, 1.97, 3.61, 5.58, 6.56, 7.21, and 7.54 ft) with an eight-channel low velocity thermal anemometer with omnidirectional velocity sensor. The measuring time was 3 min. The air velocity (in fact the air speed) was measured in the range 0.05-2 m/s (9.84-393.7 fpm) with an error of the readings of [+ or -] 0.02 m/s [+ or -] 1% ([+ or -] 3.94 fpm [+ or -] 1%). The characteristics of the anemometer were better than the required by ISO Standard 7726 (ISO 1998) and ASHRAE Standard 113 (ASHRAE 2005). The doctor (the thermal manikin) was removed during the velocity measurements. This procedure is widely applied at present research on air distribution in rooms. It provides knowledge on the background airflow. The airflow at the vicinity of the doctor is a complex interaction of the background ventilation flow and the free convection flow around the body. Identification of the velocity field around the doctor's body is an important study in itself and requires use of sophisticated measuring technique, i.e., laser Doppler anemometry (LDA), particle image velocimetry (PIV), etc. PIV and LDA are nonintrusive techniques that allow measurement of the three velocity components (in a Cartesian coordinate system) very close to the body of the thermal manikin without disturbing the flow interaction between the free convection and the surrounding room air, as well as identification of the direction of the resultant flow.
Results and discussion
Background air velocities
The velocities at the seven heights measured for the three air change rates (3, 6, and 12 [h.sup.-1]) and at the five measured locations are shown in Figure 2. As expected, the velocity in the occupied zone (up to 1.8 m [5.9 ft] above the floor) increased with the increase of the ventilation rate. All velocities at 3 [h.sup.-1], and almost all at 6 [h.sup.-1], were lower than 0.2 m/s (39.37 fpm). However, at 12 [h.sup.-1], the velocities increased to almost 0.5 m/s (98.43 fpm), and they were never below 0.2 m/s (39.37 fpm). In this case, the background ventilation flow with relatively high velocity will penetrate the free convection flow around the human body, since its maximum velocity is 0.15-0.25 m/s (29.53-49.21 fpm) (Homma and Yakiyama 1988). The results suggest that at 12 [h.sup.-1], the free convective layer around the doctor's body may be completely destroyed by the background ventilation flow and may even cause draught discomfort to the doctor (ISO 2005 [Standard 7730]). Therefore, keeping the room air temperature in the upper range of recommended indoor air temperatures listed in the present standards and guidelines (ANSI/ASHRAE Standard 55 [ASHRAE 2004], EN 15251 [European Standard 2007], and CEN CR 1752 [CEN CR 1998]) can help to reduce the risk of draught, especially under elevated air change rates. Apart from the increase in velocity, the elevated ventilation rate will lead to change in the air distribution pattern in the room. At 12 [h.sup.-1], the maximum velocity was registered from 0.5 m to 1.2 m (1.64 ft to 3.93 ft) above the floor and depended on the measuring location in the room (Figure 2). At 3 [h.sup.-1] and 6 [h.sup.-1] the vertical velocity distribution remained more or less the same or had higher velocities near the floor (below 0.5 m [1.64 ft]). This was found to be dependent on the location in the room as well.
[FIGURE 2 OMITTED]
Exposure of the doctor to coughed air--effect of distance between the doctor and the coughing patient
The impact of the distance between the coughing patient and the doctor on the exposure of the doctor was studied in a set of three experiments with distances of 0.55, 1.1, and 2.8 m (1.8, 3.61, and 9.19 ft). The excess C[O.sub.2] concentration at the mouth of the doctor facing the coughing patient obtained at the three distances under air change rates of 3, 6, and 12 [h.sup.-1] is shown in Figure 3.
Figure 3a compares the excess C[O.sub.2] concentration measured at the mouth of the "standing doctor" situated 0.55 m (1.8 ft) from the mouth opening of the coughing dummy under the three studied ventilation rates. In this case, the thermal manikin was facing the coughing dummy and was turned with back toward the second (not coughing) dummy lying in the other bed. The coughing dummy was lying on one side with mouth opening pointed against the front of the thermal manikin. Thus the "coughed" air first hit the manikin in the abdominal area and then spread over the front of its body with some of the coughed air gliding around the waist of the standing doctor, some spreading upward (toward the mouth) and some downward (toward the feet). As already stated, details and accurate identification of the airflow characteristics in this case will be studied in the future.
[FIGURE 3 OMITTED]
At 0.55 m (1.8 ft) distance, the PCT was approximately 4 s. Because of the high initial velocity of the cough and the close distance between the doctor and the sick patient, the PCT was not dependent on the background air change rate. The values of PCL for 3 [h.sup.-1] and 6 [h.sup.-1] were similar: 10,228 ppm and 10,197 ppm. The restored, after the cough convective layer surrounding the manikin entrained the diluted with room air cough and brought it into the breathing zone. The lowest PCL of 6847 ppm was for 12 [h.sup.-1]. The PCL and the slope of the following C[O.sub.2] decay depended on the air change rate in the room: the higher the ventilation rate, the lower the PCL measured and the steeper the slope (faster decay). The decay curve at 3 [h.sup.-1] makes a second smaller peak at around 16 s after the cough has been initiated, and it then slowly returns to the background concentration in the room. The secondary peak at 3 [h.sup.-1] is probably due to the fact that the background velocities are low enough and allow for the free convective layer around the manikin body to restore after the initial impact of the coughed jet and to recapture part of the coughed air re-bounced from the floor and brought back into inhalation. However, visualization and velocity measurements of the flow interaction close to the manikin's body are needed to further clarify this hypothesis. The visualization can affect the flow development at the vicinity of the body and, hence, the flow interaction. Therefore, the visualization of the flows around the human body, especially in the case of transient flow, i.e., coughing, is a challenging task that needs to be carefully investigated and documented in a separate study. At 6 [h.sup.-1] and 12 [h.sup.-1], the background velocities were higher than at 3 [h.sup.-1] and led to enhanced mixing of the cough air with the room air. As already discussed, at 12 [h.sup.-1], the velocities in the room were substantially higher than the velocity of the free convective layer surrounding the human body and may have completely peeled it off.
The results in Figure 3 show that at a ventilation rate of 12 [h.sup.-1], the PCL is reduced by only 27% compared to that at 3 [h.sup.-1] when the doctor is closest to the bed of the patient, i.e., when the doctor is performing routine medical examination (0.55 m [1.8 ft] distance from the coughing patient). It should also be noted that at this short distance, the risk from inhaling or ingesting large droplets also exists (Hodgson et al. 2009).
The results obtained at the distance of 1.1 m (3.61 ft) (Figure 3b) also show dependence of the PCL on the air change rate. However, as it can be noticed here, opposite to expectations, the peak concentration at 12 [h.sup.-1] (5251 ppm) was higher compared to that at 6 [h.sup.-1] (4542 ppm) and 3 [h.sup.-1] (4206 ppm). A possible explanation for the elevated exposure at 12 [h.sup.-1] compared to that at 6 [h.sup.-1] and 3 [h.sup.-1] is the flow interaction close to the body of the doctor, namely between the cough jet, the free convection, and the background ventilation flow. At 12 [h.sup.-1], the background velocities exceeded 0.3 m/s (59.10 fpm) at 0.6 m (1.96 ft) height, i.e., waist level of the doctor (Figure 2b), so the boundary layer around the doctor was greatly reduced or completely peeled off. Thus, when the cough hit the thermal manikin at waist level, part of it moved upward along the body and toward the breathing zone without being much diluted by the free convection and the surrounding room air. The surrounding air might have assisted the movement of the impingent cough flow upward toward the face of the doctor (manikin). At 6 [h.sup.-1] and 3 [h.sup.-1], due to the much lower background velocities (less than 0.1 m/s [19.70 fpm], Figure 2b), the free convection around the thermal manikin (doctor) was present. So, after the initial impact of the cough jet with the doctor standing at 1.1 m (3.61 ft) downstream of the sick patient, the convection flow restored and entrained room air, which led to increased dilution of the cough flow. Hence, the measured lower excess concentration of C[O.sub.2] at the breathing zone of the thermal manikin at 6 [h.sup.-1] and 3 [h.sup.-1] was lower compared to that at 12 [h.sup.-1]. However, this hypothesis needs further investigation. Similar results have been reported by Wan et al. (2009) and Sze To et al. (2009) in their measurements of dispersion and deposition of particles as a result from cough in a mock aircraft cabin. They also noticed that setting higher supply airflow rates lead to better dilution close to the source (sick coughing patient), but it also enhanced the dispersion to expiratory aerosol for those passengers seated further away (two rows upstream), leading to higher exposures at those locations. At 6 [h.sup.-l] and 12 [h.sup.-1], the PCT was around 4.8 s, while at 3 h-1, it was 5.7 s. This may suggest that proximity of the total volume supply diffuser may have affected the flow interaction. However, this hypothesis needs to be justified by further measurements and visualization experiments.
At 2.8 m (9.19 ft) distance downstream of the cough, the dilution became even bigger and the PCT measured raised just slightly above the background concentration (Figure 3c). The small peaks at 3 [h.sup.1] and 6 [h.sup.1] (141 ppm and 308 ppm) can be due to the fact that the lower background air velocities compared to 12 [h.sup.1] result in less mixing and longer penetration of the coughed jet into the room. On its way of propagation into the room, the cough first hit the bed with the exposed patient (l.3 m [4.27 ft]) and then reached the body of the doctor (2.8 m [9.19 ft]). Other factors, such as the location of one of the exhausts above the exposed patient with regard to the position of the doctor, may have had an impact as well. This needs to be studied further. The PCT ranged from 12 s to 23 s. It was shortest for 6 [h.sup.1] and longest for 3 [h.sup.1]. For 12 h-l, no clear trend for PCT or PCL could be noticed due to the small fluctuations in the excess C[O.sup.2] measured as a result from the complex air flow interaction in the room.
Exposure of the doctor to coughed air--impact of posture of coughing patient and location of doctor
The effect of posture of the coughing patient, i.e., lying on one side or on the back, on the transport of the air coughed by the sick patient toward the mouth of the "doctor" is shown in Figure 4. The figure compares the excess C[O.sub.2] concentration measured in time at the mouth of the "standing doctor" (a thermal manikin) situated 0.55 m (1.8 ft) from the mouth opening of the dummy generating the cough under three ventilation rates of 3, 6, and 12 [h.sup.1]. The measurements when the coughing dummy was placed lying on one side with mouth opening directed toward the front body plane of the doctor (horizontal cough) are shown in Figure 4a, and when the coughing dummy was lying on the back and coughing toward the ceiling (vertical cough), they are shown in Figure 4b. The results in Figure 4a (horizontal cough) show that when a cough was triggered, the "puffed" air first hit the doctor in the abdominal area and then spread over the body; some of the coughed air would glide along the waist of the standing doctor and some would spread upward (toward the mouth) and downward (toward the feet). At this position of the doctor, the PCT values almost did not depend on the air change rate, due to the high initial momentum of the coughed air. Around the fourth second after the cough, the puffed C[O.sub.2] cloud reached the breathing zone of the thermal manikin followed by decay in concentration. The PCL and the slope of the decay depend on the air change rate in the room: the higher the ventilation rate, the lower the PCL measured and the steeper the slope (faster decay). The values of PCL for 3 [h.sup.1] and 6 h-I were similar: 10,228 ppm and 10,197 ppm. The restored, after the cough convective layer surrounding the manikin's body "locked" and moved the "deflected" by the manikin's body cough toward the breathing zone. The lowest PCL of 6847 ppm was for 12 [h.sup.1]. Due to the high background velocities (12 [h.sup.1]), the convection layer around the manikin was peeled off. The surrounding air might have affected the movement of the cough flow toward the face of the doctor (manikin) and led to the lower C[O.sub.2] excess concentration compared to 3 [h.sup.1] and 6 [h.sup.1] as a result of enhanced mixing. This hypothesis, however, needs to be carefully studied.
[FIGURE 4 OMITTED]
Figure 4b shows the exposure of the doctor under the same other condition as in Figure 4a but when the infected patient was lying on the back and was coughing upward. The PCL was much lower (50 times lower) compared to the case when the cough was horizontal. Thus, the largest portion of the coughed air moved upward toward the exhaust due to its high initial momentum. The increase in the ventilation rate decreased the exposure to coughed air similarly to the case when coughing sideways. The highest PCL (200 ppm) measured around the 36th second at 3 [h.sup.1] is due to the fact that the existing convective flow around the doctor's body has entrained and moved upward toward the mouth the surrounding room air contaminated after the cough. At this air change rate, the background air velocities (lower than 0.1 m/s [19.69 fpm]) were not high enough to disturb or destroy the boundary layer (Figure 2). For 6 [h.sup.1] and 12 [h.sup.1], no clear trend for PCT or PCL could be noticed due to the small fluctuations in the excess C[O.sub.2] as a result from the complex air flow interaction in the room. Most of the cough directed upward (toward the ceiling exhaust vent) was successfully exhausted. Based on these results, it can be suggested that a good contaminant control solution in hospital rooms is to position the TV exhaust as close as possible to the polluting source--the sick coughing patient in this case. A similar arrangement has been suggested before as well (Cheong and Phua 2006; Noakes et al. 2009; Tung et al. 2009b). The impact of the posture of the coughing patient on the background C[O.sub.2] concentration will be discussed in a following sub-section of this article.
The doctor may not face only one of the patients, but as in the case when making visitations, he/she may stand by the bed to have view prospective of both patients (defined as "sideways" relative to the coughing patient, Figure 1c). This condition was studied at ventilation rate equal to 6 [h.sup.1] only. In Figure 5, the excess C[O.sub.2] concentrations at the mouth of the doctor when facing the coughing patient and when standing sideways are compared. It can be seen that there is a relatively small increase in the concentration measured at the mouth of the sideways-standing doctor after the cough. The PCT increased to nearly 10 s (more than doubled) when the doctor was turned sideways and the cough was horizontal. The PCL also drastically dropped from 10,197 ppm to 194 ppm, over 50 times decrease in peak exposure. The reason is that being turned to face both patients, i.e., sideways, the doctor's body does not play the role of an obstacle for the coughed jet. The high momentum cough jet just sweeps by the doctor. However, a small portion from the patient's cough is entrained by the boundary layer around doctor's body and is brought to the mouth, but after a longer time (PCT = 9.7 s) compared to the case when the doctor is facing the coughing patient (PCT = 4.1 s).
[FIGURE 5 OMITTED]
Exposure of the second patient to coughed air
The effect of cross-infection among patients was also studied. In this scenario, the exposed patient was facing the coughing one. The case when the exposed patient was lying on its back, i.e., facing the ceiling, was not studied, as preliminary measurements showed very low excess in the C[O.sub.2] concentration measured at the mouth of the exposed patient. The doctor was not present in the room. Instead, the second patient was simulated by the thermal manikin placed in the second bed (1.3 m [4.27 ft] away), with its head turned facing the coughing dummy's head. The results of these measurements are shown in Figure 6. The increase of the ventilation rate to 12 [h.sup.1] increased the room air movement and thus the mixing, i.e., the dilution, of the coughed air. This resulted in lower C[O.sub.2] concentration at the breathing zone of the exposed patient; the concentration of C[O.sub.2] was reduced more than twice compared to the two lower ventilation rates (3 h-l and 6 [h.sup.1]). The PCL for 3, 6, and 12 [h.sup.1] was as follows: 5518, 5073, and 1786 ppm, respectively. The PCT for 3 [h.sup.1] and 6 [h.sup.1] was around 6 s (respectively, 5.9 s and 6.1 s), while for 12 [h.sup.1], it was slightly higher--8.4 s. The high initial velocity of the coughed C[O.sub.2] and the relatively low background velocities at 3 [h.sup.1] and 6 [h.sup.1] (Figure 2) kept the PCL at the mouth of the exposed patient quite high (Figure 6). This also resulted in the shorter PCT measured for the two lower ventilation rates. The exposure of the second patient to the coughed air was substantially lower than the exposure of the doctor when facing the coughing manikin (Figure 4a). The reasons can be that the exposed patient was located at a longer distance from the coughing patient (1.3 m [4.27 ft]) than was the doctor (0.55 m or 1.1 m [1.8 ft or 3.61 ft]), and that the exposed patient was lying in the second bed at the same height from the floor as the coughing patient, which resulted in horizontal penetration of the coughed air.
[FIGURE 6 OMITTED]
Spread of coughed air in the room
The excess C[O.sub.2] concentration measured in the room close to the feet of the coughing patient (point A, Figure 1) when the doctor was standing at the three studied distances is presented in Figure 7. As already defined, the measurements were performed at height of 1.7 m [5.57 ft] above the floor. These measurements are presented and discussed in order to estimate the risk from airborne cross-infection for a person standing relatively close to the coughing person (visitors or other assisting healthcare worker).
The results in the figures reveal that the cough increases the C[O.sub.2] concentration not only at the breathing zone of the doctor facing the coughing patient but also in the background, regardless of the ventilation rate studied. The results also reveal that the elevated background air movement due to the increase of the ventilation rate may enhance the exposure to coughed air. The PCL determined for 3 [h.sup.1] was lower than at 6 h-l and 12 [h.sup.1]. Although the excess C[O.sub.2] concentration at the measured point is much lower than that at the mouth of the doctor (Figure 3), the exposure risk still exists, regardless of the background ventilation rate (up to 12 [h.sup.1] studied), i.e., the risk of cross-infection for a person standing close to the feet of the coughing patient.
The results in Figure 7 show that the excess concentration of C[O.sub.2] was measured highest at 6 [h.sup.1], regardless of the distance between the coughing person lying in bed and the doctor. One possible explanation can be the throw length of the jet supplied from the diffuser. The throw length of the supplied jet will increase with the increase of the ventilation flow, and this will change the airflow characteristics (velocity, direction, etc.) in the occupied zone. The velocity measurements at this point (Figure 2e) show that the increase of the flow rate from 3 [h.sup.1] to 6 [h.sup.1] resulted in increase of the background airflow velocity more near the floor level and less at 1.7 m (5.57 ft) height. Also, due to its direction, the generated cough transported the C[O.sub.2] away from the measurement location (Figure 1). This complex flow interaction needs to be studied separately and in detail. The positioning of the supply and exhaust diffusers is another factor affecting the contaminant distribution in the room (Wan et al. 2005; Nielsen 2009; Noakes et al. 2009). To further study this, more experiments with different locations of the ventilation diffusers are necessary.
[FIGURE 7 OMITTED]
The PCT and the PCL values determined for some of the studied combinations are listed in Table 1. The PCT at point A (Figure 1) was between 160 s and 500 s, i.e., much longer than the 4 s identified for the doctor. The PCL varied from 20 ppm to nearly 35 ppm.
The results in Table 1 show that the PCT decreases with the increase of the ventilation rate. The distance between the doctor and the coughing patient also had an influence on the PCT--the PCT increased with the increase of the distance. The measured values of PCL at point A depended not only on the location of the doctor downstream from the coughing patient but also on the background ventilation level (Table 1). A tendency for elevated PCL was measured at point A when the doctor was at a distance of 0.55 m (1.8 ft) from the coughing patient than when at distances of 1.1 m and 2.8 m (3.6 l ft and 9.19 ft) under the three ventilation rates tested (3, 6, and 12 [h.sup.1]). The blocking effect of the doctor's body on the way of the coughed jet may be the reason. The closer the doctor was standing to the bed of the coughing patient, the more of the coughed air was deflected. This resulted also in a lower PCT. However, to better explain the air pattern in the room, which results from the complex flow interactions, measurements in more locations in the room are necessary. The lowest PCL was measured at 3 [h.sup.1] and the doctor positioned at 1.1 m (3.61 ft) from the coughing patient, which was 18.42 ppm; the highest PCL value was documented at 6 h-l and the doctor standing 0.55 m (1.8 ft) from the sick patient, which was 34.36 ppm.
At both 6 [h.sub.-1] and 12 [h.sup.1], the difference in PCT when the doctor stood 1.1 m and 2.8 m (3.61 ft and 9.19 ft) away is not that big. The coughed jet expands with increase of the distance. By the time the jet reaches the doctor, part of it is already mixed into the room air and "moved" toward the feet of the sick patient. At all three ventilation rates examined, the excess of C[O.sub.2] measured at point A was slightly higher when the doctor was standing at 2.8 m (9.19 ft) from the coughing patient compared to when he was standing at 1.1m (3.61 ft; Table 1). As already discussed, the change of the ventilation rate may have changed the airflow interaction and, thus, the airflow pattern in the room, resulting in different diffusion of the coughed air. The proximity of the supply diffuser when the doctor was at a 1.1-m (3.61-ft) distance (Figure l a) may have had an impact as well by resulting in more dilution of the deflected cough jet by the manikin's body. This needs to be verified by further measurements and series of visualizations.
Figure 8 shows the level of excess C[O.sub.2] concentration in the occupied zone at 1.7 m (5.58 ft) height close to the foot of the bed with the coughing patient (point A, Figure la). When the doctor was beside the bed and close to the source of contamination (the sick patient), the highest excess C[O.sub.2] level in the occupied zone was observed at 6 [h.sup.1] and the lowest at 3 [h.sup.1] (Figure 8a). A plausible explanation can be the flow pattern in the room, which depends on the amount of air supplied, the type of diffusers, their positioning, supply air temperature, etc., hence determining the level of mixing in the space.
At all ventilation rates studied, the background C[O.sub.2] concentration after the cough was slightly lower when the coughing patient was lying on its back and coughing upward compared to the case when the coughing patient was lying on one side (Figure 8b). A possible explanation can be that the body of the doctor deflected the coughed jet when the patient coughed horizontally, so the background room velocities increased the mixing of the coughed air compared to the case when the coughing was upward.
The background C[O.sub.2] concentration was a result of the airflow interaction in the room, which depended on the direction of the cough, i.e., the posture of the coughing patient, and the airflow pattern in the room influenced by the ventilation rate. The highest excess C[O.sub.2] concentration in the occupied zone (point A, Figure 1a) was measured at 12 [h.sup.1] when the coughing patient was lying on its back and coughing upward. The airflow interaction remains to be studied in the future.
[FIGURE 8 OMITTED]
The results in Figure 8b reveal that, in general, the decrease of the ventilation rate lead to an increase of the PCT from 140 s at 12 [h.sup.1] and coughing upward to 590 s at 3 [h.sup.1] and coughing upward.
In Figure 8c, the excess C[O.sub.2] concentration in the occupied zone in point A (Figure la) at the three ventilation rates is compared in the case when only the two patients were present in the room (doctor was not in the room). In this case, the excess C[O.sub.2] concentration also increased slightly after the cough compared to the case when the doctor was in the room. Clearly the collision of the cough jet with the doctor's body promoted higher mixing and, hence, better dilution with room air, reducing the background concentration measured at point A (Figure la).
The comparison of the results (Figure 8) also revealed that the posture of the coughing patient and the presence or absence of the doctor in the room had an impact on the PCT. The impact of these factors on the PCL was even stronger. For example, when the doctor was facing the infected patient, which was coughing sideways, the highest registered PCL was at 6 [h.sup.1]. When the infected patient was coughing upward, the measured PCL was highest at 12 [h.sup.1]. As already discussed, the complex airflow interaction in the room has affected the propagation of the coughed air in the room. In general, the results of the excess C[O.sub.2] measurements reveal that, due to the interaction of the coughed flow with the background flow, the C[O.sub.2] concentration in the room will rise and will result in exposure risk, but substantially less than the direct exposure of the doctor and the second patient.
The position of the doctor--sideways or facing the coughing patient--showed influence on the spread of the coughed air as well. The excess C[O.sub.2] concentration in point A within the occupied zone was higher when the doctor was facing the coughing patient than when the doctor was turned sideways (Figure 9). In the latter case, most of the coughed flow passed by without being deflected by the body, and this resulted in the lower concentration in the background close to the feet of the coughing patient.
The present results suggest that the exposure of the doctor to the air coughed by a sick patient lying in a hospital room is a result from the interaction of many factors, such as the air flow pattern in the space, the distance between the exposed person and the sick patient, the posture of the patient and the doctor, etc. In the present set of experiments, the doctor was always standing still. Therefore, more realistic measurements with the doctor seated on the bed and/or leaning over the patient or moving around in the room should be performed in the future.
[FIGURE 9 OMITTED]
The following conclusions can be drawn based on the results of the present study.
* The cough increased the C[O.sub.2] concentration in the air "inhaled" by the doctor and the exposed patient several times above the background, suggesting that under real conditions, the high exposure will increase the risk of cross-infection for the doctor or an occupant standing on the way of the cough. The excess C[O.sub.2] concentration measured in the breathing zone of the doctor and the second patient decreased with the increase of the ventilation rate.
* The highest exposure for the doctor was documented when standing closest to the bed: 0.55 m (1.8 ft). The increase in the ventilation rate from 3 [h.sup.1] to 12 [h.sup.1] decreased the PCL from 10,228 to 6847 ppm and the time for the C[O.sub.2] concentration at the breathing zone to return back to the background C[O.sub.2] concentration level before the cough, but it had little impact on the PCT (approximately 4 s for all three tested ventilation rates).
* Contrary to the expectations, at a distance of 1.1 m (3.61 ft), the peak concentration at 12 [h.sup.1] (5251 ppm) was much higher as compared to the peak concentration at 6 [h.sup.1] and 3 [h.sup.1] (4542 ppm and 4206 ppm). Thus, elevated ventilation rates may increase the exposure of the doctor to coughed air, and thus the risk of airborne cross-infection, because of the complex flow interaction around doctor's body.
* The posture of the coughing infected patient, lying on its side or back, also had a great impact on the exposure of the doctor and the second patient; maximal exposure was documented when the doctor and the second patient were facing the coughing patient that was lying sideways and at 3 [h.sup.1]. The risk of direct exposure to coughed air was minimal when the coughing patient was lying on its back; PCL was always below 200 ppm. In this case, the risk from contamination via inhalation or ingestion of large particulate matter is also the smallest.
* The exposure of the doctor standing near the bed of the coughing patient and turned sideways was nearly 50 times lower than when the doctor was facing the patient, and it had an exposure level similar to the case when the sick patient was coughing upward toward the ceiling.
* The exposure to coughed air will also increase in the occupied zone (in this case, around the bed of the coughing patient); i.e., a risk of airborne cross-infection for medical staff not directly exposed to the coughed flow also exists. For the configurations examined in this study, the PCT ranged between 160 s and 500 s and depended on the ventilation rate, the distance between the doctor and the coughing patient, the posture of the coughing patient in the bed, and the position of the doctor--either facing or turned sideways relative to the sick occupant. The background air distribution pattern is important for the spread of the coughed air.
* For the configuration in the present study, the ventilation rate of 12 [h.sup.1] generated velocities exceeding 0.5 m/s (98.43 fpm) in the occupied zone that were above the allowable maximum velocity prescribed in the present thermal comfort standards. This may cause draught discomfort for occupants.
* Study is recommended for the exposure in the case when the doctor is seated at the bed of the coughing patient, when moving between the beds, for different lay-out of the beds in the room, positioning of the supply and exhaust diffusers, etc.
A risk of airborne cross-infection due to coughing exists in rooms with mixing ventilation, even at the air change rates of 12 [h.sup.1] that are recommended in the present standards and guidelines. With this respect, the mixing air distribution method used today is not efficient. Advanced, more efficient air distribution methods need to be developed in order to control the spread of the pollutants generated by the sick person.
This research was supported by the Danish Agency for Science, Technology and Innovation (project 09-064627).
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Zhecho D. Bolashikov, (1),* Arsen K. Melikov, (1) Wojciech Kierat, (2) Zbigniew Popiolek, (2) and Marek Brand (1)
(1) Department of Civil Engineering, Technical University of Denmark, Nils Koppels Alle, DTU, Building 402, Kgs, Lyngby, 2800, Denmark
(2) Department of Heating, Ventilation and Dust Removal Technology, Silesian University of Technology, Gliwice, Poland
* Corresponding author e-mail: email@example.com
Received March 10, 2011; accepted March 8, 2012
Zhecho D. Bolashikov, PhD, is Postdoc. Arsen K. Melikov, PhD, Fellow ASHRAE, is Associate Professor. Wojcieeh Kierat, PhD, is Adjunct. Zbigniew Popiolek, PhD, is Professor. Marek Brand, MSc, is PhD Candidate.
Table 1. PCT and PCL for the three ventilation rates and for the three distances the doctor was standing away from coughing person. Distance between doctor and coughing [PCT.sub.3 ACH], [PCL.sub.3 ACH], patient s PPM 0.55 m (1.80 ft) 380 22.27 1.10 m (3.61 ft) 460 18.42 2.80 m (9.91 ft) 500 19.92 Distance between doctor and coughing [PCT.sub.6 ACH], [PCL.sub.6 ACH], patient s PPM 0.55 m (1.80 ft) 210 34.36 1.10 m (3.61 ft) 250 26.25 2.80 m (9.91 ft) 260 28.00 Distance between doctor and coughing [PCT.sub.12 ACH], [PCL.sub.12 ACH], patient s PPM 0.55 m (1.80 ft) 160 28.32 1.10 m (3.61 ft) 190 21.24 2.80 m (9.91 ft) 200 24.19
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|Author:||Bolashikov, Zhecho D.; Melikov, Arsen K.; Kierat, Wojciech; Popiolek, Zbigniew; Brand, Marek|
|Publication:||HVAC & R Research|
|Date:||Aug 1, 2012|
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