Breathing and Cross-Infection Risk in the Microenvironment around People.
There are different pathways of pathogen transport such as direct contact with an infected person or as airborne transmission or a combination of pathways. However, the airborne transmission is a very significant form of pathogen transport; see Morawska et al. (2009). Exhalation from an infected person can spread in long distance through the airflow pattern--the macro environment - depending on the type of ventilation system and it may increase the pathogen concentration in different indoor environment areas, see Qian et al. (2006) and Nielsen et al. (2008). When people are standing close to each other the exhalation can also spread in the microenvironment, which is present close to persons. The microenvironment consists of the thermal boundary layer around persons, exhalation, flow from talking and coughing, and those flows are not fully controlled by the ventilation system, see Bjorn and Nielsen (2002), Nielsen et al. (2009), Liu et al. (2011) and Olmedo et al. (2012).
The cross-infection problem was clearly demonstrated in the worldwide SARS outbreak in 2003 (Li et al. 2004a and 2004b). A discussion of the importance of an effective ventilation system, and the possibility to protect people from airborne infection was given in a literature review by Li et al. (2007), where it was concluded, that there is a strong and sufficient evidence of a connection between ventilation and control of air flow directions in buildings and the transmission and spread of infectious diseases such as measles, TB, chicken pox, anthrax, different types of influenza, smallpox and SARS.
The paper focuses on the characteristic of cross-infection risk in a room with an air distribution system which creates fully mixed conditions. A special focus is on the aerodynamics of the exhalation flow and on the importance of the person's activity levels expressed as volume flow rate and breathing frequency.
TEST ROOM AND EQUIPMENT
Manikins and concentration distribution
The source and the target manikins are simulated by life-sized breathing thermal manikins. Both manikins can either have exhalation through the mouth or through the nose, see Nielsen et al. (2008). The direction of the exhalation from the mouth is horizontal, and the exhalation from the nose has a downward angle of 67[degrees] and a sidewise angle of 60[degrees] both measured from a horizontal plane. The source manikin's mouth has an opening of 94 [mm.sup.2] (0.15 [in.sup.2]) and a semi-ellipsoid form. The nose has two 50 [mm.sup.2] (0.08 [in.sup.2]) nostril openings. The target manikin's mouth is a circle with a ratio of 12 mm (0.47 in) and the nose has same geometry as the source manikin. The geometry is in agreement with the measurements of Gupta et al. (2010).
Adams (1993) shows the volume flow rate (MV) and breathing frequency (RF) as a function of the activity level, MET, (metabolic rate). To study the influence of the activity level two activity levels 1.2 MET and 2.0 MET are selected corresponding to standing and walking activity. The volume flow rate and breathing frequency for standing (low activity) are MV = 8.8 L/min (0.31 [ft.sup.3]/min) and RF = 16 m[in.sup.-4], and the values for walking (high activity) are MV = 15.2 L/min (0.54 [ft.sup.3]/min) and RF = 19 [mm.sup.-1].
Both manikins have a dry heat release corresponding to the activity level. The mean surface temperature of the manikin is 27.5 [degrees]C (81.5 [degrees]F) at low activity level and 30.6 [degrees]C (87.1 [degrees]F) at high activity level. N2O is used as tracer gas in the experiments, and it is supplied to the exhalation of the source manikin. The exposure to the tracer gas is measured close to the mouth of the target manikin, or in front of the source manikin when the aerodynamics of the exhalation flow is studied.
Tracer gas is not influenced by buoyancy, and the results are therefore only valid for the situation where bacteria and viruses are transported by droplets (droplet nuclei) smaller than 5-10 [micro]m. Droplet nuclei smaller than 5 [micro]m exhibit a settling velocity below 1 m/h in still air, and can therefore follow the person's exhalation flows and the ambient air flows. Large droplets are also part of the cross-infection process, but they settle either on surfaces close to the source of the infection, or they evaporate, decrease in size and follow the air flow as droplet nuclei, see Liu (2011). Furthermore, the transport of fine particles is important because they may be much more readily inhaled than the coarser particles as shown by Wells (1955).
Tracer gas concentration cannot be directly used as a measure of the health risk, but it can give an indication of this risk. The health risk can be estimated from the Wells-Riley model which, among other things, gives a link between the concentration in a person's inhalation and the connected risk of infection (Riley et al. 1978). All the measurements and discussions in this article are based on steady state conditions, however, the Wells-Riley model introduces the time as a parameter, as e.g. the number of infected sources over a period of time.
The concentration distribution is measured by an INNOVA Multi Gas Monitor, Sampler and Doser. Velocities are measured by Dantec 54R10 hot sphere anemometers, and temperatures by thermocouples type K and Helios data logger.
The geometry of the test room is 4.1 m (13.5 ft) (length) x 3.2 m (10.5 ft) (width) x 2.7 m (8.9 ft) (height). The room is ventilated by a diffuse ceiling inlet. This air distribution system generate a low velocity level in the occupied zone which means that the micro environment around the manikins is not much influenced by the air distribution system, Nielsen et al. (2010). Figure 1 shows the locations of the two manikins. They are both located along the symmetry line of the room. The load is 78.4 W (267 Btu/h) at low activity, and 110.4 W (376 Btu/h) at high activity in cases with one manikin, and 156.8 W (535 Btu/h) and 188.8 W (644 Btu/h) in cases with two manikins. Only the source manikin has a high activity level in the high activity case. The air change rate for all the experiments is adjusted to keep the room temperature close to 23[degrees]C (73.4 [degrees]F).
MEASUREMENTS AND DISCUSSION
The exhalation from a person
The exhalation flow can be considered to be a combination of an instantaneous jet and a vortex flow. The exhalation forms a non-isothermal flow with an upward direction because of the influence of the body plume and the temperature difference. The velocity distribution in the exhalation flow is measured at different distances from the mouth. Although the flow in the microenvironment around a person is turbulent it is easy to recognize the pulsating velocity from the exhalation. The average of the maximum velocity, [u.sub.x], in the pulsations is used for the calculation of flow trajectory, figure 2, and for the velocity decay of the maximum velocity, figure 3.
The concentration distribution in the exhalation flow is also measured at different distances from the mouth. The equipment for measurements of concentration is not able to measure the instantaneous values, and an averaged concentration, c, is obtained. This averaged concentration is used for the calculation of the concentration flow trajectory, figure 2, and the decay of the averaged concentration, [c.sub.x], versus distance from the mouth, figure 3.
It is interesting to study the exhalation flow, because this flow describes the transport volume for bacteria and viruses if the person is the source of an airborne infection. (It should be noted that bacteria and viruses can also be attached to large particles and be spread at a short distance as droplet infection, but this case is not considered in this paper). Figure 2 shows the trajectory of this exhalation flow from the thermal breathing manikin in the case of a low and high activity level with the corresponding heat release, exhalation flow and breathing frequency. Both the path of the maximum velocity and the averaged concentration are shown. The centre of the instantaneous flow will for example be 15 cm (5.9 in) above the height of the mouth at a distance of 50 cm (19.7 in) at low activity level. A person of the same height standing opposite to the source person will probably not be influenced by the exhalation flow.
The trajectory of exhalation flow from the manikin in the case of a high activity level is also shown. In this case the centre of the instantaneous flow will be only 5 cm (2.0 in) above the height of the mouth at a distance of 50 cm (19.7 in). A person of the same height standing opposite to the source person will probably be influenced by the exhalation flow. The flow between two persons at a low and a high activity level will be further studied in the next chapter.
MATHEMATICAL DESCRIPTION OF VELOCITY AND CONCENTRATION DISTRIBUTION
The exhalation flow can be considered to be something between a turbulent instantaneous jet and a vortex ring It has been shown that the velocity behave similarly to the velocity in a jet when the maximum velocities, or peak velocities, are used for the description of the flow, see Nielsen et al. (2009). According to Olmedo et al. (2012) we can use the following expression to describe the peak value in velocity versus the distance from the mouth:
[mathematical expression not reproducible] (1)
where [u.sub.o], [u.sub.x], [a.sub.o] and x are peak exhalation velocity, peak velocity at distance x, area of mouth, and distance x, respectively. This equation is in structure similar to the equation for steady state jet flow. [K.sub.exp] and [n.sub.1] are characteristic constants.
We know from the Reynolds analogy that we should expect a similarity between the flow, the energy and the concentration field. The following equation could be expressed for the dimensionless mean concentration:
[mathematical expression not reproducible] (2)
where [c.sub.o], [c.sub.x], cR, [a.sub.o] and x are mean exhalation concentration, mean concentration at distance x, concentration in the surroundings, area of mouth, and distance x, respectively. [K.sub.c] and [n.sub.2] are characteristic constants.
The graphical representation of equation (1) and (2) in figure 3 shows that the expression gives a good description of the time-dependent flows at different distances x. (Equation (1) and (2)are given as a straight lines with a slope close to -1 in the log-log depiction in figure 3). The factor [K.sub.exp] is 3.82 for a standing person at low activity level and 3.31 for a person at high activity level. The exponent [n.sub.1] is -0.88 and -0.82 respectively. [K.sub.c] is equal to 2.95 for low activity and equal to 2.70 for high activity, and the exponent [n.sub.2] is equal to -0.76 and -0.77, respectively.
It is typical that the dimensionless descriptions seem to be rather independent of the activity level and of [u.sub.o] and [c.sub.o]. This indicates that the velocity distribution is a fully developed turbulent flow which is not much influenced by the thermal boundary layer around the manikin. The trajectory is on the other hand influenced by the buoyancy forces or the Archimedes number.
CROSS-INFECTION RISK BETWEEN TWO PEOPLE CLOSE TO EACH OTHER
The cross-infection risk is studied for two manikins standing opposite to each other as illustrated in figure 1. The distance within the manikins varies from 110 cm (43.3 in) to 35 cm (13.8 in), which means that the smaller distances are inside the microenvironment surrounding the manikins. The influence of the activity level, height of persons and exhalation from both mouth and nose are addressed.
The exposure of the target manikin is expressed as [c.sub.exp]/[c.sub.R], where [c.sub.exp] is the concentration at the mouth of the target manikin and [c.sub.R] is the return concentration or the concentration in the surrounding air. This normalized concentration is equal to 1.0 in a fully mixed situation and it is higher than 1.0 when the target manikin is exposed to a higher level. Figure 4 shows the influence of the distance between the source and target at a low and at a high activity level of the source manikin. The target manikin has a low activity level in both cases. There is no influence of the flow in the microenvironment when the distance is equal to or larger than 80 cm (31.5 in). When the distance between the manikins gets shorter, 35 cm (13.8 in), the exposure is doubled from about 2 to 4 when the activity level is increased from 1.2 MET to 2.0 MET. It should in this connection also be realized that the dimensioned concentration in the room, [c.sub.R], will increase with increased exhalation and therefore give an increased level of dimensioned concentration, [c.sub.exp], in this situation.
The increase in exposure taking place when people are standing close to each other have been found for all types of ventilation systems, Nielsen et al. (2008), Liu et al. (2011) and Olmedo et al. (2012), but especially for stratified flow in displacement ventilation.
The measurements of the trajectory of the exhalation in figure 2, and the exposure shown in figure 4 indicate an importance of the height of the target manikin. Figure 5 shows the exposure when the two manikins have a low activity level and a distance between each other of 50 cm (19.7 in). The exposure increases from 1.6 to 5.4 when the face of the target manikin is moved to a higher location than the source manikin (16 cm) (6.3 in). See also Liu et al. (2011). It can be concluded that the trajectory of the exhalation is an important parameter in the study of the cross-infection risk between people.
The last set of measurements is about the difference in exposure of a target manikin when they are standing face to face, and the source manikin is exhaling either through the mouth or through the nose. (In reality, people will breathe through both the nose and the mouth when they have a low activity level).
Figure 6 shows the exposure for an activity level of 1.2 MET for both manikins. It is obvious that the exposure is highest when the source manikin exhales through the nose. The downward direction of this flow, 67[degrees], will be entrained in the thermal boundary layer of the target manikin below or at the height of the mouth, while it will pass above the target manikin when the exhalation is through the mouth as shown in figure 2.
The flow of the exhalation from the mouth of a single person is studied in the microenvironment at different activity levels. The room is ventilated by a diffuse ceiling inlet. The exhalation flow rises above head height when the person has a low activity level because of a temperature difference between exhalation and the surrounding air.
The peak velocity decay and the mean concentration decay in the exhalation can be described by simple expressions that show the variables as proportional to the reciprocal horizontally distance from the mouth.
Experiments with cross-infection risk between two persons (source manikin and target manikin) show that the microenvironment around the persons is of importance for the cross-infection. The exposure increases when the two persons get close to each other, and it also increases when the source person has a high activity level. Measurements around the manikins show that the relative heights of the manikins are important, and that maximum exposure will take place when the target person's face is inside the exhalation flow from the source person.
In situations with a low activity level the exhalation from the nose of a person can give a higher exposure of an opposite person that exhalation from the mouth.
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Peter V. Nielsen, PhD
Rasmus L. Jensen, PhD
Peter V. Nielsen is a professor in the Department of Civil Engineering, Aalborg University, Aalborg, Denmark. Jan Zajas is a PhD fellow in the Department of Civil Engineering, Aalborg University, Aalborg, Denmark. Michal Litewnicki is at Go4Energy, Warsaw, Poland. Rasmus Lund Jensen is an associate professor in the Department of Civil Engineering, Aalborg University, Aalborg, Denmark.
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|Author:||Nielsen, Peter V.; Zajas, Jan; Litewnicki, Michal; Jensen, Rasmus L.|
|Publication:||ASHRAE Conference Papers|
|Date:||Dec 22, 2014|
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