Indoor Air 2015; 25: 307–319 wileyonlinelibrary.com/journal/ina Printed in Singapore. All rights reserved

© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd INDOOR AIR doi:10.1111/ina.12142

Protected zone ventilation and reduced personal exposure to airborne cross-infection Abstract The main objective of this study was to examine the performance of protected zone ventilation (PZV) and hybrid protected zone ventilation (HPZV) to reduce the direct exposure to exhaled air from others’ breathing. Experimental measurements are carried out to test the performance of PZV in a full-scale office room with two breathing thermal manikins. The measurements were performed under three configurations, including two standing manikins at different distances: 0.35, 0.5, and 1.1 m. When the supply air velocity is increased to 4 m/s in the downward plane jet, the dimensionless concentration is 40% lower than for fully mixed ventilation, which can be considered as a measure of protection from the zoning condition. The measurement results showed that in both the PZV and the HPZV system it is possible to decrease the transmission of tracer gas from one manikin to the opposite manikin; therefore, it probably would reduce the risk of air borne cross-infection between two people at the same relative positions. The results suggest that PZV and HPZV may be used to reduce the exposure of people in a protected zone from indoor pollutants emitted in a source zone.

G. Cao1, P. V. Nielsen2, R. L. Jensen2, P. Heiselberg2, L. Liu2, J. Heikkinen1 1 Smart Energy and System Integration, VTT Technical Research Centre of Finland, Espoo, Finland, 2Aalborg University, Aalborg, Denmark

Key words: Protected zone ventilation; Cross-infection; Air velocity; Indoor contaminants; Exposure; Plane jet.

G. Cao VTT Technical Research Centre of Finland Tekniikantie 4 A, P.O. Box: 1000 Espoo, FI-02044 VTT, Finland Tel.: +358-40-199-3513 Fax: +358-20-722-6390 e-mail: [email protected] Received for review 29 November 2013. Accepted for publication 27 June 2014.

Practical Implications

The results of this study indicate that protected zone ventilation may be able to lower significantly personal exposure to the other person’s exhaled airflow even when two persons are standing close to each other. The use of protected zone ventilation may result in exposure values twenty times lower than obtained by displacement ventilation. In some specific conditions, for example, hospital and health-care facilities, protected zone ventilation may be useful to help people avoid cross-infection based on direct and indirect exposure to infectious agents emitted from people.

Introduction

Widely spreading epidemic respiratory diseases have emerged, such as SARS (2003), H1N1 (2009), and H7N9 (2013) (Morawska, 2006; World Health Organization, 2014 and Centres for Disease Control and Prevention, 2014). These episodes showed that infectious bacteria and viruses could spread from person to person through respiratory activities, like breathing, coughing, and sneezing. There were an estimated 8.3 million new tuberculosis cases in 2000, in which transmission is commonly a consequence of an inhaled bacterium emitted from an infectious person (Corbett et al., 2003). Yu et al. (2004) and later Li et al. (2007) provided evidence that demonstrates the associations among ventilation, air movement in buildings, and the

transmission/spread of infectious diseases such as influenza and SARS. In addition, many earlier studies have found that the effect of airflow distribution on personal exposure to an indoor contaminant varies in relation to the ventilation method (Bjørn and Nielsen, 2002; Brohus, 1997; Melikov et al., 2002; Nielsen et al., 2008; Olmedo et al., 2012, 2013). Human respiratory activities, like breathing, talking, coughing, and sneezing, produce aerosols and droplets, which may carry infectious bacteria and viruses. Gupta et al. (2010) found that the breathing flow rate variation with time is sinusoidal, and that the amplitude and frequency of the sine function are related to body height, weight, and gender. However, the interactions between the room airflow distributions, human respiratory activities and thermal conditions around people are too 307

Cao et al. complex. Holmberg and Li (1998) and later Gao and Niu (2006) found that computational fluid dynamic (CFD) simulation reveals the spread mechanisms of exhaled pollutants and simulates the dispersion and deposition of airborne particles. Wang et al. (2012) suggested that the boundary conditions are too complex to be described accurately in CFD simulations that seek to model real conditions. Moreover, thermal plumes generated by people indoors add to the complexity of the interaction of airflow distribution and aerosol particle transmission. Nielsen (2009) found that the natural convection boundary layer around a standing person will always exist, and it is only disturbed when people are standing directly below a diffuser inducing strong downward air movement. The exhalation flow from a person is able to penetrate the natural convection boundary layer and the breathing zone of another person standing nearby, though a distance of up to 1.2 m (Bjørn and Nielsen, 2002; Nielsen et al., 2008). In fact, the protective effect of the boundary layer flow around the body of a moving person in displacement ventilation will already disappear at a speed of 0.2 m/s (Bjørn and Nielsen, 2002). Even though these studies found that the indoor airflow may affect the transport of exhaled air, the challenge of effectively reducing the exposure risk of an occupant to exhaled air from other occupants remains unsolved. Mixing ventilation (MV) is used in mechanical ventilation systems, which aims to dilute the pollutants emitted from indoor sources keeping the indoor concentration at a certain level. However, it has been reported that exposure increases when the distance between the persons is reduced, to even as high as 12 times the fully mixed value when the separation distance between the source and target manikins is 0.35 m (Nielsen et al., 2012; Olmedo et al., 2012, 2013). Displacement ventilation (DV) has high ventilation effectiveness in some cases, but it is also possible to have stratified exhalation in the occupied zone. Nielsen et al. (2012) found that the vertical temperature gradient can lock the exhaled flow at a certain height, which may result in very low ventilation effectiveness in connection with cross-infection problems. Nielsen et al. (2007) also found that in rooms with MV or downward ventilation (DWF), the use of personalized ventilation (PV) will, in some cases, protect the occupants from pollution and will increase the quality of inhaled air. When applied with displacement ventilation, Melikov et al. (2003) reported that PV can substantially improve the inhaled air quality when the pollution source is not located in the vicinity of the personalized flow, as with floor pollution. These facts show that conventional ventilation methods, including MV, DV, DWF, and PV may not effectively protect people from exposure to infectious agents that originate from a nearby person. 308

Valkeap€a€a and Siren (2010) found that an air curtain may be used to prevent the invasion of cold air from a door way with a relatively high tightness efficiency, which is defined as the ability to reduce the air leakage rate through a doorway. In addition, Heiselberg and Topp (1997) carried out an experimental study on a push-pull ventilation system and found that the balance of airflow rates between supply and exhaust was critical for an efficient system. It was found that the optimum efficiency could be obtained when the exhaust airflow is equal to the airflow rate of the push jet close to the exhaust. Kulmala et al. (2007) even found that when a push air jet and an exhaust are combined in a correctly balanced ratio, capture efficiency as high as 90% could be achieved. However, these studies did not find how an air curtain or downward plane jet may reduce the direct exposure of a person to pollutants produced by exhalation. Recently, Cao et al. (2013a,b) have shown that a downward plane jet may be used to control the transmission of airborne contaminants. This technique, known as protected occupied zone ventilation (POV), was developed to protect office workers from epidemic respiratory diseases. Using POV, an internal space may be divided into different personal work areas or subzones using downward plane jets or air curtains, which separate the space and provide fresh air to the subzones. The plane jets may possibly prevent the transmission of indoor pollution from one subzone to other subzones and destroy the high concentration exhalation flow directed into the breathing zone of susceptible persons. When the exhaust flow is equal to the whole flow rate of the downward jet at exhaust level, the protection efficiency may be maximized. Cao et al. (2013a) found that the protection efficiency of POV varies from 8% to 50% depending on the exhaust location, supply air velocity and the usage of partitions. Like POV, protected zone ventilation (PZV) also uses downward plane jets to divide internal space into different personal work zones. However, PZV does not only focus on the occupied zone, it may be used for a specific area of the room locally. If the use of PZV combines other types of airflow distribution methods in subzones, like a mixing type airflow distribution or a displacement type of airflow distribution, then the system may be defined as hybrid protected zone ventilation (HPZV). In concept, HPZV, which is based on the push-pull principle, will remove contaminants locally (at the exhaust) and therefore prevent the contaminant from crossing the plane jet. However, the performance of PZV and HPZV still remains unknown for reducing the direct exposure to pollutants produced by respiratory activities. The main objective of this study was to examine the performance of PZV and HPZV to reduce the direct exposure to exhaled air during the breathing processes in a room.

Protected zone ventilation Table 3 Detailed measurement conditions Measurement conditions

Case NO. Series NO. 1

Case a a-1 a-2 a-3 Case b b-1 b-2 b-3 Case c c-1 c-2 c-3 Case d d-1 d-2 d-3 Case e e-1 e-2 e-3 Case f f-1 f-2 f-3

Series NO. 2

Series NO. 3

Reynolds number at slot

0.35 0.5 1.1

2.2 2.2 2.2

727 727 727

218  2 218  2 218  2

9.3 9.3 9.3

23.5  0.2 23.5  0.2 23.5  0.2

0.35 0.5 1.1

3.0 3.0 3.0

1013 1013 1013

300  3 300  3 300  3

11.3 11.3 11.3

23.5  0.2 23.5  0.2 23.5  0.2

0.35 0.5 1.1

4.0 4.0 4.0

1352 1352 1352

408  4 408  4 408  4

15 15 15

23.5  0.2 23.5  0.2 23.5  0.2

0.35 0.5 1.1

3.0 3.0 3.0

1013 1013 1013

108  1 108  1 108  1

4.2 4.2 4.2

21.5  0.2 21.5  0.2 21.5  0.2

0.35 0.5 1.1

4.0 4.0 4.0

1352 1352 1352

144  1.5 144  1.5 144  1.5

5.7 5.7 5.7

22.5  0.2 22.5  0.2 22.5  0.2

0.35 0.5 1.1

N/A N/A N/A

N/A N/A N/A

312  3 312  3 312  3

L/4

L/4

L/4 d/2

W = 2.0 m

Slot diffuser

W/2

W/2

L = 5.20 m

L/4

d/2 = 0.175, 0.25, 0.55 m

Exhaust

1.5 m

y

1.1 m

Slot diffuser

x = 0.03, 0.4, 0.75,1.15, 1.57, 1.67, 1.77, 1.87 and 1.92 m

x

H = 2.5 m

(b) y = –0.002, 0, 0.002 m

Supply air temperature (°C)

Jet supply velocity (m/s)

(a)

Measurement points of N2O concentraon Measurement points of air temperature Measurement points of air velocity

Fig. 2 The distribution of measurement points in the test chamber: (a) top view, (b) side view

increased when the distance between the manikins (persons) is reduced to 0.35 and 0.5 m (Nielsen et al., 2012; Olmedo et al., 2012, 2013). In this visualization

Total supply and exhaust airflow rate (m3/h)

Room air change rate

d (m)

12 12 12

23.5  0.2 23.5  0.2 23.5  0.2

section, the distance between two standing BTMs is kept as 0.3 and 0.5 m. These photos are taken from the outside the test chamber perpendicular to the length of the room. Figure 4 shows that when the velocity of the plane jet is 1.8 m/s, the exhaled airflow can penetrate the downward plane jet. The penetrating airflow can reach the breathing zone of the target manikin. When the supply velocity is increased to 2.2 m/s, the exhaled airflow can partly penetrate the download plane jet. It can be seen that the exhaled airflow was bent toward the lower part of the target manikin, which may indicate that the risk of the cross-infection between the source manikin and target manikin could be lowered due to the presence of the downward jet. The qualitative appearance of the tracer is also supported by measurements at the target manikin’s mouth. However, since the exhaled airflow reaches the lower part of the target manikin, the risk of indirect cross-infection might rise due to the thermal boundary layer of the target manikin, which will induce upward, toward the target manikin’s head, the polluted air from the lower part of the room. More visualization results can be seen in Figure S2, which shows a series of visualization results with an interval time of 0.5 s. A distance of 0.5 m between the two manikins. When the distance between the two standing breathing thermal manikins is 0.5 m, the threshold supply air velocity is found to be below 1.8 m/s for reducing the direct expo-

311

Cao et al. has a metabolic rate of 1.2 met. (A unit that describes metabolic level is 1 met, which stands for a metabolic rate for a sedentary person; 1 met is equivalent to 58.1 W/m2 of body surface area.) The volumetric breathing rate is maintained at 8.8 l/min for each manikin in this study (Nielsen et al., 2014). Measurement instrumentation

A Br€ uel & Kjær 1302 Photoacoustic Gas-Monitor and a Br€ uel & Kjær Multipoint Sampler and Doser Type 1303 were used to measure the N2O concentration in the room. Nine Dantec 54N10 omni-directional anemometers were used to measure the air velocity of the jet flow downstream of the jet slot. A Dantec 54R50 Low Velocity Analyzer was used to measure the maximum air velocity of the exhalation airflow from the source manikin. Table 2 shows uncertainty of all measurement instruments used in this study. Measurement conditions

The measurement of concentration was taken at two heights, 1.1 and 1.5 m, which represent the breathing zone of a sitting person and a standing person. The exhaust is always at floor level opposite to the downward plane jet. The ratio of the exhaust to the jet, which equals the ratio of the volume of exhaust airflow at the floor level to the volume of airflow forming the downward jet, is maintained as 2.8 in Case a, b, and c, and at 1.0 in the other cases. Table 3 shows the detailed experimental conditions. The distribution of measurement points and the location of air supply and exhaust are shown in Figure 2. The points marked with ‘O’ represent the positions of the air velocity sensors. The maximum velocity of the plane jet was measured in the room without a breathing thermal manikin and without swirl diffusers. The points marked with ‘D’ represent the positions of the air temperature sensors, which Table 1 Heating power of the two manikins

Total heat flux [W] Heating pipe for breathing [W]

Source manikin

Target manikin

72 9

72 9

were used to monitor the supply air temperature and room air temperature. The points marked with ‘□’ represent the positions of the air sampling points to measure the NO2 concentration. As PZV is designed to prevent the transmission of pollutants between two persons, pure N2O was dosed in the mouth of the source manikin toward the target manikin at a rate of 160 ml/min. The total exhalation airflow of 8.8 l/min is supplied by the mechanical lung, which corresponds to a standing person’s level of activity.

Results and discussion The decay of the centerline velocity

As for the jet velocity, the decay of the centerline velocity of a plane jet in the developed region is represented by the following: pffiffiffiffiffiffiffiffi Um =U0 ¼ K= x=h

ð3Þ

where Um(m/s) is the local maximum velocity at a distance of x (m) downstream from the slot, U0 (m/s) is the supply velocity, K is the dimensionless constant of the jet, x (m) is the distance downstream from the slot, and h (m) is the slot height. Rajaratnam (1976) recommended a value of 2.47 for K. A value of 2.4 is also used in some studies (Chen and Rodi, 1980; Kulmala et al., 2007). In this study, a value of 2.4 is used to calculate the maximum velocity decay. The turbulence intensity at the plane jet slot is about 2%, which is lower than the swirl diffuser. Cao et al. (2013a) found that the turbulence intensity of the jet will affect the K value and the jet maximum velocity decay. The plane jet use here is designed in a way to produce low turbulent airflow, which has a slower decay of maximum velocity. Figure 3 shows the dimensionless decay of measured maximum jet velocity along the center line of the plane jet. The dimensionless velocity profiles have a similar decay curve, which indicates that the plane jet performs as a Reynolds number independent jet. Visualization of the cross-infection risk between two persons with PZV

A distance of 0.35 m between the two manikins. It has been reported that exposure risk is substantially

Table 2 Uncertainty of the measurement instruments

54N60 FlowMaster

Omni-directional spherical anemometer

Temperature sensor

Air flow measurement

FCO510 Micromanometer

N2O concentration measurement

2.0% of reading

2.0% of reading 0.025 m/s (0.08 ft/s)

0.1°C

3.0% of supply air)

0.25% of reading or 0.1% of reading between 10% of lowest range and full scale  one digit

2.0% (Detection limits: 0.003 ppm)

310

Protected zone ventilation Table 3 Detailed measurement conditions Measurement conditions

Case NO. Series NO. 1

Case a a-1 a-2 a-3 Case b b-1 b-2 b-3 Case c c-1 c-2 c-3 Case d d-1 d-2 d-3 Case e e-1 e-2 e-3 Case f f-1 f-2 f-3

Series NO. 2

Series NO. 3

Reynolds number at slot

0.35 0.5 1.1

2.2 2.2 2.2

727 727 727

218  2 218  2 218  2

9.3 9.3 9.3

23.5  0.2 23.5  0.2 23.5  0.2

0.35 0.5 1.1

3.0 3.0 3.0

1013 1013 1013

300  3 300  3 300  3

11.3 11.3 11.3

23.5  0.2 23.5  0.2 23.5  0.2

0.35 0.5 1.1

4.0 4.0 4.0

1352 1352 1352

408  4 408  4 408  4

15 15 15

23.5  0.2 23.5  0.2 23.5  0.2

0.35 0.5 1.1

3.0 3.0 3.0

1013 1013 1013

108  1 108  1 108  1

4.2 4.2 4.2

21.5  0.2 21.5  0.2 21.5  0.2

0.35 0.5 1.1

4.0 4.0 4.0

1352 1352 1352

144  1.5 144  1.5 144  1.5

5.7 5.7 5.7

22.5  0.2 22.5  0.2 22.5  0.2

0.35 0.5 1.1

N/A N/A N/A

N/A N/A N/A

312  3 312  3 312  3

L/4

L/4

L/4 d/2

W = 2.0 m

Slot diffuser

W/2

W/2

L = 5.20 m

L/4

d/2 = 0.175, 0.25, 0.55 m

Exhaust

1.5 m

y

1.1 m

Slot diffuser

x = 0.03, 0.4, 0.75,1.15, 1.57, 1.67, 1.77, 1.87 and 1.92 m

x

H = 2.5 m

(b) y = –0.002, 0, 0.002 m

Supply air temperature (°C)

Jet supply velocity (m/s)

(a)

Measurement points of N2O concentraon Measurement points of air temperature Measurement points of air velocity

Fig. 2 The distribution of measurement points in the test chamber: (a) top view, (b) side view

increased when the distance between the manikins (persons) is reduced to 0.35 and 0.5 m (Nielsen et al., 2012; Olmedo et al., 2012, 2013). In this visualization

Total supply and exhaust airflow rate (m3/h)

Room air change rate

d (m)

12 12 12

23.5  0.2 23.5  0.2 23.5  0.2

section, the distance between two standing BTMs is kept as 0.3 and 0.5 m. These photos are taken from the outside the test chamber perpendicular to the length of the room. Figure 4 shows that when the velocity of the plane jet is 1.8 m/s, the exhaled airflow can penetrate the downward plane jet. The penetrating airflow can reach the breathing zone of the target manikin. When the supply velocity is increased to 2.2 m/s, the exhaled airflow can partly penetrate the download plane jet. It can be seen that the exhaled airflow was bent toward the lower part of the target manikin, which may indicate that the risk of the cross-infection between the source manikin and target manikin could be lowered due to the presence of the downward jet. The qualitative appearance of the tracer is also supported by measurements at the target manikin’s mouth. However, since the exhaled airflow reaches the lower part of the target manikin, the risk of indirect cross-infection might rise due to the thermal boundary layer of the target manikin, which will induce upward, toward the target manikin’s head, the polluted air from the lower part of the room. More visualization results can be seen in Figure S2, which shows a series of visualization results with an interval time of 0.5 s. A distance of 0.5 m between the two manikins. When the distance between the two standing breathing thermal manikins is 0.5 m, the threshold supply air velocity is found to be below 1.8 m/s for reducing the direct expo-

311

Cao et al. (a)

1 0.9

U0 is 2.2 m/s U0

0.8

U0 U0 is 3.0 m/s

(b)

Plane jet direction

0.7

Indirect exposure

0.5 m Calculated by Eq. 4

0.6 ux/u0

Indirect exposure

0.5 0.4

Source manikin

0.3

Target manikin

0.2 0.1 0 0

50 100 150 200 250 300 350 400 Dimensionless distance from the jet slot (x/h)

Fig. 3 Maximum velocity decay of the plane jet

(a)

Downward ventilation Case f

(b)

Plane jet direction

Direct exposure

Fig. 5 Visualization of the interaction between the breathing process and the download plane jet (the distance between the two standing manikins is 50 cm, orientation of the image is looking along the length of the room, and two manikins are in the centre of the room beside the downward plane jet.) (a) Average slot velocity of the plane jet: 1.8 m/s, (b) Average slot velocity of the plane jet: 2.2 m/s

Indirect exposure

0.35 m

To specify how POV protects people from exposure to indoor pollutants, which may be transferred from the polluted zone to the clean zone, the protection efficiency (PE) is defined as (Cao et al., 2013a): PE ¼ ð1

Source manikin

Target manikin

Fig. 4 Visualization of the interaction between the breathing process and the download plane jet (the distance between the two standing manikins is 35 cm, orientation of the image is looking along the length of the room, and two manikins are in the centre of the room beside the downward plane jet.) (a) Average slot velocity of the plane jet: 1.8 m/s, (b) Average slot velocity of the plane jet: 2.2 m/s

sure of the protected manikin from the source manikin. The distance between two manikins also affects the performance of the downward plane jet. The longer the distance is between two BTMs, the greater the decay of the maximum velocity of the exhalation airflow will be. The protection ability of the plane jet will be improved when the plane jet is stronger than the exhalation airflow during the interaction of the two cross-airflows. In Figure 5, when the distance between the two manikins is 0.5 m, the exhalation does not penetrate the plane jet and reach the breathing zone of the target manikin, no matter whether the supply air velocity is 1.8 or 2.2 m/s. In this condition, the risk of cross-infection might be minimized due to the distance and the plane jet in between. More visualization results with an interval time of 0.5 s can be seen in Figure S3. 312

ctzone Þ% csource

ð4Þ

where ct-zone is the pollutant concentration in the target zone, and csource is the pollutant concentration in the source zone. In this study, PE and a few values were employed to express the personal exposure to the exhalation of the source manikin. The value of cexp/cR is subsequently used to evaluate the exposure of occupants to the contaminant source as the personal exposure value, where cR is the concentration in the exhaust, and cexp is the contaminant concentration to which occupants are exposed. In addition, the values of c1.1 m/cR and ct-zone/ cR were used to express the exposure level at a height of 1.1 m from the floor and in the middle of the target room, were c1.1 m is the concentration at a height of 1.1 m from the floor. Figure 6 shows the value of cexp/cR obtained at different distances between the two manikins in Case f, where the downward ventilation or MV method was used by two swirl diffusers. The figure shows, like normal downward ventilation or mixing ventilation, the value of cexp/cR is similar to results obtained in earlier studies (Bjørn and Nielsen, 2002; Nielsen et al., 2008; Olmedo et al., 2012, 2013). Figures 7 and 8 show the comparison of the c1.1/cR values and the ct-zone/cR values, respectively. The concentration was measured at a height of 1.1 m from the floor close to the chest of the target manikin and the contaminant concentration was transported to the

Protected zone ventilation 4.4 4.0

Case f DWF (12 ACH)

3.6

Ct-zone /CR

2.8 2.4 2.0

Swirl diffuser

Swirl diffuser

d/2 = 0.175, 0.25 and 0.55 m

1.6 1.2 0.8 0.4 0.0

Source zone

d/2 d/2

Target zone

Exhaust

0.2 0.4 0.6 0.8 1.0 Distance between two manikins (m)

1.2

c1.1m/cR

Fig. 6 Comparison of the cexp/cR values obtained for different distance between the two manikins in Case f

3.0 Swirl diffuser 2.8 Swirl diffuser Case f DWF d/2 = 0.175, 0.25 2.6 (12 ACH) and 0.55 m 2.4 2.2 d/2 d/2 Target Source zone 2.0 zone Exhaust 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.0 0.2 0.4 0.6 0.8 1.0 Distance between two manikins (m)

1.2

Fig. 7 Comparison of the c1.1/cR values obtained for different distances between the two manikins in Case f (contaminant concentration at a height of 1.1 m from the floor close to the chest of the target manikin)

breathing zone via the thermal boundary layer. With a decrease in the distance between the two manikins, the values of c1.1/cR and ct-zone/cR increase. The risk of cross-infection due to the direct exposure, possibly, will decrease with the distance between two people. It can be concluded that a cross-infection risk takes place in the microenvironment around the two persons although there is some mixing flow in the room. Protected zone ventilation: Cases d and e

In Cases d and e, there was only the plane jet used in the room, which may cause a strong mixing flow in the room. The downward jet disrupts the microenvironment around the two persons, especially the jet disturbs the exhalation plume from the source manikin. The microenvironment around the two persons is changed

1.2

Distance between two manikins (m)

Fig. 8 Comparison of the ct-zone/cR values obtained for different distances between the two manikins in Case f (contaminant concentration at a height of 1.5 m from the floor in the middle of the target zone)

by the downward plane jet, which breaks the transmission of exhalation from the source manikin to the target manikin. Figures 9–11 show the values of cexp/cR, c1.1 m/cR and ct-zone/cR obtained for different distances between the two manikins in Cases d and e, respectively. The values of cexp/cR and ct-zone/cR are close to 1 at all distances, which means that the downward jet creates almost a fully mixed environment regarding airflow distribution in the room. The downward airflow did not divide the room into two zones with respect to contamination transmission between the two manikins. The vertical jet entrains contaminant from both sides, the fully mixed contaminant in the jet is then directed to both sides at the floor level and the divided flow, with the same level of contamination, is then supplied to both sides of the room. To obtain two zones it is necessary to have an optimal ratio of the exhaust airflow rate to the supply airflow rate, which means that

PEI (cexp/cR)

PEI(cexp/c R)

3.2

3.0 Swirl diffuser Swirl diffuser Case f DWF 2.8 d/2 = 0.175, 0.25 and (12 ACH) 2.6 0.55 m 2.4 d/2 d/2 2.2 Target Source zone zone 2.0 Exhaust 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.0 0.2 0.4 0.6 0.8 1.0

3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.0

Plane jet

Case d U = 3 m/s Case e U = 4 m/s

d/2 d/2

d/2 = 0.175, 0.25 and 0.55 m Target zone

Source zone Exhaust

0.2 0.4 0.6 0.8 1.0 Distance between two manikins (m)

1.2

Fig. 9 Comparison of the cexp/cR values obtained for different distances between the two manikins in Case d and Case e

313

3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.0

Plane jet

Case d U = 3 m/s Case e U = 4 m/s

d/2 d/2

d/2 = 0.175, 0.25 and 0.55 m Target zone

Source zone

PEI (cexp/cR)

c1.1 m/cR

Cao et al.

Exhaust

0.2 0.4 0.6 0.8 1.0 Distance between two manikins (m)

1.2

ct-zone/cR

Fig. 10 Comparison of the c1.1/cR values obtained for different distances between the two manikins in Case d and Case e (contaminant concentration at a height of 1.1 m from the floor close to the chest of the target manikin)

3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.0

Plane jet

Case d U = 3 m/s Case e U = 4 m/s

d/2 d/2

d/2 = 0.175, 0.25 and 0.55 m Target zone

Source zone Exhaust

0.2 0.4 0.6 0.8 1.0 Distance between two manikins (m)

1.2

Fig. 11 Comparison of the ct-zone/cR values obtained for different distances between the two manikins in Case d and Case e (contaminant concentration at a height of 1.5 m from the floor in the middle of the target zone)

the exhaust should be equal to the jet flow in the exhaust area, instead of matching the flow at the supply opening (Heiselberg and Topp, 1997). The hybrid protected zone ventilation: Cases a, b and c

Hybrid protected zone ventilation is defined as the integration of PZV and other ventilation methods. In this study, air supply systems with various supply airflow rates were investigated, including the use of a plane jet and two swirl diffusers. The measured results concerning cross-infection risk are shown in the following figures. Figure 12 shows the personal exposure in cases a, b, and c with a distance of 0.35, 0.50, and 1.10 m. It shows that the personal exposure of Case a (with a supply air velocity of 2 m/s) is higher than in Case b 314

3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.0

Plane jet

Case a U = 2 m/s

Swirl diffuser

Swirl diffuser

Case b U = 3 m/s Case c U = 4 m/s

Exhaust

0.2 0.4 0.6 0.8 1.0 Distance between two manikins (m)

1.2

Fig. 12 Comparison of the cexp/cR values obtained for cases a–c

(supply air velocity of 3.0 m/s) for a distance between the two thermal manikins of 0.35, 0.50, and 1.10 m. When the distance between the two thermal manikins is 1.1 m, the personal exposure in Case a is lowered to 1.2. In Case b, the personal exposure decreases according to the distance between the two thermal manikins. With the higher supply velocity in Case c where the supply air velocity is 4 m/s, the personal exposure is as low as 0.6, even though the distance between the two BTMs is only 0.35 m. This is an indication that the whole zone is protected from exhalations originating with the source manikin. It may be concluded that when U = 2 and 3 m/s, some destruction of the source manikin’s exhalation takes place, which indicates some degree of separation of the microenvironments around the two persons. When the supply air velocity is 4.0 m/s in Case c, the personal exposure is lowered to a value below 1 at a distance between the two thermal manikins of 0.35 and 0.50 m. The exhaled plume is disrupted and the inhalation of the target manikin is below the fully mixed value. The value of personal exposure increases to a level of a fully mixed condition when the distance between two BTMs is 1.10 m with a supply air velocity of 4 m/s. The increase of the personal exposure may indicate that the measured cR became lower at the exhaust due to the dilution of pollutant by a higher airflow rate, even though the measured cexp may not be high. The unexpected increase may also be caused by entrainment of pollutants by the uprising thermal plume generated by the target manikin from floor level. Because the airflow rate has been increased, it would enhance the mixing of room air and the pollutant and increase the room air velocity. Possibly, the room air mixed with pollutants may penetrate the thermal boundary layer of the target manikin and reach its breathing zone. However, the cause of the observed increase remains somehow unclear without knowing detailed airflow information in the room. Particle

3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.0

Case a U = 2 m/s

Swirl diffuser Plane jet

Case c U = 4 m/s

Comparison of the cross-infection risk with results from other ventilation methods

Figure 15 shows the comparison of the cexp/cR values obtained using different ventilation methods. At a distance of 0.35 m between two manikins, the value of cexp/cR could be as high as 13 by using downward flow ventilation. Displacement ventilation will also result in a high value, which can be as high as 12. In this study, the value of cexp/cR when mixing ventilation with 12 ACH is only 4, which is lower than DV and DWF. It was reported that upward flow ventilation may reduce the value further, to about 2. The value of cexp/cR in Case c can get even lower than 0.6 in this study, which is twenty times lower than using DV and downward airflow ventilation. Therefore, HPZV and PZV may be used in open-plan office environments where the space can be easily separated by downward plane jets with floor level exhaust. Another common application can be in health care facilities, such as an examination

Swirl diffuser

14.0 Case c POV

d/2 = 0.175, 0.25 and 0.55 m

Case b U = 3 m/s

3.0 Swirl diffuser Plane jet Swirl diffuser Case a U = 2 m/s 2.8 Case b U = 3 m/s d/2 = 0.175, 0.25 2.6 and 0.55 m Case c U = 4 m/s 2.4 d/2 d/2 2.2 Source Target zone 2.0 zone 1.8 1.6 Exhaust 1.4 1.2 1.0 0.8 0.6 0.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Distance between two manikins (m)

Fig. 14 Comparison of the ct-zone/cR values obtained for cases a–c (contaminant concentration at a height of 1.5 m from the floor in the middle of the target zone)

12.0

d/2 d/2

Source zone

Case d POV Case f MV

Target zone

10.0 PEI(cexp/cR)

c1.1 m/cR

image velocimetry (PIV) or CFD simulation may be needed to reveal the detailed flow structure around the target manikin and the trajectory of the exhaled pollutants in the room. Figure 13 compares the c1.1/cR values obtained for Cases a–c (contaminant concentration at a height of 1.1 m from the floor, close to the chest of the target manikin). It shows that the value remains low, lower than 0.6, when the supply air velocity is 4 m/s at distances of 35 and 50 cm. However, the value is as high as 2.7 when the supply air velocity is 2 m/s. When the velocity is either 2 or 3 m/s, the value of c1.1/cR decreases with an increasing distance between the two manikins. It can be concluded that HPZV does not protect the target manikin in all situations. Figure 14 shows the comparison of the ct-zone/cR values obtained for Cases a–c (contaminant concentration at a height of 1.5 m from the floor in the middle of the target zone). When the velocity of the plane jet is 4 m/s, the target zone or protected zone has a lower concentration than the exhaust, with about 50% of the concentration of the exhaust air. When the value of ct-zone/cR is below 1.0, zoning is achieved with a lower concentration around the target. When the jet velocity decreases, the concentration in the protected zone becomes closer to that of the exhaust air. In addition, a similar phenomenon occurs that the value of personal exposure in Case c increases when the distance between two BTMs is 1.10 m. Without measuring the detailed airflow field, however, the cause of the observed increase may not be clearly understood due to the complex interaction between the air jet and thermal plume. Further PIV or CFD may be employed to discover the detailed airflow field to get a better understanding of this phenomenon.

ct-zone/cR

Protected zone ventilation

Exhaust

DV (Olemdo et al. 2012) UWF (Olemdo et al. 2012)

8.0

DWF (Olemdo et al. 2013)

6.0 4.0 2.0

0.2 0.4 0.6 0.8 1.0 Distance between two manikins (m)

1.2

Fig. 13 Comparison of the c1.1/cR values obtained for cases a–c (contaminant concentration at a height of 1.1 m from the floor close to the chest of the target manikin)

0.0 0.0

0.2 0.4 0.6 0.8 1.0 Distance between two manikins (m)

1.2

Fig. 15 Comparison of the cexp/cR values obtained using different ventilation methods

315

Cao et al. room and operation theatre, in which a doctor would see a patient and a patient needs to be protected from exposure to indoor pollutants. These applications have distinctive attributes and different practical issues, like design, installation, commissioning, and operation, which must be addressed separately. The reduction of the exposure value may help to limit the cross-infection risk in a space where a doctor faces a patient who has a respiratory disease. However, the earlier studies are based on total volume airflow ventilation. Using the same room geometry, a fixed air-change rate of 5.6 was set to study the performance of DV, DWF, and MV (Olmedo et al., 2012, 2013). In this study of PZV, the airflow is distributed by the plane jet diffuser, which aims to separate the space into a source zone and a target zone instead of mixing the room air with the supplied air. Even though the total air change rate of the room by using HPZV varies from 4.2 to 15 ACH in this study, the ratio of the supply airflow rate through the linear slot diffuser to the exhaust should be adjusted to the level used in a push-pull principle. In this study, a linear slot diffuser with the same width as the room was used to form a full-width downward plane jet separating the room into two zones: source zone and target zone. Since the target zone was sealed to some extend by the full width downward plane jet, the transmission of contaminant will be minimized. If the width of the plane jet is less than the width of room, the contaminant may by pass the plane jet from the gap between the jet and other internal walls, which may increase the risk of cross-infection due to indirect exposure. In design phase, the plane jets should be arranged to form fully closed protection zones to avoid the possible bypassing of contaminants. As the principle of PZV and HPZV differs from other type of ventilation configurations, which only dilute the indoor pollutant level, it requires different design procedures with different room shapes.

The effect of thermal plume on the downward plane jet

To investigate the influence of the thermal plumes generated by the breathing thermal manikins, the velocity profile was measured along a horizontal line 0.1 m above the head of the BTM. The measurements were made at a distance of 0.175, 0.1, 0, 0.1 and 0.25 m from the center line of the downward plane jet (Figure 16a). The local microenvironment created around the manikins with the downward plane jet is visualized in Figure 16b. Figure 16b shows the smoke distribution with the downward plane jet, which separates the two BTMs into two zones without impinging on the head of the BTM. In particular, Figure 16a and b show that the flow above the manikin is upward due to the rising thermal plume from the BTM. The induction of room air causes the growth of the jet volume flow rate as indicated by the smoke. The boundary layer of the plane jet interacts with the outer layer of the thermal plume rising from the surroundings of the BTMs. With the presence of the plane jet between the source BTM and the target BTM, the risk of direct crossinfection is minimized. With the application of the downward plane jet between two manikins, the thermal plume would not affect the airflow distribution in the micro-environment around the person. Thus, the application of the plane jet may not cause a draught problem at the person’s head height. Moreover, the release of infectious agents from a sick person occurs by many more channels than would be reflected in an exhaled airflow. Talking (possibly), sneezing, and coughing (certainly) could produce much more vigorous jets, which may be affected by the thermal plume in a different way. Furthermore, large droplet transmission over distances on the order of a half meter might have a ballistic component that would not reflect the behavior of a tracer gas.

(a)

(b) –0.5 –0.3 –0.1

u (m/s)

0.1 0.3 0.5 0.7

Without the plane jet

0.9 1.1 1.3 1.5 –0.3

With the plane jet, U0 = 3 m/s With the plane jet, U0 = 4 m/s

–0.2 –0.1 0 0.1 0.2 0.3 Distance from the center of two manikins (m)

Fig. 16 (a) Velocities above the manikin for a distance of 0.35 m between two manikins and (b) The downward plane jet from the jet diffuser, and the upward thermal plume, visualized for Case d when the supply airflow velocity was 3.0 m/s

316

Protected zone ventilation Critical supply air velocity

As Figures 6–8 show (Downward ventilation method), Case f is like normal downward ventilation or mixing ventilation. For this case, the cexp/cR results are comparable to the values obtained in other studies (Bjørn and Nielsen, 2002; Nielsen et al., 2012; Olmedo et al., 2013). It can be concluded that a cross-infection risk occurs in the microenvironment around the two persons although there is some mixing flow in the room. In addition, the value of ct-zone increases when the two manikins are placed closer to one another. In Cases d and e, there is a strong mixing flow in the room, which is caused by the induction of the downward plane jet. The downward jet disrupts the microenvironment around the two persons and the jet also disrupts the exhaled plume from the source manikin, but it does not create two completely separate zones. It is probably not a fully ‘protected zone ventilation’ PZV, but more a division of the microenvironment around the two persons with fully mixed surroundings, in which the direct exposure to exhalation airflow is reduced. In Figures 12–14 (HPZV), with the using of both swirl diffusers and push-pull slots, the disruption of the source manikin’s exhalation plume is observed with different supply air velocities. It indicates that some division of the microenvironment around the two persons takes place. The value of ct-zone/cR shows that the two persons are exposed to somehow fully mixed surroundings with a plane jet between, when the supply air velocity is 2.0 and 3.0 m/s. When the supply air velocity is increased up to 4 m/s, the exhalation plume is fully disrupted and the concentration inhaled by the target manikin, cexp, is below the fully mixed value. The cexp/cR value of 0.6 is 40% lower than the case for fully mixed ventilation, 1.0. The plane jet and the two swirl diffusers create a protective zone with low concentration around the target. Table 4 compares the downward jet velocity at head height level (without people) with the peak flow velocity of exhalation (without downward jet). The air velocity of the downward jet is obtained using Equation 1 and the air velocity of exhalation is obtained from measurements. The peak value of the exhalation flow is much higher than the downward jet. However, exhalation is a pulsating airflow and the total volume is much smaller than in the downward jet. In this study, the measurement results show that the HPZV (with extra supply openings to the zones) works better Table 4 Velocities of the downward plane jet and exhalation airflow U (m/s)

ux downward jet (m/s)

uexh, peak value of exhalation (m/s)

2.0 3.0 4.0

0.4 0.6 0.8

2.5  0.5 2.5  0.5 2.5  0.5

if the supply air velocity is as high as 4.0 m/s, which may be considered as an effective ‘protecting velocity’. However, the ‘protecting velocity’ may be changed if the ratio of the exhaust airflow to the supply airflow changed. The good results may be obtained because of the slot and the exhaust works as a well-designed pushpull principle. In this study, supply air velocities lower than 4.0 m/s, like 2.0 and 3.0 m/s may not protect occupants effectively. However, many other factors may also influence the protection efficiency, like the width of slot, the distance between the mouth and the jet, and the occupant behavior. In this study, a pair of stationary manikins in fixed orientation with regard to the ventilation flows represents a limited view into how real people would behave in a shared indoor environment in which disease transmission might occur. For example, movement of occupants would definitely disrupt the downward plane jet flow patterns in a room with PZV or HPZV; and positioning of occupants in a manner that is different than studied with respect to the ventilation flow patterns in the room may also affect the selection of the critical protection supply air velocity. In practice, all these considerations may have an impact on the utility of the PZV or HPZV method to protect occupants against the spread of disease. Practical limitations

The study does not address many of the practical limitations that would be encountered in the use of PZV or HPZV in real indoor environments. Noise might be a serious issue of using PZV or HPZV; however, it needs specific studies under different conditions. Potential for draft to contribute to thermal discomfort is likewise a real concern in a close proximity of a person. Some specific conditions, like a hospital or aircraft, require higher supply airflow rate from 12 to 40 ACH. ASHRAE/ASHE Standard 170 (2013) even suggested some locations, like operation rooms, surgical cystoscopic rooms and delivery rooms in a hospital should have a minimum total air change rate of 20 h1. Additional energy used to operate the air curtain might be another concern, which needs more investigation in the future. In addition, the limitations of using manikins instead of real human subjects may also over or under estimate the potential benefits that were realized by the use of the breathing thermal manikins. So, future works on PZV and HPZV may take all these limitations into account, including, noise issues, thermal comfort issues, and energy efficiency issues. The downward plane jet from the linear slot is considered as a 2D flow, so the results may be valid for any width of the room. In the design phase, the necessary supply air flow rate per unit width of the room needs to be considered. In this study, all the results were obtained in a relatively narrow room, 2.0 m wide, so further studies on PZV and HPZV may consider to exam the effect of different 317

Cao et al. room shapes and the ratio of the width of the diffuser and the width of the room on the performance of PZV and HPZV as well. Conclusion

Unlike traditional ventilation systems, which are based on total volume airflow method, the principle of PZV and HPZV is to separate an internal space into different personal work areas or subzones using downward plane jets. The plane jets prevent possibly polluted air from moving from one subzone to other subzones, and additional fresh air can be delivered to each subzone via extra supply. With traditional ventilation methods, like mixing ventilation, displacement ventilation and downward airflow distribution, the risk of cross-infection between two persons is high when the distance between the two persons is as little as 0.35 m in a room. The exposure value of cexp/cR could be as high as 13 when downward flow ventilation with an airchange rate of 5.6 h1. At the same distance between two manikins, 0.35 m, using HPZV and PZV, the personal exposure cexp/cR is reduced to 0.6 and 1.6, with air-change rates of 15 and 4.2 h1, respectively. The measurement results demonstrated that in both the PZV and the HPZV system it is possible to decrease the transmission of tracer gas from one manikin to the opposite manikin; therefore it would probably reduce the risk of air borne cross-infection between two people at the same relative positions. Personal exposure when using HPZV and PZV may be twenty times lower than when using DV and downward airflow ventilation when the distance between the two manikins is 0.35 m. The associated reduction of exposure may indicate that PZV and HPZV could be used to reduce air- borne cross-infection in a space, in which a doctor faces a patient who has a respiratory disease. The protecting effect of the PZV system is attributable the disruption of the direct exhalation flow in the microenvironment around the two manikins. When the distance between two standing BTMs is 0.5 m, the threshold supply air velocity is even found to be below 1.8 m/s, reducing the direct exposure of the protected manikin to the source manikin. The distance between the two manikins also affects the performance of the downward plane jet. The protection ability of PZV and HPZV can be improved when the principle of ‘push-pull’ ventilation is applied, which requires an optimal ratio of the supply airflow to the amount of exhaust.

The transmission of pollutant from exhalation generated by one BTM to another is also influenced by the air movement around the BTMs. The supply air velocity of the plane jet has significant effect on the performance of the HPZV. When the supply air velocity increases up to 4 m/s, the exposure indicator (cexp/cR) is 40% lower than for the fully mixed ventilation. The local microenvironment created around the manikins with the downward plane jet was observed and no collision of the rising thermal plume and the plane jet was found. The personal exposure increases substantially when the supply airflow rate is high and the distance between the two manikins is 1.1 m. However, the cause of the increase remains somehow unclear without detailed knowledge of the airflow conditions in the room. Further study may use PIV or CFD simulations to examine the detailed flow structure around the target manikin and the trajectory of the exhaled pollutants in the room. Acknowledgements

The authors thank for the financial support from the Academy of Finland and VTT Technical Research Centre of Finland through the post-doctoral project POWER-PAD (NO. 259678). Supporting Information

Additional Supporting Information may be found in the online version of this article: Figure S1. The dimensions of the breathing thermal manikin (BTM) and the location BTM with different combination of air supply and exhaust: (a) source BTM, (b) target BTM, (c) the location of air supply and exhaust in Series NO. 1, (d) the location of air supply and exhaust in Series NO. 2, (e) the location of air supply and exhaust in Series NO. 3. Figure S2. Visualization of the interaction between the breathing process and the download plane jet (the distance between the two standing manikins is 35 cm, orientation of the image is looking along the length of the room, and two manikins are in the centre of the room beside the downward plane jet. Figure S3. Visualization of the interaction between the breathing process and the download plane jet (the distance between the two standing manikins is 50 cm, orientation of the image is looking along the length of the room, and two manikins are in the centre of the room beside the downward plane jet).

References Adams, W.C. (1993) Measurement of Breathing Rate and Volume in Routinely Performed Daily Activities, Sacramento, CA, California Environmental Protection Agency.

318

Bjørn, E. and Nielsen, P.V. (2002) Dispersal of exhaled air and personal exposure in displacement ventilated rooms, Indoor Air, 12, 147–164.

Brohus, H. (1997) Personal exposure to contaminant sources in ventilation. PhD Thesis, Aalborg University.

Protected zone ventilation Cao, G.Y., Kandzia, C., M€ uller, D., Heikkinen, J., Kosonen, R. and Ruponen, M. (2013a) Experimental study of the effect of turbulence intensities on the maximum velocity decay of an attached plane jet, Energ. Buildings, 65, 127–136. Cao, G.Y., Siren, K. and Kilpel€ ainen, S. (2013b) Modelling and experimental study of performance of the protected occupied zone ventilation, Energ. Buildings, 68, 515–531. Centres for Disease Control and Prevention (2014) http://www.cdc.gov/flu/weekly/ summary.htm. Chen, J. C. and Rodi, W. (1980) Turbulent buoyant jets – a review of experimental data. HMT, vol. 4, Pergamon, London. Corbett, E.L., Watt, C.J., Walker, N., Maher, D., Williams, B.G., Raviglione, M.C. and Dye, C. (2003) The growing burden of tuberculosis – global trends and interactions with the HIV epidemic, Arch. Intern. Med., 163, 1009– 1021. Gao, N.P. and Niu, J.L. (2006) Transient CFD simulation of the respiration process and inter-person exposure assessment, Build. Environ., 41, 1214–1222. Gupta, J.K., Lin, C.H. and Chen, Q.Y. (2010) Characterizing exhaled airflow from breathing and talking, Indoor Air, 20, 31–39. Heiselberg, P. and Topp, C. (1997) Removal of airborne contaminants from a surface tank by a push-pull system. Proceedings of the 5th international symposium on ventilation for contaminant control, Ottawa, Canada. ISSN 1395-7953 R9746. Holmberg, S. and Li, Y.G. (1998) Modelling of the indoor environment – particle dispersion and deposition, Indoor Air, 8, 113–122. Kulmala, I., Hynynen, P., Welling, I. and S€a€am€anen, A. (2007) Local ventilation

solution for large, warm emission sources, Ann. Occup. Hyg., 51, 35–43. Li, Y., Leung, G.M., Tang, J.W., Yang, X., Chao, C.Y.H., Lin, J.Z., Lu, J.W., Nielsen, P.V., Niu, J., Qian, H., Sleigh, A.C., Su, H.J.J., Sundell, J., Wong, T.W. and Yuen, P.L. (2007) Role of ventilation in airborne transmission of infectious agents in the built environment – a multidisciplinary systematic review, Indoor Air, 17, 2–18. Melikov, A.K., Radim, C. and Milan, M. (2002) Personalized ventilation: evaluation of different air terminal devices, Energ. Buildings, 34, 829–836. Melikov, A.K., Cermak, R., Kovar, O. and Forejt, L. (2003) Impact of airflow interaction on inhaled air quality and transport of contaminants in rooms with personalized and total volume ventilation. Proceedings of Healthy Buildings 2003, Singapore, 592–597. ISBN 9810499744, 9789810499747. Morawska, L. (2006) Droplet fate in indoor environment, or can we prevent the spread of infection? Indoor air, 16, 335– 347. Nielsen, P.V. (2009) Control of airborne infectious diseases in ventilated spaces, J. R. Soc. Interface, 6, 747–755. Nielsen, P.V., Hyldg ard, C.E., Melikov, A., Andersen, H. and Soennichsen, M. (2007) Personal exposure between people in a room ventilated by textile terminals: with and without personalized ventilation, HVAC Res., 13, 635–644. Nielsen, P., Buus, M., Winther, F.V. and Thilageswaran, M. (2008) Contaminant flow in the microenvironment between people under different ventilation conditions, ASHRAE Trans., 1, SL -08-064, 632–638. Nielsen, P.V., Olmedo, I., de Adana, M.R., Grzelecki, P. and Jensen, R.L. (2012) Air-

borne cross-infection risk between two people standing in surroundings with a vertical temperature gradient, HVAC Res., 18, 1–10. Nielsen, P.V., Zajas, J., Litewnicki, M. and Jensen, R.L. (2014) Breathing and crossinfection risk in the microenvironment around people, ASHRAE Papers CD: 2014 ASHRAE Winter Conference. Olmedo, I., Nielsen, P.V., de Adana, M.R., Jensen, R.L. and Grzelecki, P. (2012) Distribution of exhaled contaminants and personal exposure in a room using three different air distribution strategies, Indoor Air, 22, 64–76. Olmedo, I., Nielsen, P.V., de Adana, M.R. and Jensen, R.L. (2013) The risk of airborne cross-infection in a room with vertical low-velocity ventilation, Indoor Air, 23, 62–73. Rajaratnam, N. (1976) Turbulent Jets, Amsterdam, The Netherlands, Elsevier. Valkeap€ a€ a, A. and Siren, K. (2010) The influence of air circulation, jet discharge momentum flux and nozzle design parameters on the tightness of an upwards blowing air curtain, Int. J. Vent., 8, 337–346. Wang, M., Lin, C.H. and Chen, Q.Y. (2012) Advanced turbulence models for predicting particle transport in enclosed environments, Build. Environ., 47, 40–49. World Health Organization (2014) http:// www.who.int/influenza/surveillance_ monitoring/updates/latest_update_GIP_ surveillance/en/. Yu, I.T.S., Li, Y.G., Wong, T.W., Tam, W., Chan, A.T., Lee, J.H.W., Leung, D.Y.C. and Ho, T. (2004) Evidence of airborne transmission of the severe acute respiratory syndrome virus, N. Engl. J. Med., 350, 1731–1739.

319