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Protection of occupants from exhaled infectious agents and floor material emissions in rooms with personalized and underfloor ventilation.

The performance of two personalized ventilation systems supplying air at the breathing zone was tested in conjunction with underfloor ventilation generating two different airflow patterns in a full-scale test room. Two breathing thermal manikins were used to simulate occupants. The distribution of pollutants associated with exhaled air and floor material emissions was evaluated at various combinations of personalized and underfloor airflow rates. Compared to underfloor ventilation alone, personalized and underfloor ventilation provided excellent protection of seated occupants from any pollution, while the concentration of exhaled air pollution increased in the room. The two types of personalized ventilation performed differently. Subsequent analyses of airborne infection transmission risk indicated that personalized ventilation could become a supplement to traditional methods of infection control.

INTRODUCTION

Air exhaled by people when coughing, sneezing, or talking contains aerosol droplets up to 100 [micro]m in diameter (Brosseau et al. 1994; Duguid 1945) and respirable particles 0. 1-1 [micro]m in diameter (Hinds 1999) that may carry viral and bacterial agents. After expulsion, the larger droplets (larger than 60-100 [micro]m) settle out from the air or evaporate to droplet nuclei (up to 10 [micro]m). Such nuclei have very low settling velocities and, together with respirable particles, are transported by the room air. Thus, there is a possibility for the airborne transmission of respiratory infectious agents between occupants. A recent literature review shows strong evidence that the airborne transmission of some infectious diseases is associated with building ventilation (Li et al. 2006).

Floor surface area is typically large compared to other objects in rooms. Studies (Mendell 1993; Wolkoff et al. 1995; Jaakola et al. 1999, 2000) have associated the prevalence of building- related symptoms with the presence of carpeting, polyvinyl chloride (PVC), and linoleum, which emit a variety of gaseous pollutants, including phthalates. Phthalates in home dust have been shown to increase the risk of asthma and allergies among children (Bornehag et al. 2004). Therefore, studying exposure to floor material emissions is very important.

Personalized ventilation (PV) is an air distribution principle that supplies air at each workplace, aiming to improve the inhaled air quality as well as the thermal comfort of each occupant. Because occupants are allowed to adjust the rate and sometimes temperature of the supply air, other means of ventilation and air conditioning, e.g., total-volume ventilation, must be provided together with PV to ensure acceptable air quality and thermal environment in the room.

The inhaled air quality depends on the interaction of personalized airflow, room airflow, and pollution sources (Melikov 2004a, 2004b). PV supplying clean air at the breathing zone is able to decrease the inhaled pollutant concentration 2 to 50 times compared to total-volume ventilation alone (Melikov et al. 2002; Cermak 2004; Cermak et al. 2006). At the same time, PV was shown to promote mixing of room air in non-uniform environments. Cermak et al. (2006) documented that in a room with personalized and displacement ventilation, occupant exposure to agents associated with exhaled air could increase.

Underfloor ventilation, also referred to as underfloor air distribution (UFAD), delivers air directly to the occupied zone through floor diffusers or grilles. Air is returned at the ceiling. The height above the floor where the supply airstream velocity decreases to 0.25 m/s is referred to as the maximum penetration height or the throw height. A short throw has been associated with a displacement-like airflow pattern, while a long throw is said to promote mixing. An extensive report on UFAD was given in Bauman and Daly (2003).

Knowledge concerning the performance of PV in conjunction with UFAD is limited. A pilot study (Cermak and Melikov 2003, 2004) suggested an overall similarity between personalized and displacement ventilation (Cermak et al. 2006). Detailed analyses of the airflow and pollutant interactions on the air quality remain to be made.

In this study, the performance of two PV systems was tested in conjunction with a UFAD system generating two different throw heights in a full-scale test room. The system performance was tested with regard to the pollutants released through human exhalation and emitted by the floor surface. The measurements of inhaled air concentrations and vertical profiles of concentration, temperature, and velocity are reported for different scenarios of PV use. The risk of airborne infection transmission was assessed based on the model by Rudnick and Milton (2003).

METHOD

Measurements were performed in a full-scale experimental room (5.4 x 4.8 x 2.6 m) in which there were two workplaces. Each workplace consisted of a desk with an air terminal device for PV, a computer tower and monitor, a lamp, and a seated breathing thermal manikin (Figure 1). Six fixtures located on the ceiling provided the overall lighting. The room was built in a laboratory hall in which the air temperature was maintained close to the room air temperature in order to reduce the heat flux through the walls. The walls were sealed prior to the measurements. The room was ventilated by means of a combination of PV and a UFAD system.

[FIGURE 1 OMITTED]

Personalized Ventilation

Two types of air terminal devices for PV were tested (Figure 2). The round movable panel (RMP) consisted of an air distribution box with a round supply opening mounted at the end of a movable arm duct. The supply opening had a diameter of 0.185 m and was equipped with a honeycomb flow straightener. The second type was a vertical desk grille (VDG), which was a slot diffuser sized 0.02 x 0.22 m mounted at the front desk edge. Units of the same type were mounted on the desks at the same time and tested. They were adjusted to discharge air directly to the breathing zone of the seated occupants. The positioning (also the studied airflow rate, see Table 1) corresponded to the settings most often preferred by occupants (Kazcmarczyk et al. 2002, 2004).

Underfloor Ventilation

The UFAD system consisted of four diffusers positioned on the floor so as not to directly affect the workplaces nearby (Figure 1). There was a swirl element in the diffusers that made it possible to adjust the direction of supply jets from horizontal (parallel to floor) to vertical (upward). Only the vertical discharge was used in the present study. The throw height of the swirl jets was adjusted by the supply airflow rate.

Breathing Thermal Manikins

Two thermal manikins with a controlled surface temperature (comfort mode [Tanabe et al. 1994]) were used. They were identical in shape and were seated upright at the desks with a breathing zone height of 1.1 m (Figure 2). They were equipped with artificial lungs that simulated human breathing (Melikov et al. 2000). The breathing frequency was set at 10 inhale/exhale cycles per minute with a pulmonary ventilation of 6 L/min, or 0.6 L per breath. The exhaled air was mixed with a tracer gas and heated to a density similar to the density of the gas exhaled by people (1.144 kg/[m.sup.3]). The air was exhaled through the nose.

Pollution Sources

The performance of the combined systems was tested with regard to pollution associated with exhaled breath, i.e., respiratory infectious agents and pollution emitted by floor covering. Constant doses of sulphur hexafluoride (S[F.sub.6]) and nitrous oxide ([N.sub.2]O) were released to the air exhaled from the front and the rear manikins, respectively. The floor emissions were simulated by carbon dioxide ([CO.sub.2]); a rectangular grid of tubing perforated with a module of 0.6 m ensured a uniform release of [CO.sub.2] over the floor area. The tubing layout was optimized for the uniformity of tracer-gas distribution.

[FIGURE 2 OMITTED]

Experimental Conditions

Four combinations arranged from two types of PV and two throw heights of UFAD were tested at various combinations of airflow rates (Table 1). The throw heights tested were 0.3 and 1.0 m, corresponding, respectively, to supply flow rates of 12.5 and 20 L/s per diffuser (total airflow rates were 50 and 80 L/s, respectively). The patterns of PV use involved either one or both manikins using the same type of PV at a rate of 15 L/s per desk. Both PV and UFAD provided clean outdoor air (concentration of tracer gases was monitored throughout the experiments). The personalized air temperature was 20[degrees]C. The underfloor air temperature was adjusted as listed in Table 1 so as to maintain the exhaust air temperature at 26[degrees]C (calculated).

Criteria for Assessment

Performance was tested with regard to the pollutant concentration in the air inhaled by the thermal manikins: [CO.sub.2]--floor pollution; S[F.sub.6]--air exhaled by the front manikin; [N.sub.2]O--air exhaled air by the rear manikin. In the room, the pollutant concentration, velocity, and temperature were measured at two locations (Figure 1). The locations were selected arbitrarily near the workplaces out of the immediate reach of breathing, convection flows generated by the human body and heat sources, and ventilation flows (distance from the center of the nearest floor-based terminal was 0.95 m in both cases). The concentration was measured at six heights (0.1, 0.6, 1.1, 1.4, 1.7, and 2.2 m above the floor) with a real-time gas monitor based on the photoacoustic principle of measurement. The velocities and temperatures were measured at eight heights (0.05, 0.1, 0.2, 0.6, 1.1, 1.4, 1.7, and 2.2 m above the floor) with a set of 16 omnidirectional, low-velocity transducers connected to two eight-channel thermal anemometers. Strictly speaking, these transducers measure speed (i.e., magnitude of velocity), but in this paper, for simplicity, the term velocity is used. Characteristics of the anemometers complied with the recommendations of ISO (1998) and ASHRAE (2005). The parameters of the supply air, the exhaust air, and the laboratory conditions were monitored throughout the experiments as well.

Both the concentration and temperature data were expressed in terms of their normalized values. This was generally defined as (x - [x.sub.S])/([x.sub.E] - [x.sub.S]), where x is the concentration or temperature at a point (room or inhaled air), [x.sub.S] is the supply air concentration or temperature, and [x.sub.E] is the exhaust air concentration or temperature.

Procedure

The experimental conditions were set up in the evening prior to each experiment to stabilize overnight. The tracer-gas dosing apparatus was turned on in the morning. The measurements began when the steady-state condition in the experimental room was reached. Because the sampling channels of the gas monitor were limited to six, the measurement locations were split into groups of six and analyzed sequentially. Ten to twelve readings were taken at each location. The velocity and temperatures were measured when the group measurements finished (approximately at two-hour intervals). The sampling frequencies and the duration of the velocity and temperature measurements were, respectively, 5 Hz and 5 min.

Uncertainty of Measurement

The data were treated in accordance with the ISO guideline for the expression of uncertainty (ISO 1993). Possible variations in temperature or velocity during the day were accounted for by including the process stability component in the combined uncertainty analyses (variance of the mean values). The instability was extremely low. The absolute expanded uncertainty of the mean, with a level of confidence of 95%, was: normalized tracer-gas concentration less than [+ or -]0.1 (depending on the gas concentration level, typically less than [+ or -]2% of the measured absolute mean), normalized air temperature less than [+ or -]0.1 ([+ or -]0.3[degrees]C for the measured absolute mean), and velocity [+ or -]0.03 m/s.

RESULTS

Airflow Pattern

Figure 3 shows the contours of the mean air velocity for different tested conditions near the UFAD diffusers. It documents that the variations in supply air temperature had a much smaller impact on the throw height than the variations in the flow rate (Table 1).

The airflow pattern in front of the manikins was a result of the complex interaction of the flow from PV, the free convection flow around the manikins, and the transient exhalation flow. At the supply opening, the airflow from the RMP was laminar (due to the flow straightener) with a mean air velocity of 0.56 m/s and a turbulence intensity of 10% at 15 L/s. Measurements without the manikins at the desks showed that the mean air velocity at the center of a free jet did not decrease below 90% of its initial level at a distance of 0.4 m from the opening. The mean air velocity at the VDG opening was 3.41 m/s at 15 L/s. The airflow from the VDG was with rectangular initial cross section and turbulent (flow straightener absent) and had fast velocity decay.

Pollutant Distribution

The concentration profiles measured under different tested conditions were very similar (1) at locations A and B and (2) at two locations for matching (reversed) patterns of PV use. For example, the profiles of S[F.sub.6] emitted by the front manikin when only the front manikin used PV were similar to the profiles of [N.sub.2]O emitted by the rear manikin when only the rear manikin used PV. Therefore, in order to allow for generalization, the experimental conditions were regrouped as shown in Table 2 and the concentration profiles for the matching patterns and the two locations were averaged and presented.

[FIGURE 3 OMITTED]

Exhaled Air (Infectious Agents)

In the reference cases (without PV applied), two-zonal distribution of exhaled air pollution existed in the room, with low concentrations near the floor and higher concentrations near the ceiling (Figures 4 and 5). The interfacial layer, which separated the two zones, was slightly lower for the short throw (0.8-1.2 m) than for the long throw (1.1-1.4 m). Most likely this was due to the different supply rates. The length of the throw had a strong impact on the concentration near the floor--low concentrations have been associated with a short throw and vice versa.

The RMP used by a nonpolluting manikin alone did not affect the vertical distribution of exhaled pollution at any throw height (Figure 4). The flow was too weak to cause major disturbances to the room airflow pattern. For the same scenario, the VDG increased the concentration of exhaled pollutants in the lower occupied zone and decreased their concentration below the ceiling. This was apparent especially in conjunction with a short throw. The similarity of concentration profiles measured at different workplaces implied that the location of a source did not have an impact on the pollutant distribution.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The distribution of exhaled pollution changed completely when only the polluting manikin used PV (Figure 5). The RMP airflow reached the breathing zone of the manikins and brought the pollution down to the lower occupied zone despite its low mean velocity and low turbulence intensity. The distribution of exhaled pollution was almost uniform with the RMP and thus similar to a typical distribution in rooms with mixing ventilation. Although less apparent, the results were somewhat similar for the VDG.

The inhaled air analyses, which give a more accurate assessment of occupant exposure, identified clear differences between the systems. The concentration of exhaled pollution was four to five times lower with the short throw than with the long throw (Figures 6 and 7).

As expected, PV supplying clean air to the breathing zone decreased the pollutant concentration in inhaled air in most cases (Figure 6). Regardless of the throw height, the lowest concentrations were achieved with the RMP due to its long and wide core region of clean air. The VDG was also able to decrease the concentration but only in conjunction with the long throw. Because of large entrainment of polluted room air, the inhaled air did not decrease with the VDG and the short throw.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

A polluting manikin using PV and an exposed manikin not using PV characterized the worst scenario. The measurements identified a clear increase in the inhaled concentration for the exposed manikins compared to the reference cases (Figure 7). Lower concentrations were associated with a short throw of UFAD. Further, the concentrations were lower with VDG than with RMP.

The impact of PV on the exhaled pollution diffusion was also tested at a rate of 7 L/s. The results show that the VDG outperformed the RMP yet again (Figure 8). Therefore, limiting the airflow rate up to a certain level is not likely to remove the impact of PV on the pollutant distribution. Neither could the airflow rate limit level out the difference between the PV types. At the lower rates, as previous results on the performance of PV revealed (e.g., Melikov et al. 2002), the inhaled air quality for occupants using PV decreases (the benefit is balanced with a loss).

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Floor Pollution

Pollution emitted from floor covering was studied as well. In the reference case, the vertical distribution (not presented) was almost uniform; thus it was similar to the distribution with mixing ventilation (see Cermak et al. [2006] for details on displacement and mixing ventilation). The inhaled concentration was comparable to the exhaust concentration (Figure 9). The improvement with PV was substantial. Contrary to the exhaled pollution, the use of PV at one desk did not affect the concentration at the other desk (Figure 9, right). Neither did the length of the throw have an impact on the inhaled concentration.

Velocity and Temperature

The mean air velocities and temperatures were compared for UFAD alone and in conjunction with PV used at 15 L/s per person, which is when the impact of PV was greatest (Figures 10 and 11). Very low velocities of around 0.05 m/s were measured in all cases. The impact of PV on the room velocity distribution was practically negligible.

As expected, the temperature gradient was positive along the room height. The normalized temperature near the floor was 0.7[degrees]C with both throw heights. The room air temperature increased when PV was used, but complete temperature uniformity was never established. The impact of the VDG was larger than the impact of the RMP, most likely because of its stronger air jet, which creates high supply air velocity and turbulence intensity.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

DISCUSSION

Inhaled Air Quality and Thermal Comfort

The inhaled air quality with regard to exhaled pollution has been shown to depend on the relation between the vertical throw height of UFAD and the elevation of the interfacial layer. As demonstrated by Yamanaka et al. (2002), a low pollutant concentration was achieved in the occupied zone when the vertical throw was shorter than the layer of interface and vice versa. Therefore, maintaining the throw lower than the interfacial layer height is the key to achieving high air quality with UFAD systems.

The ability of personalized ventilation to protect occupants from pollution was tremendous. Compared to UFAD with a long throw height alone, the RMP and the VDG terminals decreased the inhaled air concentration approximately 25 and 2.5 times, respectively. Compared to the exhaust air, the inhaled air concentration decreased, respectively, 50 and 5 times. Such improvements are comparable or higher than the improvements reported previously for various types of PV (Faulkner et al. 1999, 2004; Melikov et al. 2002; Bolashikov et al. 2003).

PV appears to be the only principle that is able to decrease occupant exposure to floor pollution. The present results, accompanied by previous results (Cermak et al. 2006), reveal that a total-volume ventilation system commonly used today (mixing, displacement, underfloor ventilation) could not achieve such a low concentration of floor pollution in inhaled air. The improvement with PV will depend on the characteristics, direction, and rate of personalized air, and it will be independent of the total-volume ventilation principle.

The RMP was superior to the VDG in all respects, except when used in the vicinity of a pollution source. At rates of 7 and 15 L/s, the RMP diffused the exhaled pollution throughout the occupied zone and increased the inhaled concentration for occupants not using PV. Therefore, in rooms with strict requirements regarding exhaled pollution (infection) control, the VDG terminal may be preferred. Otherwise, owing to its outstanding performance in other situations, the application of RMP can be recommended.

It is difficult to generalize concerning the impact of airflow direction and supply air velocity of PV. The two factors are connected (VDG: upward airflow, high velocity; RMP: horizontal/downward airflow, low velocity). Because RMP caused a higher transmission of exhaled pollution than VDG, despite its much lower supply air velocity (which was consistent at both 7 and 15 L/s), the airflow direction seems to be more important than the supply air velocity for the distribution of exhaled air pollutants.

The layout of an office may have a great influence on the airflow pattern in rooms and, thus, the distribution of pollutants. The office arrangement must be considered in rooms with displacement ventilation, where the spread of exhaled pollution can be directional (Cermak et al. 2006; Bjorn and Nielsen 2002). The measurements with the VDG (Figure 7) showed that the transmission of exhaled pollution between the two workplaces was independent of the location of the workplace where the pollution was generated. A small difference in exposure was found for the RMP in conjunction with the short throw. The reason is most likely the airflow direction, which caused the pollution to spread below and above the breathing zone, respectively, with the two terminals. The free convection flow around the human body projected the spatial differences near the floor to the inhaled air (RMP), whereas below the ceiling the air was exhausted (VDG).

Reversibility of experiments (Table 2) and similarity of results (Figures 6 and 7, with minor inconsistency for the RMP) implies that the exposure of occupants to exhaled air was independent of the location of the polluting workplace. This is an important result that could make it possible to extend the present data to larger, open-plan offices with various arrangements of workplaces and a greater number of occupants. Although the cross-interference of PV systems was not studied in detail, the air quality benefits of PV (Figure 6), which are limited to workplaces, are likely independent of the office layout, too.

The normalized air temperatures recorded near the floor (0.7[degrees]C) agreed well with the accounts of Bauman and Daly (2003). Compared to displacement ventilation (Cermak et al. 2006), the temperatures were higher with UFAD--the slope of the temperature gradient was less steep. This gives UFAD an advantage over displacement ventilation in that potentially fewer occupants will be dissatisfied with draft and a high temperature difference between head and ankles.

The measurements of the thermal environment did not indicate a substantial cross-influence of PV systems used at different desks, apart from a small increase in room air temperature when both PV units were used. Because large changes in room air temperature could affect the thermal comfort of occupants, air terminal devices with a small impact on the environment should be preferred. The present study favors the RMP as opposed to the VDG.

Risk of Airborne Infection Transmission

Rudnick and Milton (2003) derived an equation that determines the risk of transmission of infectious diseases using [CO.sub.2] concentration as a marker for exhaled-breath exposure. In order to demonstrate the impact of PV and other systems on the risk of transmission, the reproductive number for an infectious disease was calculated (the concept of normalized concentration was introduced in the model; see Appendix). The reproductive number represents the number of secondary infections that arise when a single infectious case is introduced into a population where everyone is susceptible.

In the example documented in Figure 12, the typical values of a normalized concentration of exhaled pollution inhaled by the thermal manikins were used to calculate the reproductive number at two ventilation rates of outdoor air. One of the rates, 10 L/s, corresponds to the ventilation rate of outdoor air required by the present standards (CEN 1998; ASHRAE 2004). A quantum generation rate of 100 quanta per hour, estimated by Rudnick and Milton (2003) for influenza, was used in the calculation. It was also assumed that 30 persons occupy the room for eight hours (chosen arbitrarily).

[FIGURE 12 OMITTED]

The calculations showed that in the case of mixing ventilation and a supply rate of 10 L/s per person, it is likely that 7 out of 30 occupants will contract influenza after a working day. The number of possibly infected persons decreases to just two (one already infected and one secondarily infected) if either the ventilation rate is increased to 40 L/s per person or a UFAD system with a short throw is employed. The use of PV would most likely result in complete protection of occupants.

Apart from occupant protection, PV may also transport the exhaled pollution between the occupants. The calculations, assuming a normalized concentration of 0.7 and an airflow rate of 10 L/s per person, suggest that there is a probability of as many as three new cases of influenza (in a total five of cases) if the infector uses PV while the other occupants do not use PV for protection. This is potentially important and should not be neglected, although the exposure risks due to the infector's use of PV will always be lower than or comparable with mixing ventilation alone.

The later calculation is based on assumptions that are not entirely supported by present data. A normalized concentration of 0.7 was measured at a rate of 40 L/s per person. If 10 L/s per person were used, personalized and underfloor ventilation supplying, respectively, clean (outdoor, purified) and recirculated air is most likely. However, such a concept was not studied. Associated exposures are expected to be higher than those reported because the room concentration gradient would shift toward higher values. Mixing is considered the limit case.

It has been assumed that the loss of viability, filtration, settling, and other mechanisms are small compared with removal by ventilation (discussed by Rudnick and Milton [2003]). An additional assumption is that the efficiency of a ventilation system will be independent of the room size and the space layout. This was fulfilled in the present study because the inhaled concentrations were similar for the front and the rear manikins when exposed to the reverse patterns of use (patterns II and III, Table 1).

The infectiousness of a disease is crucial for the risk of transmission analyses. With a less infectious agent such as rhinovirus (Rudnick and Milton [2003] estimated the generation rate at 4 quanta per hour), the infection would not spread in any of the cases outlined in Figure 12. This implies that the ability of PV to increase the transmission of exhaled pollution between occupants would not be a problem. On the other hand, it is very difficult to prevent the spread of highly infectious diseases such as measles (Rudnick and Milton's estimate was 570 quanta per hour). For the given conditions it was calculated that the spread of measles would cease at a normalized concentration of 0.02. This is comparable to the top performance of the RMP terminals tested.

Given the unique ability to protect occupants from airborne pollution, PV could become a supplement to traditional methods of infection control (ultraviolet radiation, highly efficient filtration). Direct comparison of different engineering control methods remains to be made.

The main application of PV can be seen for spaces where occupants (seated or standing) spend a relatively long time at a particular workplace, such as offices, schools, theaters, hospital and hotel reception desks, cashiers in supermarkets, post offices, call centers, vehicles, etc. It should be noted that the studied design of PV may not be applicable in environments with relatively high activity of the occupants, e.g., health care environments, where susceptible workers as well as infectors (patients) may move around and so have difficulty accessing the control of the PV.

The results indicate that efficient air distribution, and PV in particular, has a potential to mitigate the risk of airborne transmission of contagious respiratory diseases. This could prove beneficial in cases such as the recent SARS epidemic (Li et al. 2004, 2005) and so in the future should be considered by the building and transport industries in light of the potential threats of avian flu outbreaks and an influenza pandemic.

Disease transmission is a complex issue. Both large droplets and smaller droplet nuclei contribute to the exposure of occupants to respiratory infectious agents by airborne route. Tracer gases, which were used to simulate the agents in the present study, could not simulate properly the transport of larger droplets that are expelled with momentum, do not follow air currents, and are likely to settle out from air. Furthermore, only expulsion during breathing was considered, whereas transmission pathways associated with coughing and sneezing were not. Therefore, the given risk analyses may underestimate the real case.

CONCLUSIONS

* PV is able to protect occupants from inhaling airborne contaminants associated with exhaled breath (e.g., virulent agents) as well as contaminants emitted from floor covering. The concentration of contaminants in inhaled air depends on the design of an air terminal device and its use.

* The use of PV may cause mixing of contaminants generated in its vicinity, such as exhaled air. PV does not affect the distribution of contaminants emitted from floor covering.

* The direction of personalized airflow has an impact on the exposure to exhaled air when the throw height of underfloor ventilation is short. Airflow directed upward causes a lower transmission of exhaled air between occupants than the airflow directed downward, both at high and medium airflow rates.

* The exposure of occupants to exhaled air due to mixing caused by PV was nearly independent of the location of the workplace where the contaminants were generated.

ACKNOWLEDGMENTS

This work was supported by the Danish Technical Research Council (STVF) as part of the research program of the International Centre for Indoor Environment and Energy established at the Technical University of Denmark for the period 1998-2007.

NOMENCLATURE

C = concentration, g/[m.sup.3]

T = temperature, [degrees]C

Subscripts

E = exhaust air

S = supply air

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APPENDIX

The reproductive number ([R.sub.A0]) in a building environment can be derived (see Rudnick and Milton [2003]) as

[R.sub.A0] = (n - 1)[1 - exp(-f'qt)], (1)

where

n = number of persons in a ventilated space;

f' = volume fraction of inhaled air that was exhaled by infectors;

q = quantum generation rate by an infected person, quanta per hour; and

t = total exposure time, h.

In the present study, the volumetric fraction of inhaled air that was exhaled by infectors was calculated as

f' = [[V.sub.e]/V]C(-) = [C-[C.sub.0]]/[C.sub.a], (2)

where

[V.sub.e] = pulmonary ventilation, [m.sup.3]/s;

V = clean air ventilation rate, [m.sup.3]/s;

C(-) = normalized concentration, dimensionless;

C = volume fraction of the exhaled air tracer in inhaled air;

[C.sub.0] = volume fraction of the tracer in supply air; and

[C.sub.a] = volume fraction of the tracer added to the exhaled breath during breathing.

This was different from the original model by Rudnick and Milton (2003), where [CO.sub.2] produced by every person in a shared space was used as the exhaled air marker. In the present study, a single manikin produced exhaled pollution simulated by a tracer gas.

Radim Cermak, PhD

Arsen K. Melikov, PhD

Fellow ASHRAE

Received April 4, 2006; accepted July 31, 2006

Radim Cermak is a refrigeration systems specialist at Ingersoll Rand Climate Control Technologies, R & D Center, Prague, Czech Republic. Arsen K. Melikov is an associate professor at the International Centre for Indoor Environment and Energy, Department of Mechanical Engineering, Technical University of Denmark, Lyngby, Denmark.
Table 1. Experimental Conditions: Two Types of Air Terminal Devices for
Personalized Ventilation Were Tested for Condition Listed

 Supply Exhaust
 Outdoor Airflow Rate, L/s Temperature, Airflow
Pattern PV--Front PV--Rear [degrees]C Rate, Temperature,
of Use UFAD Manikin Manikin UFAD PV L/s [degrees]C

I 50 0 0 16.4 -- 50 26
II 50 0 15 18.2 20 65 26
III 50 15 0 18.2 20 65 26
IV 50 15 15 20 20 80 26
V 50 7 0 17.2 20 57 26
I 80 0 0 20 -- 80 26
II 80 0 15 21.1 20 95 26
III 80 15 0 21.1 20 95 26
IV 80 15 15 22.2 20 110 26

Table 2. Two Experiments (Defined by Different Combinations of Airflow
Rates and Tracer Gases) Were Used to Analyze Different Scenarios in
Terms of PV Use

Pattern of Use
Airflow Rate, L/s
PV-- PV-- Tracer
Front Rear Gas--Front
Manikin Manikin Manikin Comment (Scenario)

 0 0 S[F.sub.6] Neither manikin uses PV
 0 15 S[F.sub.6] Only exposed manikin uses PV
15 0 S[F.sub.6] Only polluting manikin uses PV
15 15 S[F.sub.6] Both manikins use PV

Reversed Pattern
Airflow rate, L/s
PV-- PV-- Tracer
Front Rear Gas--Rear
Manikin Manikin Manikin Comment (Scenario)

 0 0 [N.sub.2]O Neither manikin uses PV
15 0 [N.sub.2]O Only exposed manikin uses PV
 0 15 [N.sub.2]O Only polluting manikin uses PV
15 15 [N.sub.2]O Both manikins use PV
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Date:Jan 1, 2007
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