ASHRAE Journal - June 2011 - 88

iaq applications
some situations, the inhalation dose of such a low momentum puff may still be low due to the entrainment of clean air.9 The third unfavorable condition is when disturbances to the room air are strong, such as in multi-bed general wards and in waiting areas. Multiple health-care workers move around, causing disturbances in the room and leading to the room airflow to be quasi-mixing. This can be an issue if the ventilation rate was designed to be lower than the fully mixing ventilation requirement. A large number of studies recommend using displacement ventilation in non-hospital environments. Hillerbrant and Ljungqvist10 were probably the first to demonstrate the superior performance of displacement ventilation in a health-care setting. More recent studies recommend using displacement ventilation in a one-patient ward.11 Brohus and Nielsen3 also reported improved performance of displacement ventilation for a passive pollutant source at the head level of a thermal manikin. One difference between these studies and Qian, et al.,7 is that the latter considered the situations when the patient exhaled both upward and sideways.

Mixing Zone

Stratification Zone

Puff Weak Plume

Gravity Current

Figure 4: Exhaled puff and a weak plume trapped in the thermally stratified zone with other stronger plumes.
2. Li, Y., M. Sandberg, L. Fuchs. 1993. “Effects of thermal radiation on airflow with displacement ventilation: an experimental investigation.” Energy and Buildings 19(4):263 – 274. 3. Brohus, H., P.V Nielsen. 1996. “Personal exposure in displace. ment ventilated rooms.” Indoor Air 6(3):157 – 167. 4. Friberg, B., et al. 1996. “Inefficiency of upward displacement operating theatre ventilation.” Journal of Hospital Infection 33(4):263 – 272. 5. Lai, A.C.K., Y.C. Cheng. 2007. “Study of expiratory droplet dispersion and transport using a new Eulerian modeling approach.” Atmospheric Environment 41(35):7473 – 7484. 6. Bjørn, E., P.V Nielsen. 2002. “Dispersal of exhaled air and person. al exposure in displacement ventilated rooms.” Indoor Air 12:147–64. 7. Qian, H., et al. 2006. “Dispersion of exhaled droplet nuclei in a two-bed hospital ward with three different ventilation systems.” Indoor Air 16(2):111 – 128. 8. Mundt, E. 2008. “Air quality in the near zone of humans with different ventilation strategies.” The 29th AIVC Conference. 9. Li, X., J. Niu, N. Gao. 2011. “Spatial distribution of human respiratory droplet residuals and exposure risk for the co-occupant under different ventilation methods.” To be published in HVAC&R Research. 10. Hillerbrant, B., B. Ljungqvist. 1990. “Comparison between three air distribution systems for operating rooms.” Swiss Contamination Control, No 4a:336 – 339. 11. Yin, Y, et al. 2009. “Experimental study on displacement and mixing ventilation systems for a patient ward.” HVAC&R Research 15(6):1175 – 1191. 12. Nielsen, P.V et al. 2010. “Risk of cross-infection in a hospital ward ., with downward ventilation.” Building and Environment 45(9): 2008 – 2014. 13. Qian, H., Y. Li. 2010. “Removal of exhaled particles by ventilation and deposition in a multibed airborne infection isolation room.” Indoor Air 20(4):284 – 297. 14. Atkinson J, et al. 2009. “Natural Ventilation for Infection Control in Health-Care Settings. WHO Guidelines” World Health Organization.

summary
Displacement ventilation performs much better than mixing ventilation in certain conditions and performs worse in others when considering control of exhaled substances. If a study focuses only on the preferable conditions, it is likely that displacement ventilation will be found to perform superior to traditional mixing ventilation and other designs. Based on the available data and our understanding at present, and for protecting the health-care workers and/or other patients/visitors, we do not recommend the use of displacement ventilation in hospitals in either single-patient or multiple-patient wards for control of exhaled substances or any harmful infectious aerosols. Further field data on the locking-up phenomenon, weak plume trapping, particle deposition in the thermally stratified layers, the effect of occupant height variation, plume strength, droplet size and human movement, total exposure and risk analysis are needed. Possible alternative ventilation solutions for such situations include downward ventilation with upper level return7,12,13 and natural ventilation.14 Obviously, when the exposure to exhaled substances is not a major hygiene issue as in infection control, displacement ventilation may still be an attractive energyefficient solution for ventilation, e.g., in offices, classrooms and movie theaters or non-critical rooms in hospitals.

acknowledgments
Work on this column was supported by a grant from the Research Fund for the Control of Infectious Diseases (RFCID) of the Hong Kong SAR Government, commissioned by the Hospital Authority (Project no. HA-NS-07). We thank Andrew Persily, Chandra Sekhar and Jianlei Niu, Elisabeth Mundt, Magnus Mattsson for useful comments and Qian Hua for the photo in Figure 1.

References
1. Stymne, H., M. Sandberg, M. Mattsson. 1991. “Dispersion pattern of contaminants in a displacement ventilated room—implications for demand control.” The 12th AIVC Conference. 88 ASHRAE Journal

Yuguo Li, Ph.D., Fellow ASHRAE, is a professor at the Department of Mechanical Engineering, University of Hong Kong, Hong Kong, China; Peter V. Nielsen, Ph.D., Fellow ASHRAE, is a professor at the Department of Civil Engineering, Aalborg University, Aalborg, Denmark; Mats Sandberg, Ph.D., is a professor at the Research School, University of Gävle, Gävle, Sweden.
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