The cost of nosocomial infections in the United States is estimated to be $4 billion to $5 billion annually. Applying a scientifically based analysis to disease transmission and performing a site specific risk analysis to determine the design of the ventilation system can provide real and long term cost savings. Using a scientific approach and convincing data, this paper hypothetically illustrates how a ventilation system design can be optimized to potentially reduce infection risk to occupants in an isolation room based on a thorough risk assessment without necessarily increasing ventilation airflow rate. A computational fluid dynamics (CFD) analysis was performed to examine the transport mechanism, particle path and a suggested control strategy for reducing airborne infectious disease agents. Most studies on the transmission of infectious disease particles have concentrated primarily on air changes per hour (ACH) and how ACH provides a dilution factor for possible infectious agents. Although increasing ventilation airflow rate does dilute concentrations better when the contaminant source is constant, it does not increase ventilation effectiveness. Furthermore, an extensive literature review indicates that not every exposure to an infectious agent will necessarily cause a recipient infection. The results of this study suggest a hypothesis that in an enclosed and mechanically ventilated room (e.g., an isolation room), the dominant factor that affects the transmission and control of contaminants is the path between the contaminant source and exhaust. Contaminants are better controlled when this path is uninterrupted by an air stream. This study illustrates that the ventilation system design, i.e., when it conforms with the hypothesized path principle, may be a more important factor than flow rate (i.e., ACH). A secondary factor includes the distance from the contaminant source. This study provides evidence and supports previous studies that moving away from the patient generally reduces the infection risk in a transient (coughing) situation, although the effect is more pronounced under higher flow rate. It is noted that future research is needed to determine the exact mode of transmission for most recently identified organisms.
Agonafer D, Liao G-L, Spalding DB (1996). The LVEL turbulence model for conjugate heat transfer at low Reynolds numbers. In: Application of CAE/CAD Electronic Systems, EEP-Vol. 18. New York: American Society of Mechanical Engineers.
American College Health Association. Recommendation on meningococcal meningitis vaccination. Available at: http://www.acha.org/projects_programs/meningitis/index.cfm. Accessed Aug. 24, 2011.
ASHRAE (2003). Risk Management Guidance for Health, Safety and Environmental Security Under Extraordinary Incidents. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
ASHRAE (2005). ASHRAE Handbook-Fundamentals. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
ASHRAE/ASHE Standard, 170-2008 (2008). Ventilation of Health Care Facilities. Atlanta: American Society of Heating, Refrigerating and Air-conditioning Engineers.
Beggs CB, Kerr KG, Noakes CJ, Hathway EA, Sleigh PA (2008). The ventilation of multiple-bed hospital wards: Review and analysis. American Journal of Infection Control, 36: 250–259.
Beigel JH, Farrar J, Han AM, Hayden FG, Hyer R, de Jong MD, Lochindarat S, Nguyen TK, Nguyen TH, Tran TH, Nicoll A, Touch S, Yuen KY; Writing Committee of the World Health Organization (WHO) Consultation on Human Influenza A/H5 (2005). Avian influenza A (H5N1) infection in humans. New England Journal of Medicine. 353: 1374–1385.
Bennett WD (2002). Effect of beta-adrenergic agonists on mucociliary clearance. Journal of Allergy and Clinical Immunology, 110(6 Supp): S291–297.
Bolashikov ZD, Kierat W, Melikov AK, Popiołek Z (2010). Exposure of health care workers to coughed airborne pathogens in a hospital room with overhead mixing ventilation: Impact of the ventilation rate and the distance downstream from the coughing patient. In: Proceedings of IAQ 2010, Airborne Infection Control—Ventilation, IAQ & Energy, Kuala Lumpur, Malaysia.
Brachman PS (1971). Nosocomial infection—airborne or not? In: Brachman PS, Eickhoff TC (eds), Proceedings of the International Conference on Nosocomial Infections. American Hospital Association (pp. 189–192), Chicago, USA.
CDC (2005). Guidelines for preventing the transmission of mycobacterium tuberculosis in health-care settings, 2005. Morbidity and Mortality Weekly Report (MMWR), 54(17): 1–141.
Chao CYH, Wan MP, Sze To GN (2008). Transport and removal of expiratory droplets in hospital ward environment. Aerosol Science and Technology, 42: 377–394.
Chen C, Zhao B, Cui W, Dong L, An N, Ouyang X (2009). The effectiveness of an air cleaner in controlling droplet/aerosol particle dispersion emitted from a patient’s mouth in the indoor environment of dental clinics. Journal of the Royal Society Interface, 7: 1105–1118.
Cheong KWD, Phua SY (2006). Development of ventilation design strategy for effective removal of pollutant in the isolation room of a hospital. Building and Environment, 41: 1161–1170.
Cole EC, Cook CE (1998). Characterization of infectious aerosols in health care facilities: an aid to effective engineering control and preventive strategies. American Journal of Infection Control, 26: 453–464.
Couch RB (1981). Viruses and Indoor Air Pollution. Bulletin of the New York Academy of Medicine, 57: 907–921.
Duguid JP (1945). The size and the duration of air-carriage of respiratory droplets and expelled from the human respiratory tract during expiratory activities. Journal of Aerosol Science, 40: 256–269.
Edwards DA, Man JC, Brand P, Katstra JP, Somerer K, Stone HA, Nardell E, Scheuch G (2004). Inhaling to mitigate exhaled bioaerosols. PNAS, 101: 17383–17388.
Fairchild CI, Stamper JK (1987). Particle concentration in exhaled breath. American Industrial Hygiene Association Journal, 48: 948–949.
Fennelly KP, Martyny JW, Fulton KE, Orme IM, Cave DM, Heifets LB (2004). Cough-generated aerosols of Mycobacterium tuberculosis: A new method to study infectiousness. American Journal of Respiratory and Critical Care Medicine, 169: 604–609.
Fisk WJ (2000). Review of health and productivity gains from better IEQ. In: Proceedings of Healthy Buildings 2000 (vol. 4, pp. 23–34), Espoo, Finland.
Fitzgerald D, Hass DW (2005). Mycobacterium tuberculosis. In: Mandell GL, Bennett, JE, Dolin R (eds), Principles and Practice of Infectious Diseases, 6th edn. Philadelphia: Churchill Livingston, pp. 2852–2886.
Gupta JK, Lin CH, Chen Q (2009). Flow dynamics and characterization of a cough. Indoor Air, 19: 517–525.
Gupta JK, Lin CH, Chen Q (2010). Characterizing exhaled airflow from breathing and talking. Indoor Air, 20: 31–39.
Haas JP (2006). Measurement of infection control department performance: state of the science. American Journal of Infection Control, 34: 545–549.
Habel K (1945). Mumps and chickenpox as airborne diseases. American Journal of the Medical Sciences, 209: 75–78.
Hoppe P (1981). Temperature of expired air under varying climatic conditions. International Journal of Biometeor, 25: 127–132.
Kaushal V, Saini PS, Gupta AK (2004). Environmental control including ventilation in hospitals. JK Science, 6: 229–232.
Kierat W, Bolashikov ZD, Melikov AK, Popiolek Z, Brand M (2010). Exposure to coughed airborne pathogens in a double bed hospital patient room with overhead mixing ventilation: Impact of posture of coughing patient and location of doctor. In: Proceedings of ASHRAE IAQ 2010.
Kosar D (2002). The answer is 3. Engineered Systems, 2002, July: 60–70.
Kowalski WJ (2007). Air-treatment systems for controlling hospital-acquired infections. HPAC Engineering, 79: 28–48.
Langmuir AD (1980). Changing concepts of airborne infection of acute contagious diseases: a reconsideration of classic epidemiologic theories. Annals of the New York Academy of Sciences, 353: 35–44.
Launder BE, Spalding DB (1974). The numerical computation of turbulent flows. Computer Methods in Applied Mechanics and Engineering, 3: 269–289.
Li Y, Huang X, Yu ITS, Wong TW, Qian H (2005). Role of air distribution in SARS transmission during largest nosocomial outbreak in Hong Kong. Indoor Air, 15: 83–95.
Li Y, Leung GM, Tang JW, Yang X, Chao CYH, Lin JZ, Lu JW, Nielsen PV, Niu J, Qian H, Sleigh AC, Su H-JJ, Sundell J, Wong TW, Yuen PL (2007). Role of ventilation in airborne transmission of infectious agents in the built environment—a multidisciplinary systematic review. Indoor Air, 17: 2–18.
Maki DG, Alvarado CJ, Hassemer CA, Zilz MA(1982). Relation of the inanimate hospital environment to endemic nosocomial infection. New England Journal of Medicine, 307: 1562–1566.
Memarzadeh F (2011a). Literature review of the effect of temperature and humidity on viruses that cause epidemics & pandemics. ASHRAE Transactions, 117(2): 24–37.
Memarzadeh F (2011b). The Environment of Care and Health Care-Associated Infections: An Engineering Perspective. Chicago: American Society of Health Care Engineers.
Morawska L (2006). Droplet fate in indoor environments, or can we prevent the spread of infection? Indoor Air, 16: 335–347.
Morawska L, Johnson GR, Ristovski ZD, Hargreaves M, Mengersen K, Corbett S, Chao, CYH, Li Y, Katoshevski D (2009). Size distribution and sites of origin of droplets expelled from the human respiratory tract during expiratory activities. Journal of Aerosol Medicine, 40: 256–269.
Nicas M, Nazaroff WW, Hubbard A (2005). Toward understanding the risk of secondary airborne infection: emission of respirable pathogens. Journal of Occupational and Environmental Hygiene, 2: 143–154.
Nielsen PV, Olmedo I, Ruiz de Adana M, Grzelecki P, Jensen RL (2011). Airborne cross infection between two people in a displacement ventilated room. HVAC & R Research. (in presss)
Noakes CJ, Fletcher LA, Sleigh PA, Booth WB, Beato-Arribas B, Tomlinson N (2009). Comparison of tracer techniques for evaluating the behaviour of bioaerosols in hospital isolation rooms. In: Proceedings of Healthy Buildings 2009, Syracuse, USA.
Olmedo I, Nielsen PV, de Adana MR, Jensen RL, Grzelecki P (2011). Distribution of exhaled contaminants and personal exposure in a room using three different air distribution strategies. Indoor Air, doi: 10.1111/j.1600-0668.2011.00736.x
Papineni RS, Rosenthal FS (1997). The size distribution of droplets in the exhaled breath of healthy human subjects. Journal of Aerosol Medicine, 10: 105–116.
Riley EC, Murphy G, Riley RL (1978). Airborne spread of measles in a suburban elementary school. American Journal of Epidemiology, 107: 421–432.
Salah B, Dinh Xuan AT, Fouilladieu JL, Lockhart A, Regnard J (1998). Nasal mucociliary transport in healthy subjects is slower when breathing dry air. European Respiratory Journal, 1: 852–855.
Schaal KP (1991). Medical and microbiological problems arising from airborne infection in hospitals. Journal of Hospital Infection, 18(Suppl. A): 451–459.
Scott RD (2009). The direct medical costs of healthcare-associated infections in U.S. hospitals and the benefits of prevention. Report: Centers for Disease Control and Prevention.
Stone PW, Braccia D, Larson E (2005). Systematic review of economic analyses of health care-associated infections. American Journal of Infection Control, 33: 501–509.
Streifel A (1999). Hospital Epidemiology and Infection Control, 2nd Edn, Chapter 80. Philadelphia: Lippincott Williams & Wilkins.
Sun W, Ji J (2007). Transport of Droplets Expelled by Coughing in Ventilated Rooms. Indoor and Built Environment, 16: 493–504.
Sze To GN, Wan MP, Chao CYH, Wei F, Yu SCT, Kwan JKC (2008). A methodology for estimating airborne virus exposures in indoor environments using the spatial distribution of expiratory aerosols and virus viability characteristics. Indoor Air, 18: 425–438.
Tang JW, Noakes CJ, Nielsen PV, Eames I, Nicolle A, Li Y, Settles GS (2011). Observing and quantifying airflows in the infection control of aerosol- and airborne-transmitted diseases: an overview of approaches. Journal of Hospital Infection, 77: 213–222.
Tung YC, Shih YC, Hu SC (2009a). Numerical study on the dispersion of airborne contaminants from an isolation room in the case of door opening. Applied Thermal Engineering, 29: 1544–1551.
Tung YC, Hu SC, Tsai TI, Chang IL (2009b). An experimental study on ventilation efficiency of Isolation room. Building and Environment, 44: 271–279.
Waffaa NS, Iman A, Pachachi AI, Almashhadanii WM (2006). The effect of montelukast on nasal mucociliary clearance. The Journal of Clinical Pharmacology, 46: 588–590
Wan MP, Chao CYH (2007). Transport characteristics of expiratory droplet nuclei in indoor environments with different ventilation airflow patterns. Journal of Biomechanical Engineering, 129: 341–353.
Wan MP, Sze To GN, Chao CYH, Fang L, Melikov A (2009). Modeling the fate of expiratory aerosols and the associated infection risk in an aircraft cabin environment. Aerosol Science and Technology, 43: 322–343.
Wells WF, Wells MW, Wilder TS (1942). The environmental control of epidemic contagion. I. An epidemiologic study of radiant disinfection of air in day schools. American Journal of Hygiene, 35: 97–121.
Wells WF (1955). Airborne Contagion and Air Hygiene: An Ecological Study of Droplet Infections. Cambridge, USA: Harvard University Press.
Xie X, Li Y, Chwang ATY, Ho PL, Seto H (2007). How far droplets can move in indoor environments—revisiting the Wells evaporation-falling curve. Indoor Air, 17: 211–225.
Yin Y, Xu W, Gupta JK, Guity A, Marmion P, Manning A, Gulick RW, Zhang X, Chen Q (2009). Experimental study on displacement and mixing ventilation systems for a patient ward. HVAC&R Research, 15: 1175–1191.
Zhu S, Kato S, Yang JH (2006). Investigation into airborne transport characteristics of airflow due to coughing in a stagnant room environment. ASHRAE Transactions, 112(1): 123–133.
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Memarzadeh, F., Xu, W. Role of air changes per hour (ACH) in possible transmission of airborne infections. Build. Simul. 5, 15–28 (2012). https://doi.org/10.1007/s12273-011-0053-4
- infection transmission and control
- risk assessment
- air change rate (ACH)
- computational fluid dynamics (CFD)
- patient room
- ventilation system design