ASHE Monograph

T his literature review explores the role that ventilation, assessed by air changes per hour (ACH), plays in infection transmission and disease control. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Position Document on Airborne Infectious Diseases (2009) notes the dilution effect that ACH has on infectious disease transmission. The Wells-Riley equation supports the premise that increasing the volume of “clean” air dilutes the room air and thus exposes people in the room to fewer potentially infectious particles. ACH is the only variable in the Wells-Riley equation that can be quantified by direct measurement. Our knowledge of the effect of the many other variables (e.g., infectious dose, relative humidity, temperature, host susceptibility, chain of transmission) on disease transmis­ sion is limited. The majority of studies conducted to assess the role of ventilation in infec­ tion transmission and disease control have been either observational or based on animal models that are not particularly applicable to the study of human disease. Case studies on aircraft and other vehicles, schools, and “sick” build­ ings and hospitals, as well as some animal studies, suggest that regardless of actual ACH, ventilation rate is just one of many factors that affect the trans­ mission of infectious disease. The ability to precisely predict particulate movement is needed to increase our understanding of how airborne disease organisms move within ventilated facilities. A recently published empirical study (Faulkner et al. 2013) indi­ cates that in some situations there may be limits to the improvements in the environment that can be achieved by increasing ACH in the occupied zones of rooms.