How Diseases Spread

Airborne Spread of Disease

ASHRAE’s position document contains a valuable synopsis of control measures such as dilution ventilation, pressure differentials, exhaust ventilation, air cleaning, ultraviolet germicidal irradiation (UVGI) and even temperature and humidity. These techniques have broad applicability to any disease that is airborne. Because of the difficulties in separating out the relative importance of transmission modes, health-care facilities often focus on “infection control bundles” (i.e., use of multiple modalities simultaneously) and err on the side of caution. The need for action may go beyond health-care facilities to include passenger transportation buildings and conveyances, jails, homeless shelters and schools. The Ebola outbreak illustrates how vulnerable we all are to new infectious agents, a future one of which might be airborne. Tuberculosis, in some cases influenza, the common cold, and other diseases spread by the airborne route. Four worldwide (pandemic) influenza outbreaks occurred in the last 100 years: 1918, 1957, 1968, and 2009. There were also three notable epidemics: 1947, 1976 and 1977. The 1918 Spanish flu was the most serious pandemic in recent history and was responsible for the deaths of an estimated 50 million or more people. The most recent H1N1 pandemic in 2009 resulted in thousands of deaths worldwide. Statistically, it seems like we have had the recent good fortune of avoiding a truly devastating pandemic such as these historical ones. Another way of saying this is that such a tragic event is long overdue.


Lawrence J. Schoen, P.E., is president and principal engineer, Schoen Engineering Inc., Columbia, Md. He is a member and past chair of ASHRAE’s Environmental Health Committee and chaired the committee responsible for the most recent position document in 2014.


As I write this, the Ebola outbreak in Africa and the events in Texas are the major news
stories. I can’t speak directly to that crisis. However, I do know that you need access
to essential information from ASHRAE on HVAC, and its potential role in the spread
of infectious disease. One good source is the recently released “ASHRAE Position
Document on Airborne Infectious Diseases.