ASHRAE Journal - November 2014 - 86

COLUMN IAQ APPLICATIONS

What ASHRAE Says
About Infectious Disease
BY LAWRENCE J. SCHOEN, P.E., FELLOW ASHRAE

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."
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.

How Diseases Spread
Direct contact is any surface contact such as touching, kissing, sexual contact,
contact with oral secretions or skin lesions.
Indirect contact involves contact with an intermediate inanimate surface
(fomite), such as a doorknob or bedrail that is contaminated.
Exposure through the air occurs through (1) droplets, which are released and
fall to surfaces about 3 ft (1 m) from the infected and (2) small particles, which
stay airborne for hours at a time and can be transported long distances. When
droplets become small particles by evaporation, they may be called droplet
nuclei. This is illustrated in Figure 1.
Tuberculosis and in some cases influenza, the common cold, and other
diseases spread by the airborne route.
An epidemic affects the population in a limited geographic area, whereas a
pandemic affects a large geographic area or the entire world.
FIGURE 1 Droplet suspension: aerobiology of droplets and small airborne par-

ticles produced by an infected patient.
Airflow

Droplets are swept by the
air motion and fall due to
gravity. Their fall is resisted Gravity
by a drag force.
Ballistic trajectory of
large droplets from
sneezing mode.
Droplets land on a
surface and dry to become
desiccated. Bed-making
activities can eject dry
particles into the air.

Droplets evaporated to
Drag become droplet nuclei.
Force
Turbulence mixing
dilutes the droplet
concentration.

Dilution of contaminated
air from breathing mode.
Bed
1 to 2 m

Floor

ASHRAE's Position

ASHRAE's expertise, while the latter is not. ASHRAE's
position document recommends that designers and

ASHRAE takes no position on the issue of the relative
importance of precautions for airborne exposure vs.
those for direct contact. The former is clearly within

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.

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ASHRAE Journal - November 2014

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