BY LAWRENCE J. SCHOEN, P.E., FELLOW/LIFE MEMBER ASHRAE
The HVAC systems in most non-medical buildings play only a small role in infectious
disease transmission, including COVID-19.1
Knowledge is emerging about COVID-19,
the virus that causes it (SARS-CoV-2), and how the disease spreads. Reasonable, but
not certain, inferences about spread can be drawn from the SARS outbreak in 2003
(a virus genetically similar to SARS-CoV-2) and, to a lesser extent, from transmission
of other viruses. Preliminary research has been recently released, due to the urgent
need for information, but it is likely to take years to reach scientific consensus.
Even in the face of incomplete knowledge, it is critically
important for all of us, especially those of us in positions
of authority and influence, to exercise our collective
responsibility to communicate and reinforce how personal choices about social distancing and hygiene affect
the spread of this disease and its impact not just on ourselves, but on our societal systems and economy. The consequences of overwhelming the capacity of our healthcare systems are enormous and potentially tragic. The
sooner we “flatten the curve,”2 the sooner we can return
to safer and normal economic and personal lives.
According to the WHO (World Health Organization),
“The COVID-19 virus spreads primarily through droplets
of saliva or discharge from the nose when an infected
person coughs or sneezes….” Talking and breathing can
also release droplets and particles.3 Droplets generally fall
to the ground or other surfaces in about 1 m (3 ft), while
particles (aka aerosols), behave more like a gas and can
travel through the air for longer distances, where they
can transmit to people and also settle on surfaces. The
virus can be picked up by hands that touch contaminated
surfaces (called fomite transmission) or be re-entrained
into the air when disturbed on surfaces.
SARS infected people over long distances in 2003,4
SARS-CoV-2 has been detected as an aerosol in hospitals,5 and there is evidence that at least some strains of
it remain suspended and infectious for 3 hours,6 suggesting the possibility of aerosol transmission. However,
other mechanisms of virus dissemination are likely to
be more significant, namely,
• direct person to person contact
• indirect contact through inanimate objects like
• through the hands to mucous membranes such as
those in the nose, mouth and eyes
Lawrence J. Schoen, P.E., is president and principal engineer at Schoen Engineering,
Inc., in Columbia, Md. He was chair of the committee that wrote the most recent version of the “ASHRAE Position Document on Airborne Infectious Diseases.”
COLUMN IEQ APPLICATIONS
This article was published in ASHRAE Journal, May 2020. Copyright 2020 ASHRAE. Posted at http://www.ashrae.org. This article may not be copied and/or distributed
electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit http://www.ashrae.org.
This peer-reviewed column does not represent official ASHRAE guidance. For more information on ASHRAE resources on COVID-19, visit ashrae.org/COVID19.
MAY 2020 ashrae.org ASHRAE JOURNAL 73
• droplets and possibly particles spread between
people in close proximity.
For this reason, basic principles of social distancing (1
to 2 m or 3 to 6.5 ft), surface cleaning and disinfection,
handwashing and other strategies of good hygiene are
far more important than anything related to the HVAC
In the middle-Atlantic region of the United
States where I work, malls, museums, theaters, gyms
and other places where groups of people gather are
closed and there are “stay at home”8 orders. This is a
“game” of chance, and the fewer individuals who come
in close contact with each other, the lower the probability for spread of the disease. Since symptoms do not
become apparent for days or weeks, each of us must
behave as though we are infected.
Other public buildings, considered essential to varying
degrees, remain open. These include food, hardware
and drug stores, and of course, hospital and health-care
facilities (which are beyond the scope of this article).
Anecdotally, some universities are allowing some or all
faculty, staff and graduate students to conduct essential
research and online classes. Banks and other service
organizations are open to staff and are receiving customers by appointment only, and private and government
workplaces are open with work at home for some or all
encouraged or mandated.
For those buildings that remain open, in addition to
the policies described above, non-HVAC actions include:
• Increase disinfection of frequently touched surfaces.9
• Install more hand sanitation dispensers, assuming
they can be procured.
• Supervise or shut down food preparation and
warming areas, including the office pantry and coffee
• Close or post warning signs at water fountains in
favor of bottle filling stations and sinks, or even better,
encourage employees to bring their water from home.
Once the basics above are covered, a few actions
related to HVAC systems are suggested, in case some
spread of the virus can be affected:
• Increase outdoor air ventilation (use caution in
highly polluted areas); with a lower population in the
building, this increases the effective dilution ventilation
• Disable demand-controlled ventilation (DCV).
• Further open minimum outdoor air dampers, as
high as 100%, thus eliminating recirculation (in the mild
weather season, this need not affect thermal comfort or
humidity, but clearly becomes more difficult in extreme
• Improve central10 air filtration to the MERV-1311
or the highest compatible with the filter rack, and seal
edges of the filter12 to limit bypass.
• Keep systems running longer hours, if possible 24/7,
to enhance the two actions above.
• Consider portable room air cleaners with HEPA filters.
• Consider UVGI (ultraviolet germicidal irradiation),
protecting occupants from radiation,13 particularly in
high-risk spaces such as waiting rooms, prisons and
Construction sites present unique challenges. Much,
but not all, construction work has the recommended
social distancing; much, but not all, is outdoors or in
partially enclosed and therefore well-ventilated buildings; and many, but not all, workers already use personal protective equipment such as masks14 and gloves.
Governments in some locations have mandated closure
of construction sites, while in others work proceeds.15
Engineers who perform field observations, commissioning or special inspections must consider what work can
be postponed, performed remotely, or conducted using
photographic documentation, and what personal precautions to take when site visitation is unavoidable.
If you, the reader, are called upon to advise building
operators, please use the above general guidance, and be
sure to combine it with knowledge of the specific HVAC
system type in a building and the purpose and use of
the facility. Like all hazards, risk can be reduced but not
eliminated, so be sure to communicate the limitations
of the HVAC system and our current state of knowledge
about the virus and its spread.
We all have a role to play to control the spread of this
disease. HVAC is part of it and even more significant are
social distancing, hygiene and the influence we can have
on personal behavior.
Thanks to William P. Bahnfleth, Ph.D., P.E., Presidential
Member/Fellow ASHRAE, Lew Harriman, Fellow ASHRAE,
Yuguo Li, Ph.D., Fellow ASHRAE, Andrew K. Persily, Ph.D.,
Fellow ASHRAE, and Pawel Wargocki, Ph.D., Member
ASHRAE for their review of preliminary drafts of this
article. Any errors that remain are the author’s alone.
- According to the WHO, COVID-19 is the name given on February
11, 2020 by the ICTV (International Committee on Taxonomy
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74 ASHRAE JOURNAL ashrae.org MAY 2020
of Viruses) to the disease caused by SARS-CoV-2, aka the novel
coronavirus. Hospitals and health-care facilities are beyond the scope
of this article, though many of the same principles apply to them.
- “NIH Director: ‘We’re on an Exponential Curve’”, The Atlantic,
Peter Wehner, March 17, 2020.
- Bischoff 2013. References cited with only a name and date
in this article are taken from ASHRAE Position Document on Airborne
Infectious Diseases, 2014, 2020 (“PD”). According to the PD, there is no
exact size demarcation between droplets and particles, and this “… is
less important than knowing that large droplets and small particles
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behave differently and that the latter can remain airborne.”
- Yu et al. 2004; Li et al. 2005a, 2005b. References from ASHRAE
Position Document on Airborne Infectious Diseases.
- “Aerodynamic Characteristics and RNA Concentration of SARSCoV-2 Aerosol in Wuhan Hospitals during COVID-19 Outbreak,” nonpeer reviewed preprint. bioRxiv, posted March 10, 2020.
- “Aerosol and Surface Stability of SARS-CoV-2 as Compared
with SARS-CoV-1,” letter to the editor New England Journal of Medicine,
March 17, 2020, DOI: 10.1056/NEJMc2004973.
- Non-health care workplaces fall into the medium and lower
exposure risk categories described in
Guidance on Preparing Workplaces for COVID-19,
U.S. Department of Labor, Occupational
Safety and Health Administration OSHA
- Stay at home orders have been
incorrectly called “shelter in place.” The
latter is a more extreme action advised
when there is an outdoor contaminant,
which is not the COVID-19 situation. See
- To paraphrase the words of the singersongwriter Joe Jencks, janitors, cleaners,
nurses, aides, doctors, EMTs and other first
responders are our “viral firefighters,” the
advance guard in the fight against this virus.
- This applies to systems that deliver
air to the occupied space and, to a lesser
extent, room air returned to rotary heat
exchangers, in order to reduce cross
- MERV-13 is approximately equivalent
to F7, a rating used in the EU. More effective
filters with reasonable pressure drop are
available, and some increased pressure
drop often results in an acceptably small
effect on system performance.
- Consider the filters contaminated,
protect personnel changing them, and seal
them in plastic bags for disposal.
- There is research that shows UVGI
in both the upper-room and in-duct
configurations can inactivate some
disease-transmitting organisms. Either
of these takes time to plan and install. For
more information, see the ASHRAE PD
previously cited and Chapter 62 Ultraviolet
Air and Surface Treatment of 2019 ASHRAE
- N95 masks are in short supply and
some contractors are donating them to
medical facilities, according to Engineering
News Record March 18, 2020 “Construction
Firms Respond to Mask Donation Need.”
- Engineering News Record, March 19,
- “Pennsylvania Halts All ‘Non-LifeSustaining Business” Operations, Including