ASHRAE Journal - January 2021 - 23

TECHNICAL FEATURE

make sense of this virus may fall back upon training for
handling familiar risks, including mold or Legionella,
but signiﬁcant differences exist between these threats
that should drive an entirely different response. First,
understand that SARS-CoV-2, like all viruses, is not a living organism. Viruses are a combination of nucleic acid
(RNA or DNA) and other " chemicals " that take over the
cellular machinery of a living host and use that machinery to replicate themselves. This replication is entirely
dependent upon having a living host to support them.
This means that viruses cannot replicate and proliferate
on any inanimate surface in the environment, including
the surfaces within an HVAC system.
It has long been recognized that respiratory viruses can
be transmitted via droplets that are generated by coughing or sneezing.3 Further, this virus relies on a protective
envelope of mucous in the respiratory droplets to aid it
in achieving " primary host " to " secondary host " transmission. Upon initial expulsion into the general environment via a cough or a sneeze, most of these droplets
are in the size range of 30 µm to 100 µm in diameter and
will start to settle out of the air quite rapidly and deposit
on nearby surfaces.4
However, as noted in a recent study in Environment
International,5 " immediately after droplets are expired,
the liquid content starts to evaporate, and some droplets
become so small that transport by air current affects
them more than gravitation. Such small droplets are free
to travel in the air and carry their viral content meters
and tens of meters from where they originated. "
Although the number of these aerosol particles may
be relatively small compared to the number released
from the original " cloud, " and the fraction that contains
active viruses may be even smaller, effective ventilation
systems will be important in minimizing potential infections. As stated in the newly released " ASHRAE Position
Document on Infectious Aerosols, " 6 " the risk of pathogen
spread, and therefore the number of people exposed,
can be affected both positively and negatively by the airﬂow patterns in a space and by [HVAC] and local exhaust
ventilation (LEV) systems. "
Unlike a virus, mold is a living organism capable of
self-replication upon environmental surfaces. This
means that mold can multiply itself and colonize on
HVAC system surfaces under the right conditions.
Similarly, Legionella is a living bacterium capable of colonizing suitable environments and replicating on its own

without the assistance of a host. Although disinfectants
can be used to kill these living organisms as well as viral
particles, viruses will lose the ability to cause infection
on their own without a living host.

Science on Transmission

The science is continuing to evolve on the understanding of the long-term viability and risk of the SARS-CoV-2
virus on surfaces. Two preliminary studies on this topic
have indicated that the amount of time before the
SARS-CoV-2 virus degrades depends upon the surface
to which it attaches. A recent study showed that after
inoculation on stainless steel, the concentration of
SARS-CoV-2 was reduced by 50% after 1.8 days and by
90% after 5.96 days.7 A second study, published in a letter in The Lancet Microbe,8 found no traces of infectious
virus on stainless steel surfaces at seven days (half-life of
14.7 hours).
Based on this current science of the virus' infectivity,
the CDC has recommended9 that no disinfection beyond
normal cleaning is required on surfaces within buildings that have had no occupants for seven days. Within
days, if not hours, of landing on a typical surface, the
SARS-CoV-2 virus begins to degrade and signiﬁcantly
reduce its viral load. This is one instance where time can
be a useful disinfectant.
The current science10 also indicates that infectious
doses of the SARS-CoV-2 virus don't appear to travel
long distances through the air. While there is not yet
a complete picture around the virus' behavior, early
research11 indicates this virus does not present a farﬁeld aerosol transmission problem. However, the guidance from authoritative sources can sometimes appear
to be inconclusive. That is primarily due to the different
risk potential that can be encountered in various environments. For example, the WHO says, " in these events,
short-range aerosol transmission, particularly in speciﬁc indoor locations, such as crowded and inadequately
vented spaces over a prolonged period of time with
infected persons, cannot be ruled out. " 11 However, in
its guidance for health care the WHO states, " COVID-19
appears to spread most easily through close contact with
an infected person. When someone who has COVID-19
coughs or sneezes, small droplets are released and, if
you are too close, you can breathe in the virus. " 12
CDC recommends modiﬁed airborne, in addition to
standard (contact) and droplet precautions, for the care
JAN UARY 2021

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ASHRAE Journal - January 2021

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