ASHRAE Journal - May 2021 - 20

COLUMN IEQ APPLICATIONS

droplets produced by a cough or sneeze concentrated
and trapped in the breathing zone, raising exposure
for anyone who might encounter it. The promotion of
mixing, perhaps by use of existing ceiling fans operating at reduced speed, can help achieve a more uniform,
lower concentration in the breathing zone and disperse
concentrated aerosol clouds without creating high speed
horizontal airflows.

Recommendation 4: HVAC System Operation
Maintain temperature and humidity design
setpoints.
Although evidence exists5 that survival time of
SARS-CoV-2 virus is higher at low temperature and
humidity, it is not clear that manipulation of either
temperature or humidity as risk mitigation measures
will have a major impact compared to other controls.
While ASHRAE guidance recommends6 maintaining
humidity within a 30% to 60% relative humidity range,
installation of humidification systems in buildings that
are not humidified is costly and potentially harmful to

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ASHRAE JOURNAL

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M AY 2021

the building, its occupants and its systems as a result of
moisture damage and mold growth, hence the recommendation to maintain as-designed conditions.
Maintain equivalent clean air supply required for
design occupancy whenever anyone is present in the
space served by a system.
This recommendation addresses two disparate concerns. Early guidance for building operation recommended 24/7 operation of HVAC systems, which creates
the potential to increase energy use substantially. On
the other hand, some buildings may disable ventilation
outside of normal business hours, but when custodial
or maintenance personnel still may be present. Current
guidance is that while systems need not run when no
one is present, they should run at their full-occupancy
levels whenever anyone is in the building.
When necessary to flush spaces between occupied periods, operate systems for a time required
to achieve three air changes of equivalent clean air
supply.
The purpose of flushing spaces between occupancy
periods is to clear the air of residual infectious aerosol.
The initial amount of contaminant in the air of a wellmixed space falls by 95% after three air changes of clean
air are supplied. Initial ETF guidance recommended a
two-hour flush at the beginning and end of the day. The
current recommendation prescribes only one flush per
occupied period and only if it is determined to be necessary, e.g., between class periods in an academic building. Flushing can take credit for air recirculated through
filters or the operation of air cleaners, so the time to
achieve three air changes in some buildings or spaces
may be less than 30 minutes.
Limit reentry of contaminated air that may reenter
the building from energy recovery devices, outdoor
air intakes and other sources to acceptable levels.
While reentry concern initially focused mainly on
energy recovery wheels and outdoor air intakes, other
possible routes for reentry exist. Recent studies of
COVID-19 outbreaks in South Korea concluded that in
one case infectious fecal aerosol flowed from one apartment to another through sanitary plumbing via dry
traps7 and in another that the pathway was reentry from
a natural ventilation shaft due to stack effect.8 Similar
incidents occurred during the 2003 SARS epidemic in
Hong Kong, including suspected building-to-building
transmission at the Amoy Gardens apartment complex.9

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ASHRAE Journal - May 2021

Table of Contents for the Digital Edition of ASHRAE Journal - May 2021