ASHRAE Journal - September 2020 - 26

TECHNICAL FEATURE

Filtration

Newly reported surface sampling inside recirculating HVAC systems in an Oregon hospital provides an
indicator of filter efficiency. The amount of virus collected on the prefilter, final filter and supply air damper
allows for comparison of virus in mixed air before and
after prefilters (MERV 10) and after discharge from the
final filters (MERV 15). The amount of virus collected on
surfaces decreased by approximately 70% after passing
through the prefilters but did not decrease further after
the final filters.9
A modeling study of influenza spread found that
higher efficiency air filters could lower flu infection
risk.24 Although COVID-19 response measures now
include installation of filters with higher MERV ratings,
there have been no studies to determine whether filter rating makes any difference in transmission of the
disease.
Experience suggests that, with appropriate design and
positioning, HEPA filtration units within a space could
potentially reduce SARS-CoV-2 exposure by directly
capturing air in the vicinity of infected patients or by
removing airborne virus near susceptible individuals.
However, designers must also be aware that discharge
air could also blow virus between occupants and resuspend settled virus from surfaces.

Air Disinfection

Ultraviolet (UV) light systems for infection control can
either be in the space (upper-room) or HVAC system (induct). These systems are occasionally used in hospitals
to help control airborne infectious agents but are rarely
used in non-healthcare facilities.
Upper-room UV was found in one study to prevent the
spread of measles, mumps and chicken pox. However,
the authors also noted that it might not be effective in
the protection of susceptible individuals against other
pathogens.25 No studies were found related to the use of
UV to control SARS-CoV-2.

Humidity Control

Coronaviruses survive under dry conditions, and occupants can be more susceptible to respiratory infection at
lower relative humidity (RH). Humidification has been
suggested as a means of controlling flu. Comparison of
initial COVID-19 cases by region or country suggested
that infection rates might be greater in areas with higher

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SEPTEM BER 2020

humidity.26 However, COVID-19 also spread during initial stages of the pandemic in humid areas.27
Continuing high rates of COVID-19 in the United States
this summer further suggest that humidity is not an
important factor. The National Academy of Sciences concluded that differences in humidity only explain a small
fraction of the overall variation in COVID-19 transmission rates.28

Wastewater

SARS-CoV-2 has been found in feces, urine, sewage,
toilet surfaces and air in restrooms. The release of droplets containing virus from these sources is likely, but no
monitoring results were found.

Mechanical Hygiene

SARS-CoV-2 on surfaces inside HVAC systems was
recently documented where hospital systems recirculated air in zones with COVID-19 patients. The virus was
detected on prefilters receiving mixed air, the surface of
final filters receiving air after prefiltration and supply
air dampers after the final filters.9 In an earlier study,
the surface of an exhaust outlet in a Singapore hospital
tested positive.12

Preliminary Conclusions

Further study is needed to determine whether HVAC
systems actually transmit COVID-19 infection and if
HVAC modifications can help control its spread. Review
of available information revealed very little detail in this
regard.
Airborne Transmission. Limited sampling has established that SARS-CoV-2 is airborne and can expose (but
not necessarily infect) occupants well beyond 2 m (7 ft).
Recent findings further establish that the virus can circulate through some HVAC systems. While several outbreaks of COVID-19 infection have suggested airborne
transmission, these have generally been in areas with
poor ventilation, raising the possibility that dilution
by code-required building ventilation inhibits disease
transmission.
Ventilation. Increasing outdoor air dilutes the concentration of airborne SARS-CoV-2. Minimum acceptable
ventilation rates have not been established for operation
of buildings where infected occupants may be present.
It is not known whether increasing ventilation rates
above ASHRAE minimums actually reduces disease

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ASHRAE Journal - September 2020

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