Our knowledge of the
COVID-19 virus and the engineering controls associated with mitigating its
transmission have evolved considerably since the pandemic hit the US earlier
this year. We now know that it is highly unlikely for COVID-19 to travel
through a code-compliant, multi-zone, hospital HVAC system (return ductwork,
central air handling unit coils, final filters, and terminal units). There are
no known instances of COVID-19 transmission through this pathway as of this
writing.
The use of 100 per
cent exhaust (where there's no recirculation via central air handlers) is not
justified, except for rooms where aerosol generating procedures are performed
and in the Emergency Department, where there are significant numbers of people
under investigation. The use of central air systems with MERV 14-16 filters or
local HEPA filtration have replaced most of the recommendations for 100 per
cent exhaust.
Maintaining a space
with an exterior wall or roof, such as most patient rooms, at negative
pressure, where more air is extracted from the room than is supplied to it, can
lead to infiltration of outside air through the building envelope, which can
lead to serious health problems, most notably mold growth in the envelope
construction.
The widespread use of
individual room negative pressure relationships isn't justified, except for
patient and exam rooms equipped with anterooms. There is limited protection
afforded to healthcare workers once the door to an infected patient’s room is
opened, no matter how negative the pressure relationship. Increased exhaust
requires an enhanced intake of outside air, which results in system capacity
issues and higher operating costs. In many cases, failure to maintain pressure
relationships and air balance can cause loss of temperature and humidity
control.
The current ASHRAE
Standard 170 guidelines on Ventilation of Health Care Facilities have served us
well and there is little evidence that going beyond those requirements yields
cost-justified improvements in COVID-19 viral transmission mitigation. There is
probably no safer place in a US built environment than a code compliant,
well-operated and maintained hospital.
We have learned that
the “normal” hierarchy of controls needs to be inverted in the case of this
pandemic. Instead of PPE being the least effective, we have found that it is
the most effective means of mitigating transmission. Elimination and
substitution are simply impractical if not functionally impossible. Since the
virus is transmitted person-to-person, it is impractical to use engineering
controls to interrupt that pathway. The single most effective strategy is
source control.
We
advocate for a layered approach. This includes:
⦁
Normal mode where Airborne Infectious Isolation Rooms (AIIRs) may be limited to
patients receiving aerosol generating procedures
⦁
Small scale surge capacity mode where it may be necessary to create additional
dedicated rooms or temporary patient observation rooms with HEPA filtration and
and negative pressure
⦁
Large scale surge where an established dedicated ward or even a separate
building with its own set of protocols may be needed.
An alternative
strategy to installing a HEPA filter unit in the ceiling and routing ductwork
in the ceiling cavity, is to provide a ventilated headboard like the CDC
suggests . This can be built on site.
When
patient numbers grow beyond small scale surge capacity, hospitals could
consider these options:
⦁
Cohort and Convert units, wings or floors where all patients in “hot” zones are
considered infected
⦁
Using anterooms/vestibules to segregate “hot” from “cold” zones
⦁
Rather than converting individual rooms one at a time, use HEPA machines to
create pressure difference
⦁
Create one-way flow if possible, with PPE donning in one anteroom and removing
in another. These should be large enough to accommodate two caregivers, working
in a “buddy system” and there should be handwash available
⦁
If necessary, create positive pressure zones for staff respite inside the hot
zone, using HEPA machines to establish pressure differences, and using
anterooms or vestibules if practical
Credit:
Kim Shinn, PE, LEED
Fellow and BEMP is a principal at TLC Engineering Solutions.