ASHRAE Journal - July 2020 - 26

TECHNICAL FEATURE

guidelines or standards. ANSI/ASHRAE/ASHE Standard
170, Ventilation of Health Care Facilities, is also a great
resource in the design of these types of rooms.
Typical requirements of a well-designed negative pressure isolation room located in an acute care hospital are
as follows:
* Minimum of 12 air changes per hour;
* Minimum of 0.01 in. w.c. (2.5 Pa) of negative pressure to the adjacent corridor;
* A setpoint of 0.03 in. w.c. (7.5 Pa) of negative pressure to the adjacent corridor;
* Inclusion of an anteroom;
* Positive airflow from the corridor to the anteroom;
* Positive airflow from the anteroom to the isolation
room;
* Minimum airflow difference of 150 cfm (71 L/s) into
the space;
* Exhaust to the outside without mixing with the
general exhaust;
* Ceiling supply air diffuser at the front of the patient's feet; and
* Lower wall-mounted exhaust air grille near the
patient's head.
Does a makeshift negative pressure room meet all the
requirements of a licensed isolation room? Absolutely
not. Does it provide a possible temporary safeguard
against the spread of infection? Yes. Our approach
closely follows the OSHPD Policy Intent Notice PIN-4,
"Review of Existing Facilities for Airborne Infection
Isolation Rooms and Projects Related to Isolation of TB
Patients."3 PIN-4 was originally established in 1996 to
address tuberculosis (TB) patients and was updated in
2011. The 2020 executive orders issued by OSHPD authorized the use of PIN-4 for implementing makeshift negative pressure rooms during the COVID-19 pandemic.
Here is an excerpt from OSHPD PIN-4:
"Because of the expense of building new Airborne Infection
Isolation Rooms, the increasing number of suspected and confirmed tuberculosis (TB) cases requiring isolation at public hospitals, the need to admit TB patients to different areas of the hospital
at different times, and the need to use 'TB rooms' for non-isolation
patients some of the time, many hospitals are turning to a variety
of methods to isolate TB patients that are generally consistent with
publications of the Centers for Disease Control and Prevention
(CDC)."
TB is an airborne disease;4 it is currently debated
whether COVID-19 has the characteristics of an airborne
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disease. The CDC states that the virus is thought to
spread through respiratory droplets.5 The well-known
6 ft (2 m) guideline established for social distancing is
simply to avoid the transmission of COVID-19 through
droplets. Droplet particles are greater than 5 μm to
10 μm in diameter. Airborne transmission is different
from droplet transmission, as it refers to the presence
of microbes within droplet nuclei, which are generally
considered to be particles less than 5 μm in diameter.
Droplet nuclei can remain in the air for long periods of
time.6
What if the COVID-19 virus is also transmitted through
microbes within droplet nuclei? While unlikely, scientists do not know enough to make a conclusive recommendation at this time. Many health-care institutions
are treating COVID-19 as an airborne disease and, as
HVAC engineers, we should treat it similarly to TB until
we know otherwise. An ASHRAE issued statement indicates that "transmission of SARS-CoV-2 through the air
is sufficiently likely that airborne exposure to the virus
should be controlled."7 Below are some of the recommendations of PIN-4 that were successfully implemented in our approach:
* Provide a portable high-efficiency particulate air
(HEPA) exhaust fan unit for each patient room. Each
unit's HEPA filter shall have a minimum of 99.97% dioctyl phthalate efficiency or minimum efficiency reporting
value (MERV) of 17.
* HEPA exhaust fan units are not hardwired,
plumbed or directly attached to the building structure,
floors, walls or ceilings.
* HEPA exhaust fan units are installed in existing intensive care rooms or existing medical-surgical nursing
rooms.
* Exhaust is through windows, either directly or via
duct through a fixed window panel.
* Monitor the room pressure. Gages shall be readable
from the corridor and annunciate locally at the door
when air balance is disrupted except for time delays for
the normal opening of the doors.
* Close windows and seal doors to the extent practical
for all air penetration leaks into the room.
* Ensure alterations do not compromise or alter fire
protection systems.
* Verify that exhaust outlets of any portable unit
exhausted to the building exterior is at least 10 ft (3 m)
away from any opening. This may require securing some

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ASHRAE Journal - July 2020

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