ASHRAE Journal - April 2021 - 15

TECHNICAL FEATURE

well-mixed condition, i.e., 1.0). Though displacement
ventilation has a theoretical exposure value less than
mixing ventilation, the researchers calculated a median
exposure value of 1.23 (23% higher than a well-mixed
condition). Chen, et al., attributed this higher-thanexpected value to the inclusion of many cases with a
short inter-person distance that negated the benefits of
the displacement ventilation.23
One last concept used to control the spread of pathogens is pressure differential. ASHRAE Standard 1702017 calls for a minimum pressure differential of 2.5 Pa
(0.01 in. w.c.) across the envelope and between adjacent
rooms for both airborne infectious isolation (AII) rooms
and protective environment (PE) rooms.31 While schools
may not have the same requirements as an AII or PE
room, areas such as the nurse's office and toilet rooms
may benefit from the use of a negative pressure differential between these spaces and adjoining spaces to
prevent virus migration.

Conclusions
This article looked at the airborne transmission of
viruses and their propagation by the air distribution
system. As discussed, displacement ventilation can
provide a very high theoretical ventilation efficiency,
but the presence of a lock-up layer and poor protection
from expiratory jets may negate some of this ventilation efficiency in regard to virus exposure near an
infected person. As discussed in Part 1, if the infected
person is wearing a face mask, the ventilation efficiency could be much higher.24 Mixing ventilation, in
general, provides uniform exposure to contaminants
in a space and the potential for greater spread at
diluted levels.24

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