ASHRAE Journal - April 2021 - 12

TECHNICAL FEATURE

and could then be transported by the thermal plume of
someone standing nearby to the breathing zone.15
Sometimes equipment located in the space can be a
source of particle generation, which can act as a vehicle
to increase the carriage and intake of aerosolized droplets. Laser printers have been found to be the main
source of particulate matter in office buildings (where
tobacco smoking is not allowed).16 When a laser printer
makes copies, only about 75% of the toner material is
effectively transferred to the drum. The remaining
material can be released to the air as electrostatically
charged fine particles that can remain airborne for a
long time.16 These charged particles can attract virusladen aerosols, droplets and droplet nuclei that when
inhaled are more likely to be deposited in the lungs due
to their charge.16
Many preschool and kindergarten classrooms contain
toilet rooms. These toilets can also be a source of droplet
generation. Gravity flow, pressure-assisted gravity flow,
and pressure valve systems have been compared, and
each of these systems have been found to produce both
droplets and droplet nuclei.17 As water enters the bowl
after flushing, the turbulence creates large droplets
that can contaminate the toilet seat, surrounding floors
and nearby surfaces. A toilet plume of aerosol particles
(<3 μm) traveling at 5 m/s (16 fps) or greater is also produced that can entrain viruses and carry them away
from the toilet and remain airborne for an extended
period of time.17,18 The vomit and feces of an infected
person can contain high pathogen concentrations19 and
can continue to produce virus-laden aerosols even after
several flushes.17 In general, studies have shown greater
aerosol production occurs with higher flush energy.17
Pathogens like coronaviruses, norovirus and rotavirus have all been shown to be spread by the fecal-oral
route.18
A recent study has also shown urinals may pose a
similar issue to that of a toilet.20 Urinals have long been
discounted as a source of exposure, but researchers have
recently extracted a SARS-CoV-2 virus particle from
the urine of an infected patient.20 When the urinal was
flushed, more than 57% of the particles produced traveled away from the urinal, and the particles reached a
height of 0.84 m (33 in) in only 5.5 seconds.20 While the
viral load of urine may require more research, saliva is
commonly deposited in urinals and could be dispersed
by the flush.
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ASHRAE JOURNAL

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APRI L 2021

FIGURE 1 Zone air distribution effectiveness.21
Ez =

Ce − Cs
C − Cs

where
Ez = Zone air distribution effectiveness
C = Average contaminant concentration at the breathing zone
Ce = Average contaminant concentration at the exhaust
Cs = Average contaminant concentration at the supply

Effect of Air Distribution and Pressure Differential

Air distribution can play an important role in the
movement of droplet nuclei in the space. Many classrooms use mixing type ventilation in which high velocity cool air is supplied at the ceiling level and returned
either at the ceiling level or the floor. Displacement ventilation and underfloor ventilation are two other types
of ventilation methods often used in high performance
schools.
Displacement ventilation is a stratified air system
that supplies low speed, cool air near the floor level
that slowly moves across the room. As this air reaches a
heat source (person, equipment, etc.), the cold, dense
air is warmed, creating a thermal plume that draws the
clean air up the source, forcing polluted air up above
the breathing zone where it can be returned. Underfloor
ventilation introduces air from diffusers at the floor
level, but unlike displacement ventilation, which relies
on buoyancy, the air is typically supplied using high
velocity jets, and mixing occurs in the occupied zone.
Above the occupied zone, stratification can still occur
and, similar to displacement ventilation, pollutants can
be carried by the upward flow to a return in the ceiling.
ASHRAE Standard 62.1-2019 defines the zone air distribution effectiveness (Ez) (Figure 1) as " the ratio of the
change of contaminant concentration between the air
supply and air exhaust to the change of contaminated
concentration between the air supply and the breathing
zone. " 21 Table 6-4 in ASHRAE Standard 62.1-2019 lists the
zone air distribution effectiveness of several systems. For
an ideal mixed air system, the Ez equals 1.0. For a displacement ventilation system, Xie, et al., calculated an
Ez of 1.3 for a classroom and 1.25 for an underfloor ventilation system.22 Table 6-4 also shows that when the location of the return diffusers is above 5.5 m (18 ft), a higher
Ez can be obtained. CFD modeling of these systems also
shows indirect exposure under displacement ventilation is generally lower than the exposure under mixing
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ASHRAE Journal - April 2021

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