ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 4

(the coil surface) is stationary, so exposure, or "residence,"

Organization to acknowledge that SARS-CoV-2 can be

time is continuous. As a result, the intensity of the UV-C

transmitted through air as an aerosol.10

energy striking the surface can be relatively low. In the
case of a moving air stream, however, exposure time is
limited-a mere fraction of a second, in some cases-so

ACH equivalents increase with germicidal intensity
(Figure 1).
UV-C can supplement protocols for disinfection,
sterilization, and manual cleaning, providing a level of

UV-C intensity must be much greater.
It is important to note that microbe inactivation is a

protection in the event a protocol fails. Facility managers

nonlinear function of UV-C exposure. In other words, "If a

are encouraged to implement a layered approach incor-

certain UV exposure kills 90 percent of a bacterial popu-

porating multiple infection-control measures to ensure

lation (frequently referred to as 'one-log kill'), doubling

that any pathogen that cannot be removed by one method

the exposure time or intensity can kill only 90 percent of

(e.g., ﬁltration, cleaning) is inactivated by another (UV-C).1

the residual 10 percent, for an overall germicidal efﬁcacy

Applying UV-C Energy

of 99 percent ('two-log kill')."7
In addition to reducing HVAC-surface and airborne

There are three primary means of applying UV-C energy

bacteria, germicidal UV can be used to supplement and

to protect HVAC surfaces and air streams against infec-

improve other infection-control strategies, such as room

tious agents: upper-room/air systems, HVAC air-stream

air exchange. When a required number of air changes

disinfection, and HVAC coil/surface irradiation (Table 1).

per hour (ACH) cannot be achieved using outside-air

Upper-room/air systems. One of the oldest appli-

ventilation alone, upper-room UV systems can perform

cations of germicidal UV for space infection control,

germicidal "equivalent" ACH. "It has been estimated that

upper-room/air systems work by effectively intercepting

µW/cm2

is present

pathogens and viruses at their source in room air. These

in the upper room, 63 percent of airborne tuberculosis

ﬁxtures are efﬁcient against droplet nuclei from coughing,

germs that arrive there will be killed in 24 sec (the germi-

sneezing, or talking, as well as pathogens circulated by

cidal equivalent of one room air change), and, therefore,

drafts, pressure differentials, or the movement of people.

99 percent will be killed in 2 min (equivalent of ﬁve air

Airborne droplets containing infectious agents can

changes)."8 This is important, as pathogenic aerosols can

remain in room air for 6 min or longer. Operating 24/7/365,

be spread through HVAC systems.9 In fact, in July 2020,

upper-room/air germicidal ﬁ xtures can inactivate these

more than 200 scientists petitioned the World Health

microbes in a matter of seconds.

when an average UV intensity of 10

Equivalent air changes per hour

Upper-room/air UV-C ﬁ xtures utilize the natural rise
20

and fall of convection or mechanical air currents to circulate airborne infectious agents into the upper room, where

15

they are exposed to UV-C radiation and killed. Studies
have shown that one hour of use of an upper-room/air

10

UV-C ﬁ xture can be equivalent to 10 to 16 air changes.11
Wall-mounted at a height above 7 ft, these ﬁ xtures use

5

non-reﬂective bafﬂes to direct UV-C energy upward and
outward, ensuring that stray emissions do not enter the

0

0

20
40
60
80
100
Average upper-room UV ﬂuence rate (μW/cm 2)

occupied portion of the room. First-pass kill or inactivation ratios of up to 99 percent have been modeled, with
concentrations further reduced with each subsequent pass

FIGURE 1. UV-C-induced inactivation of Mycobacte-

of recirculated air ("multiple dosing"). The goal, relative

rium parafortuitum in a test room under well-mixed

to coverage, is to maintain a UV-C irradiance level of at

conditions at 50-percent relative humidity.11

least 50 µW/cm2 in the upper room.

4

2020 A M CA i n m o t i o n

w w w. a m c a .o r g
http://www.amca.org
ASHRAE Journal Supplement - AMCA InMotion - October 2020

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