Effectiveness of Germicidal UV Radiation for Reducing Fungal Contamination within Air-Handling Units

Abstract

Levels of fungi growing on insulation within air-handling units (AHUs) in an office building and levels of airborne fungi
within AHUs were measured before the use of germicidal UV light and again after 4 months of operation. The fungal levels following
UV operation were significantly lower than the levels in control AHUs.

Full text

APPLIED AND ENVIRONMENTAL MICROBIOLOGY,
0099-2240/01/$04.00⫹0 DOI: 10.1128/AEM.67.8.3712–3715.2001
Aug. 2001, p. 3712–3715 Vol. 67, No. 8
Copyright © 2001, American Society for Microbiology. All Rights Reserved.
Effectiveness of Germicidal UV Radiation for Reducing Fungal
Contamination within Air-Handling Units
ESTELLE LEVETIN,
1
* RICHARD SHAUGHNESSY,
2
CHRISTINE A. ROGERS,
1
†AND ROBERT SCHEIR
3
Faculty of Biological Science
1
and Department of Chemical Engineering,
2
The University of Tulsa,
Tulsa, Oklahoma 74104, and Steril-Aire, Inc., Cerritos, California 90703
3
Received 27 October 2000/Accepted 16 May 2001
Levels of fungi growing on insulation within air-handling units (AHUs) in an office building and levels of
airborne fungi within AHUs were measured before the use of germicidal UV light and again after 4 months of
operation. The fungal levels following UV operation were significantly lower than the levels in control AHUs.
Fungal contamination of air-handling units (AHUs) is a
widespread phenomenon in buildings with central heating,
ventilation, and air-conditioning (HVAC) systems and is a
potential source of contamination for occupied spaces (1, 8, 16,
20). Fungi have been found growing on air filters, insulation,
and cooling coils, as well as in ducts. This contamination often
contributes to building-related diseases, including both infec-
tious diseases and hypersensitivity diseases, such as allergic
rhinitis, asthma, and hypersensitivity pneumonitis (4, 11, 13).
In addition, acute toxicosis and cancer have been attributed to
respiratory exposure to mycotoxins (5).
Control of fungi in indoor environments has traditionally
focused on source control, ventilation, and air cleaning. Source
control emphasizes the reduction or elimination of moisture to
limit fungal growth. Although this can be effective in many
areas, it is not achievable in HVAC systems during cooling. By
design, air-conditioning systems cause moisture to condense
from air. As a result, other methods are needed to reduce
fungal contamination. Ventilation relies on using filtered out-
door and recirculated indoor air. Ventilation is ineffective,
however, when unfiltered outdoor air introduces outdoor bio-
aerosols or when the HVAC system itself is contaminated. Air
cleaning has focused on using properly maintained high-quality
filters within HVAC systems as well as portable air-cleaning
devices. Recently, there has been renewed interest in the use of
germicidal UV irradiation to disinfect indoor environments for
control of infectious diseases in hospitals, other health care
facilities, and public shelters (14, 15, 18, 19).
Although it has been known for many years that UV light
has various effects on fungi (3, 9, 10), only a few studies have
specifically focused on the effects of germicidal UV light (2, 7,
12, 17, 22, 23). Currently, various manufacturers are marketing
germicidal UV lamps for controlling contamination, including
fungal contamination in indoor environments, as well as AHUs
and ducts. Studies have shown that these measures may be
effective for controlling the spread of bacterial diseases (14, 15,
18, 19); however, little is known about the effectiveness of
UV-C radiation for controlling fungal contamination. The
present investigation was undertaken to determine the effec-
tiveness of germicidal UV radiation for reducing fungal con-
tamination within AHUs.
This investigation was conducted in a 286,000 square-foot
office building in Tulsa, Okla. The building was originally con-
structed in the 1920s and was completely remodeled in 1976.
Each floor of this four-story building is equipped with four
primary AHUs and two perimeter units; these units were in-
stalled when the building was remodeled. Beginning in 1996,
the air handlers were retrofitted with germicidal UV lamps.
During the fall of 1996 all the AHUs in the building were
inspected. At this time UV lamps were installed in AHUs on
one floor, and work was progressing to install them on a second
floor. Acoustical insulation within many of the AHUs exhibited
abundant mold growth, as did drain pans. Preliminary air sam-
ples and insulation samples were collected to develop the sam-
pling protocols used in this study.
AHUs on two floors were selected for further investigation;
no UV lamps had been installed in these AHUs. The floors
were designated the study floor and the control floor. Only the
four main AHUs on each of these floors were used for the
remainder of the investigation. In May 1997, air samples and
insulation samples were collected from the eight AHUs. UV
lamps were installed on both floors, but they were activated
only in the AHUs on the study floor. Each AHU was retrofit-
ted with 10 lamps, which were installed downstream of the
coils. The output of each lamp was 158 ␮W/cm
2
at1mor10
␮W/cm
2
for every 2.54 cm of tube length at 1 m (21). The
lamps were operated 24 h a day throughout the summer and
early fall in the AHUs on the study floor. On the control floor,
no UV lights were operated. Throughout the building, air
conditioning was in use during this period. In late September,
samples were collected from all eight AHUs.
Preliminary data showed that air sampling in the AHUs
conducted while the AHUs were running resulted in collection
of few or no fungal spores because the high airflow rate pro-
duced nonisokinetic conditions. For this reason the supply fan
in each AHU was shut off prior to sampling. Although this
action caused some mechanical disturbance, it provided a
method for estimating the potential load of fungal propagules
available for dispersal.
Air samples were collected in duplicate by using paired sin-
gle-stage Andersen (N-6) samplers with malt extract agar
* Corresponding author. Mailing address: Faculty of Biological Sci-
ence, The University of Tulsa, 600 S. College, Tulsa, OK 74104. Phone:
(918) 631-2764. Fax: (918) 631-2762. E-mail: estelle-levetin@utulsa
.edu.
† Present address: Department of Environmental Health, Harvard
School of Public Health, Boston, MA 02115.
3712

plates for viable fungi and paired Burkard personal samplers
for total spores. Two-minute Andersen samples and 5-min
Burkard samples were collected approximately 40 cm down-
stream of the cooling coils 30 s after the supply air fan in each
AHU was turned off. All samples were started simultaneously,
but the Andersen samplers were switched off after 2 min.
Samples were obtained from each AHU at least twice in both
the spring and the fall.
Plates from the Andersen samplers were incubated at room
temperature for 5 to 7 days. Colonies were counted, fungi were
identified, and concentrations were expressed in CFU per cu-
bic meter of air. Burkard slides were made permanent by using
a lactophenol-polyvinyl alcohol mounting medium, and the
slides were examined microscopically at a magnification of
⫻1,000. Spores were identified and counted. Counts were con-
verted into atmospheric concentrations and expressed in num-
bers of spores per cubic meter of air. Data from all samples for
each AHU were averaged for each time period.
For each AHU, pieces of fiberglass insulation (approximate-
ly 60 cm
2
) were cut from the insulation directly opposite the
cooling coils, approximately 1 m from the base, 2 m from the
end wall, and less than 30 cm from the UV lights. The insula-
tion samples were individually sealed in sterile plastic bags for
transport to the laboratory. In the laboratory, a smaller square
of each insulation sample (6.5 cm
2
) was cut from the center of
the larger piece. The small square was soaked in 10 ml of
sterile distilled water for 20 min. The suspension was vortexed
for 30 s and then dilution plated in triplicate on malt extract
agar plates. The plates were incubated at room temperature
for 5 to 7 days. Colonies were counted, fungi were identified,
and concentrations were expressed in CFU per square centi-
meter. Data from replicate samples were averaged for each
AHU.
For each type of sample collected (viable spores, total
spores, and insulation) the concentrations obtained for each
AHU were averaged to determine means for the study floor
and means for the control floor. Mann-Whitney U tests were
used to compare the means in May and in September by using
Statistica 5.0 software.
The dominant fungi found within the AHUs for both the air
samples and the insulation samples included Penicillium cory-
lophyllum, Aspergillus versicolor, and a strain of an unidentified
Cladosporium species which was somewhat similar to Clado-
sporium sphaerospermum (6) and may be a strain of this spe-
cies. These three taxa accounted for more than 90% of all
viable fungi isolated. Other fungi identified included Acremo-
nium spp., Cladosporium cladosporioides, Cladosporium spha-
erospermum, Cladosporium elatum, and Hyalodendron sp. Occa-
sionally other Aspergillus and Penicillium species also occurred
in the samples.
In May before the UV lights were turned on, the mean
concentrations of the total fungi isolated from the insulation
samples on the two floors were similar (Table 1), and there was
no significant difference (P⬎0.05). In the fall the mean con-
centration on the study floor had decreased, while on the
control floor the concentrations had increased and were sig-
nificantly greater than the concentrations on the study floor
(P⬍0.05). In September the mean concentrations of both A.
versicolor and the unknown Cladosporium species were signif-
icantly lower in the AHUs on the study floor (P⬍0.05).
Similar results were obtained with the air samples (Table 2).
In the spring before the UV lights were turned on, the mean
concentrations of total viable airborne fungi in the AHUs on
the two floors were not significantly different (P⬎0.05). In the
fall, the mean concentration of viable fungi in the AHUs on
study floor was an order of magnitude lower, while on the
control floor the concentration of viable fungi in the AHUs
had increased. The total concentrations of viable fungi in the
AHUs on the study floor and the control floor in the fall were
significantly different (P⬍0.05). Because many of the AHUs
contained high concentrations of viable fungi, there were fre-
quently multiple impactions and multiple colonies at each im-
paction point on a culture plate. As a result, it was not always
possible to identify each colony to the species level. Therefore,
the concentration data in Table 2 are only genus level data.
The concentrations of Penicillium, Aspergillus, and Cladospo-
rium were significantly lower in the AHUs on the study floor
than in the AHUs on the control floor after the use of UV
lights (P⬍0.05).
The total spore levels obtained with the Burkard samplers
TABLE 1. Mean concentrations of fungi isolated from insulation samples in AHUs before and after installation of germicidal UV lamps
Fungal taxon isolated
Concn (10
3
CFU/cm
2
)
Study floor
a
Control floor
May
b
September May
b
September
Acremonium 0.65 (0.65) 5.81 (5.81) 23.81 (23.68)
Aspergillus versicolor 64.87 (38.56)
c
0.96 (0.56)
d
87.58 (32.95) 1,765.46 (1,702.1)
d
Cladosporium (unknown) 135.28 (50.38) 8.42 (5.22)
d
22.68 (10.19) 95.31 (37.74)
d
Cladosporium cladosporioides 0.26 (0.26) 5.04 (5.04) 0.65 (0.39) 228.59 (226.92)
Cladosporium (other) 0.13 (0.13) 1.72 (1.60)
Curvularia 0.05 (0.05)
Hyalodendron 4.65 (3.84) 13.95 (13.95) 83.96 (83.10) 109.66 (72.09)
Penicillium 8.16 (4.35) 1.05 (0.63) 9.27 (8.11) 16.0 (15.59)
Sporothrix 0.01 (0.01)
Nonsporulating colonies 0.04 (0.04) 1.94 (1.94)
Total 213.27 (82.53) 30.51 (24.85)
d
211.89 (130.80) 2,240.55 (1,622.4)
d
a
UV lamps were used only on the study floor.
b
May concentrations were measured before the UV lamps were turned on.
c
Mean (standard error).
d
The concentrations on the control floor and the study floor were significantly different after the use of germicidal UV lamps (P⬍0.05).
VOL. 67, 2001 GERMICIDAL UV RADIATION AND FUNGAL CONTAMINATION 3713

were far greater than the viable spore levels (Table 3). Prior to
the use of UV lights, there was not a significant difference (P⬎
0.05) between the mean levels of total spores in the AHUs on
the two floors. In September, the total concentrations on the
study floor were significantly lower than the total concentra-
tions on the control floor (P⬍0.05). The fungal taxa identified
were consistent with the data obtained with the Andersen
sampler and also with the insulation data. However, because it
is not possible to differentiate Penicillium and Aspergillus
conidia without conidiophores, the two genera are combined
as Penicillium-Aspergillus in Table 3. The concentrations of
Cladosporium and Penicillium-Aspergillus on the two floors
were significantly different in September (P⬍0.05).
The types of fungi found in the air samples were the same as
the types found in the insulation. Outdoor fungal taxa were
rarely found in either the control floor AHUs or the study floor
AHUs. This suggests that few outdoor spores passed through
the filters in the units and also that the source of the airborne
spores was the contaminated insulation in the units when dis-
turbance occurred, such as the disturbance caused when the
supply fans were shut off. As a result, we cannot say that the
UV-C radiation had a direct effect on spores in the air stream.
In addition, the effectiveness of UV lamps seemed to be local-
ized, because visual inspection indicated that there was con-
spicuous fungal growth in the downstream duct insulation lin-
ing. Nevertheless, the significant decrease in the insulation
certainly had an impact on the resultant air stream and also
had an impact on downstream concentrations. Further studies
are needed to examine downstream effects and the resultant
air quality in occupied spaces, especially in problem buildings.
The results of this study were similar to the results of a pilot
study performed by Menzies et al. (17). These authors found
that using germicidal UV lamps resulted in elimination of
bacterial and fungal growth on surfaces within an AHU. How-
ever, the study of Menzies et al. was performed from October
to December in Montreal, Canada, when operation of the
HVAC system in the heating mode would normally result in
reduced contamination. During the preliminary phase of this
study in 1996, we found that once the units were switched from
the air-conditioning mode to the heating mode, fungal con-
tamination dramatically decreased.
While the present investigation indicated that concentra-
tions of fungi were significantly lower when UV lamps were in
use, the study did not show what stages of fungal growth were
most susceptible, nor did it show whether there was a reduction
in spore viability. Also, we were not able to show if all the fungi
obtained from the AHUs were susceptible to the UV light. In
addition, this study was limited to the species found in the
building investigated. Asthana and Tuveson (2) showed that
germicidal effects were highly selective for certain species.
Clearly, more work is needed to determine the direct effects of
UV-C radiation on fungi capable of growing in HVAC systems.
In summary, this study indicated that germicidal UV irradi-
ation may be an effective approach for reducing fungal con-
tamination within AHUs. The use of germicidal UV lamps in
AHUs resulted in significantly lower levels of fungal contam-
ination in the fiberglass insulation lining of study floor AHUs
than in the insulation of control floor units. Also, there were
significantly lower levels of viable and total airborne fungi than
in the study floor units than in the control floor units when
samples were taken during periods of disturbance.
Partial support for this project was provided by a grant from Steril-
Aire, Inc., Cerritos, Calif.
We thank Melinda Sterling Sullivan, Jodi Keller, and Mary Petty-
john for assisting with sampling and/or culturing activities. We also
acknowledge the unending support and accommodations provided by
Tom McKain, Building Supervisor, and Argel Johnson, Maintenance
Director, throughout this study.
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TABLE 2. Mean concentrations of viable airborne fungi during
disturbance sampling within AHUs before and after installation of
germicidal UV lamps
Fungal taxon
isolated
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2
CFU/m
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Study floor
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Control floor
May
b
September May
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c
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d
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d
Cladosporium 15.64 (8.83) 1.28 (0.5)
d
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d
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UV lamps were used only on the study floor.
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a
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May
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b
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c
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d
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d
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d
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d
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Smuts 0.03 (0.01) 0.01 (0.01) 0.04 (0.02)
Other 0.70 (0.25) 0.24 (0.06) 0.47 (0.2) 1.46 (1.09)
Total 57.92 (25.09) 12.41 (4.47)
d
25.19 (16.73) 255.54 (82.27)
d
a
UV lamps were used only on the study floor.
b
May concentrations were measured before the UV lamps were turned on.
c
Mean (standard error).
d
Concentrations on the control floor and the study floor were significantly
different after the use of germicidal UV lamps (P⬍0.05).
3714 LEVETIN ET AL. APPL.ENVIRON.MICROBIOL.

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