Efficiency Of Bacterial Disinfection
By A
Duct Mounted UV-Aire Air Purifier
By:
Kane Environmental Assays
Sanitary & Environmental Microbiology
Bernard E. Kane, Ph.D.
1706 Canterbury Rd
Greenville, NC 27858
Ph. 252.355.6789
Oysterdoctor@aol.com
For:
2
Table of Contents
Background ..........................................................................................................................3
Organism.........................................................................................................................3
Testing Structure.............................................................................................................3
Testing Airflow Rate........................................................................................................3
Organism Applicator........................................................................................................4
UV Device .......................................................................................................................4
Air Sampling Method.......................................................................................................4
Test Apparatus.....................................................................................................................4
Testing Procedure................................................................................................................5
Tables............................................................................................................................... 6-7
Table 1: Control Data......................................................................................................6
Table 2: UV-18 Test Data Results..................................................................................6
Table 3: UV-18X Test Data Results................................................................................7
Conclusion............................................................................................................................8
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Background
This product study evaluates the effectiveness of the UV-Aire air purifier in reducing the
levels of bacteria with a single pass through a simulated air duct system. This device is
designed to irradiate the air as it circulates through the home, so the single pass
evaluation is the worst-case scenario use of this device. The air in the home will pass
through the heating and air conditioning system many times a day, as the air is
circulated throughout the home. Knowing the effectiveness of the UV-Aire in a single
pass application, enables us to project how effectively the device will treat the air with
multiple passes a day.
UV light technology has been successfully used for the disinfection of drinking water for
years. Applications for air disinfection with the use of UV light technology include:
commercial air treatment in hospitals, clean rooms, meat packing plants, bakeries,
dairies, breweries, bottling plants and large commercial HVAC systems.
ORGANISM:
Serratia marcescens (ATCC 14756) was chosen as the test bacterium. The distinctive
red colonies made it easy to evaluate from any background organisms. A raw test
suspension of the organism of approximately 95,000 CFU/ml was used. As dispersed
into the test system, this suspension yielded bacterial counts of 269 CFU/ft3 @ 500 ft/min
airflow and 107.5 CFU/ft3 @ 1000 ft/min airflow. (CFU = Colony Forming Units)
TESTING STRUCTURE:
An 18” x 18” galvanized air duct, 38 feet long was constructed as the test chamber (see
Figure 1). A fan was mounted at the exit end of the chamber and the treated air
exhausted to the outdoors. To reduce contamination of the intake air, all air intakes on
the exhaust side of the building were sealed. The exhaust fan was equipped with a flow
adjustment to allow for adjustable air speeds measured in feet per minute (FPM) through
the duct.
TESTING AIRFLOW RATE:
The airflow rate through the ductwork was adjusted to two nominal velocities of 500 ft/min
and 1000 ft/min. The airflow velocities were measured at the center of the duct at the
intake end of the test duct.
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ORGANISM APPLICATOR:
An atomizing humidifier spray nozzle mounted at the center of the test duct intake was
used to distribute the organism into the air stream. The application flow rate was 0.45
gallons per hour.
UV DEVICE:
A Field Controls UV-Aire air purifier model UV-18 was mounted onto the center of the
side of the test duct 6 feet from the exit end of the chamber. The lamp is a UVC
germicidal lamp (non ozone producing) 18 inches long with a UV output rating of 73
µW/cm2 at 1 meter from the lamp.
AIR SAMPLING METHOD:
An Andersen N6 single stage “bioaerosal” sampler was used to take the air samples and
distribute the sampled air onto agar medium. The test medium was Tryptic Soy Agar
from PathCon, Inc. The air sampling pump airflow rate was 1 CFM.
The Anderson sampler method requires corrections to the actual colony counts on the
plates. This provides a more accurate measure of the bacteria per cubic foot of the
sample air. In the following tables, the Serratia marcescens Positive Hole Count values
are the actual plate counts and the Corrected Particle Count values are corrected value
based on Anderson correction tables.
Test Apparatus
Figure 1
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Testing Procedure
The testing was performed in two stages. The first stage operated the test chamber with
the lamp off. (See table 1) This developed the control data or the base line bacterial
levels for the comparison. The second stage operated the test chamber with the lamp on.
(See table 2)
Two airflow rates were used to evaluate the lamp effectiveness based on exposure time.
Airflow velocities through the ducts of a typical residential heating and cooling system
range from 300 to 500 feet per min (fpm). For this study a base air velocity of 500 fpm
was used. To decrease the exposure time, a second test was conducted with the airflow
in the duct doubled to 1000 fpm. Since the effectiveness of UV lamps is based on the UV
light output and exposure time, doubling the airflow reduces the effectiveness of the lamp.
The bacterium was cultured and the cells harvested to provide a suspension of known
cell density. This was further diluted to provide gallon quantities of a test suspension
containing an estimated 95,000 CFU/ml. This suspension was pumped through the spray
nozzle mounted in the center of the duct inlet.
Five air samples were taken for each of the test velocities at short intervals (typically ½ to
2 minutes). This produced a large sample volume of air and reduced the levels of back
ground bacteria and molds counts. The plate counts (colony forming units or CFU) for
each of the five tests were totaled and divided by the total test volume of air. This
produced the comparison value of (269 CFU/FT3 of air) for the 500 FPM airflow and
(107.5 CFU/FT3 of air) for the 1000 FPM airflow. Due to apparent efficiency losses in the
sampling method at the 1000 FPM velocity, the bacterium count yielded a 60% drop
instead of the anticipated 50% reduction due to the velocity change.
Four air samples were taken at 1, 2.5, 3, 5, 6 & 10 minute intervals for each of the test
velocities with the lamp on. The longer sample times with the lamp on were needed to
obtain plate counts which would provide reliable estimates of the efficiency of disinfection,
but with this, more background organisms were found. The plate counts were (18.00
CFU/FT3 of air for the UV-18 and 2.56 CFU/FT3 of air for the UV-18X) at 500 FPM airflow.
They were 31.18 CFU/FT3 of air for the UV-18 and 10.40 CFU/FT3 of air for the UV-18X
at 1000 FPM airflow.
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Table 1: Control Data (testing with lamp off)
Sample
Number
Air Sampling
Duration (min)
1 1 500 181 241
2 1 500 193 263
3 1 500 208 294
4 0.5 500 117 138
5 0.5 500 118 140
Total min. = 4
1 2 1000 168 218
2 2 1000 167 216
3 2 1000 169 220
4 1 1000 91 103
5 1 1000 92 103
Total min. = 8
Airflow
Velocity
fpm
Serratia marcescens
Positive hole count
Total Corrected
Particle counts
Total Corrected
Particle counts
Corrected Particle
Counts
CFU/FT3of air
(count/min)
= 1076 269.00
= 860 107.50
Table 2: UV-18 Test data and results (testing with lamp on)
Air
Sample
Number
1
2
3
4
Sampling
Duration
(min)
1 1000 30 31
1 1000 32 33
3 1000 88 99
6 1000 145 180
Total min = 11
1
2
3
4
1 500 13 13
1 500 19 19
3 500 57 61
6 500 92 105
Total min = 11
Airflow
Velocity
(fpm)
Serratia
marcescens
Positive hole
count
Total Corrected
Particle Counts
Total Corrected
Particle Counts
of air
3
%Survival
CFU/Control
Log
Reduction
%
Effective
Corrected
Particle
Counts
CFU/FT
(count/min)
= 343 31.18 29.01 0.54 70.99
Control:
107.50
= 198 18.00 6.69 1.17 93.31
Control:
269.00
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Table 3: UV-18X Test data and results (testing with lamp on)
Air
Sample
Number
1
2
3
4
Sampling
Duration
(min)
2.5 1000 21 22
2.5 1000 27 28
2.5 1000 28 29
5 1000 48 51
Total min = 12.5
1
2
3
4
5 500 8 8
5 500 10 10
5 500 17 17
10 500 28 29
Total min = 25
Airflow
Velocity
(fpm)
Serratia
marcescens
Positive hole
count
Total Corrected
Particle Counts
Total Corrected
Particle Counts
of air
3
%Survival
CFU/Control
Log
Reduction
%
Effective
Corrected
Particle
Counts
CFU/FT
(count/min)
= 130 10.40 9.67 1.01 90.33
Control:
107.50
= 64 2.56 0.95 2.02 99.05
Control:
269.00
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Conclusion
UV-Aire
Model
UV-18 500 93.31 6.69 1.17
UV-18 1000 71.99 29.01 0.54
UV-18X 500 99.00 0.95 2.02
UV-18X 1000 90.33 9.67 1.01
The testing showed the UV-Aire lamp yields at least a 90% reduction of the test bacteria
with a single airflow pass through a duct system at typical airflow rates. This efficiency
will not be the same for all bacteria and molds since each organism requires different
exposure times at the same UV output energy level.
At the higher velocity, the lamp still reduced the bacterial levels by at least 71 % at a 50%
decrease in the exposure time. Since the reduction efficiency is based on lamp UV
output and exposure time, the assumption can be made that decreasing the exposure
time to the UV light is similar to testing an organism that requires a higher UV energy
requirement to kill the bacteria. The log reductions in bacterial levels were very close to
theoretical values. Within the limits of testing accuracy, twice as many log reductions
(0.54 vs. 1.17 and 1.01 vs. 2.02) occurred with twice the exposure time.
This testing and the results clearly show that the exposure of the air to the UV light of the
UV-Aire will reduce levels of airborne bacteria.
Airflow
velocity
(fpm)
Percent Reduction
of Bacteria
Percent Survival of
Bacteria
Log Reductions
of Bacteria
Form #4291 08/01
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