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Kane Environmental Assays
Sanitary & Environmental Microbiology
Bernard E. Kane, Ph.D.
1706 Canterbury Rd,
Greenville, NC 27858
Ph. 252.355.6789
Oysterdoctor@aol.com

 

EFFICIENCY OF BACTERIAL DISINFECTION BY
AN IN DUCT MOUNTED
air health™ uv home air sanitizer

By
Bernard Kane, PhD
Kane Environmental Assays
1706 Canterbury Road
Greenville, NC 27858


Introduction:

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.

This product testing study evaluates the effectiveness of air health™ air
sanitizer in reducing the levels of bacteria with a single pass through a simulated
air duct system. This device is designed to irradiate air as it circulates
through the home, so the single pass evaluation is the worst-case scenario use
of this device. air in the home will pass through the heating and air
conditioning system many times a day, as air is circulated throughout the
home. Knowing the effectiveness of air health™ in a single pass application,
enables us to project how effectively the device will treat air with multiple
passes a day.

 

Material and Methods:

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/ft 3 @ 500 ft/min airflow and 107.5 CFU/ft 3 @
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. airflow rate through the ductwork was adjusted to two
nominal velocities of 500 ft/min and 1000 ft/min. airflow velocities were
measured at the center of the duct at the intake end of the test duct.

Organism applicator. An atomizing humidifier spray nozzle mounted at the
center of the test duct intake was used to distribute the organism into air
stream. The application flow rate was 0.45 gallons per hour.

UV device. A home health products air health™ air sanitizer model AH-1 was mounted
onto the center of the side of the test duct 6 feet from the exit end of the chamber.
The bulb is a UVC germicidal bulb (non ozone producing) 16 inches long with a
UV output rating of 62 mW/cm 2 at 1 meter from the bulb.

Air sampling method. An Andersen N6 single stage "bioaerosal" sampler was
used to take air samples and distribute the sampled air onto agar medium.
The test medium was Tryptic Soy Agar from PathCon, Inc. 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 sampled 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.

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Testing Procedure:

The testing was performed in two stages. The first stage operated the test
chamber with the bulb 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 bulb on. (See table 2)

Two airflow rates were used to evaluate the bulb 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 was
conducted with airflow in the duct doubled to 1000 fpm. Since the
effectiveness of UV bulbs is based on the UV light output and exposure time,
doubling airflow reduces the effectiveness of the bulb.

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 1/2 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/FT 3
of air) for the 500 FPM airflow and (107.5 CFU/FT 3 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, 3 & 6 minute intervals for each of the test
velocities with the bulb on. The longer sample times were used to obtain higher
plate counts, but with this more background organisms were found. The plate
counts yielded a value of (26.27 CFU/FT 3 of air) for the 500 FPM airflow and
(36.09 CFU/FT 3 of air) for the 1000 FPM airflow.

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Results:

The testing showed the air health™ bulb yields 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 bulb still reduced the bacterial levels by 66% at a 50%
decrease in the exposure time. Since the reduction efficiency is based on bulb
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 (1.01 vs. 0.47) occurred with twice the exposure time.

This testing and the results clearly show that the exposure of air to the UV light
of air health™ will reduce levels of airborne bacteria.

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