Ultraviolet Germicidal Irradiation (UVGI)
The use of ultraviolet germicidal irradiation (UVGI)
for the sterilization of microorganisms has been studied since the 1930s.
Microbes are uniquely vulnerable to the effects of light at wavelengths at or
near 2537 Angstroms (253.7 nanometers) due to the resonance of this wavelength
with molecular structures. Looking at it another way, a quanta of energy of
ultraviolet light possesses just the right amount of energy to break organic
molecular bonds. This bond breakage translates into cellular or genetic damage
for microorganisms. The same damage occurs to humans, but is limited to the skin
and eyes.
|
 The ultraviolet component of
sunlight is the main reason microbes die in the outdoor air. The die-off
rate in the outdoors varies from one pathogen to another, but can be
anywhere from a few seconds to a few minutes for a 90-99% kill of viruses or
contagious bacteria. Spores, and some environmental bacteria, tend to be
resistant and can survive much longer exposures. UVGI systems typically use
much more concentrated levels of ultraviolet energy than are found in
sunlight.
|
|
 Some
properly designed, and well-maintained, UVGI installations have proven
highly effective, as in certain hospitals, and some studies performed in
schools. CDC guidelines recommend the use of UVGI only with the simultaneous
use of HEPA filters and high rates of purge airflow. The germicidal effects
can also be species-dependent. |
Laboratory tests have achieved extremely high rates of
mortality under idealized conditions. In actual applications, many factors can
alter the effectiveness of UVGI, including the following :
 |
Exposure time (the air velocity must allow
for a sufficient dose). |
|
|
Room air mixing (for non-powered
applications like ceiling units). |
|
|
Power levels. |
|
|
The presence of moisture or particulates
provide protection for microbes. |
|
|
Dust settling on light bulbs can reduce
exposures, maintenance is necessary. |
One especially effective application of UVGI is the control
of microbial growth in air handling unit cooling coil and filter assemblies. The
constant exposure has been found to be very effective at controlling fungal
growth, either because the spores are inactivated, or perhaps because mycelial
growth cannot be sustained under continuous exposure.
Certain types of UVGI designs seem to provide a much higher
rate of disinfection than standard models operating at nearly identical
spectrums, the difference being the result of improvements in the electrical
power controls and regulation of internal plasma temperature, resulting in the
generation of a more constant energy density at a distance from the light
source.
Viruses are especially susceptible to UVGI, more so than
bacteria, but are also very difficult to filter. Some studies have shown that
viruses are more sensitive to ultraviolet radiation at wavelengths somewhat
above the normal UVGI broad-band wavelength of 2537 A (Rauth 1965; Setlow 1961).
A combination of filtration for bacteria and spores, with UVGI for viruses may
be an optimum combination if all components are sized appropriately.
UVGI THEORY & RATE CONSTANTS FOR AIRBORNE PATHOGENS
- UVGI inactivates pathogens according to the standard decay equation
S = exp(-kt)
In this equation S represents the fraction of the
original population that survives exposure at time t. The rate
constant k has been determined experimentally for a number of bacteria,
viruses and spores, at different power levels. Summarized below are many of the
known rate constants for the indicated pathogens. Since many researchers have
obtained values that differ, they have all been included. The variations in rate
constants have been charted, and also compared in terms of the time to achieve a
99% kill at 25 microwatts/sq.cm. (μW/cm 2).
The source documents may be found in the references.
|
UVGI RATE CONSTANTS AT 25 microwatt/cm2 |
|
Microorganism |
Microbial Group |
Rate Constant (1/sec.) |
Time for 99% Kill
(seconds) |
Reference |
Intensity μW/cm 2 |
| |
|
|
|
|
|
|
Adenovirus |
Virus - DNA |
6.2518 |
0.03 |
Jensen 1964 |
0.538 |
| |
|
0.0047 |
39.19 |
Rainbow 1973 |
1389 |
|
Coxsackievirus |
Virus - RNA |
6.7779 |
0.03 |
Jensen
1964 |
0.538 |
| |
|
2.2532 |
0.08 |
Hill
1970 |
11.6 |
|
Influenza A |
Virus - RNA |
8.967 |
0.02 |
Jensen
1964 |
0.538 |
| |
|
0.001 |
184.21 |
Westinghouse data |
|
|
Echovirus |
Virus - RNA |
2.6969 |
0.07 |
Hill
1970 |
11.6 |
|
Vaccinia |
Virus - DNA |
9.4165 |
0.02 |
Jensen
1964 |
0.538 |
|
Reovirus |
Virus - RNA |
2.0095 |
0.09 |
Hill
1970 |
11.6 |
|
Staphylococcus aureus |
Bacteria - GP |
0.1243 |
1.5 |
Wells
1955 |
|
| |
|
0.0921 |
2.0 |
Sharp 1939 |
|
| |
|
0.0886 |
2.1 |
Mitscherlich (T45) |
|
| |
|
0.0028 |
65.8 |
Kundsin 1968 |
25 |
| |
|
0.0012 |
153.5 |
Gates
1929 |
1.1 |
| |
|
0.0009 |
204.7 |
Westinghouse data |
|
|
Mycobacterium tuberculosis |
Bacteria - GP |
0.0981 |
1.9 |
David
1973 |
4 |
| |
|
0.0774 |
2.4 |
Wells 1955 |
|
| |
|
0.0011 |
167 |
Collins
1971 |
40 |
| |
|
0.0007 |
263 |
Westinghouse data |
|
|
Mycobacterium kansasii |
Bacteria - GP |
0.009 |
20.5 |
David
1973 |
4 |
| Mycobacterium avium-intra. |
Bacteria - GP |
0.0097 |
19.0 |
David 1973 |
4 |
|
Streptococcus pyogenes |
Bacteria - GP |
0.1066 |
1.7 |
Mitscherlich (T45) |
|
| |
|
0.0006 |
307.0 |
Lidwell |
2.6 |
|
Bacillus anthraci |
Bacteria - GP |
0.0509 |
3.6 |
Mitscherlich (T45) |
|
| (spores?) |
|
0.0008 |
230.3 |
Westinghouse data |
|
|
Corynebacterium diptheriae |
Bacteria - GP |
0.0683 |
2.7 |
Mitscherlich (T45) |
|
| |
|
0.0679 |
2.7 |
Sharp 1939 |
|
| |
|
0.0011 |
167.5 |
Westinghouse data |
|
| Serratia marcescens |
Bacteria - GN |
0.0022 |
83.7 |
Collins 1971 |
40 |
|
Neisseria meningitidis |
Bacteria - GN |
0.0028 |
65.8 |
Kundsin
1968 |
25 |
|
Moraxella-Acinetobacter |
Bacteria - GN |
0.000002 |
92103 |
Keller 1982 |
0.22 |
|
Haemophilus influenzae |
Bacteria - GN |
0.00005 |
3684 |
Mongold
1992 |
1 |
| Pseudomonas aeruginosa |
Bacteria - GN |
0.0419 |
4.4 |
Mitscherlich (T45) |
|
| |
|
0.0026 |
70.8 |
Collins
1971 |
40 |
| |
|
0.0007 |
263.2 |
Westinghouse data |
|
| |
|
0.0005 |
368.4 |
Abshire
1981 |
100 in soln. |
| Legionella pneumophila |
Bacteria - GN |
0.003 |
61.4 |
Antopol 1079 |
50 |
| |
|
0.0019 |
97.0 |
unidentified Figure |
1 |
| |
|
0.0018 |
102.3 |
Westinghouse data |
|
| E.
coli (reference) |
Bacteria - GN |
0.0921 |
2.0 |
Sharp
1939 |
|
| |
|
0.0008 |
230.3 |
Westinghouse data |
|
|
Mycoplasma pneumoniae |
Bacteria |
0.0046 |
40.0 |
Kundsin
1968 |
25 |
|
Cryptococcus neoformans |
Fungi |
0.0002 |
921 |
Wang
1994 |
|
| Mucor ramosissimus
spores |
Fungi |
0.0002 |
921 |
Westinghouse data |
|
|
Penicillium expensum spores |
Fungi |
0.0003 |
614 |
Westinghouse data |
|
| Aspergillus niger
spores |
Fungi |
0.00002 |
9210 |
Westinghouse data |
|
|
Aspergillus flavus spores |
Fungi |
0.00004 |
4605 |
Westinghouse data |
|
| Aspergillus glaucus
spores |
Fungi |
0.00005 |
3684 |
Westinghouse data |
|
|
Rhizopus nigricans spores |
Fungi |
0.00002 |
9210 |
Westinghouse data |
|
|
Selective Averages |
of Rate Constants
|
|
Microbial Group |
Average Rate Constant (1/s)
|
|
Viruses |
5.26582 |
|
Gram Negative Bacteria |
0.01235 |
|
All Bacteria |
0.02174 |
|
Gram Positive Bacteria |
0.03307 |
|
Spores |
0.00012 |
Kill Factors for Various Pathogens
Energy Needed for kill factor
(Microwatt seconds per square centimeter)
|
Mold Spores |
Color |
90% Kill |
100% Kill |
- Aspergillius flavis
|
Yellowish green |
66,000 |
99,000 |
- Aspergillius glaucus
|
Bluish green |
44,000 |
88,000 |
- Aspergillius niger
|
Black |
132,000 |
330,000 |
|
Mucor racemosus A |
White gray |
17,000 |
352,000 |
|
Mucor racemosus B |
White gray |
17,000 |
352,000 |
|
Oospora lactis |
White |
5,000 |
11,000 |
|
Penicillium expansum |
Olive |
13,000 |
22,000 |
|
Penicillium roqueforti |
Green |
13,000 |
26,400 |
|
Penicillium digitatum |
Olive |
44,000 |
88,000 |
|
Rhisopus nigricaans |
Black |
111,000 |
220,000 |
Energy Needed for kill factor
(Microwatt seconds per square centimeter)
|
Bacteria |
90% Kill |
100% Kill |
|
Bacillius anthracis |
4,520 |
8,700 |
|
Bacillius magaterium sp. (spores) |
2,730 |
5,200 |
|
Bacillius magaterium sp. (veg.) |
1,300 |
2,500 |
|
Bacillius paratyphusus |
3,200 |
6,100 |
|
Bacillius subtilis spores |
11,600 |
22,000 |
|
Bacillius subtilis |
5,800 |
11,000 |
|
Clostridium tetani |
13,000 |
22,000 |
|
Corynebacterium diphtheriae |
3,370 |
6,500 |
|
Eberthella typosa |
2,140 |
4,100 |
|
Escherichia coli |
3,000 |
6,600 |
|
Leptospira Canicoal – infectious Jaundice |
3,150 |
6,000 |
|
Microccocus candidus |
6,050 |
12,300 |
|
Microccocus spheriodes |
1,000 |
15,400 |
|
Mycobacterium tuberculosis |
6,200 |
10,000 |
|
Neisseria catarrhalis |
4,400 |
8,500 |
|
Phtomonas tumeficiens |
4,400 |
8,000 |
|
Proteus vulgaris |
3,000 |
6,600 |
|
Pseudomonas aeruginosa |
5,500 |
10,500 |
|
Pseudomonas fluorescens |
3,500 |
6,600 |
|
Salmonella enteritidis |
4,000 |
7,600 |
|
Salmonella enteritidis |
4,000 |
7,600 |
|
Salmonella paratyphi-enteic fever |
3,200 |
6,100 |
|
Salmonella typhosa – typhoid fever |
2,150 |
4,100 |
|
Salmonella typhimurium |
8,000 |
15,200 |
|
Sarcinia lutea |
19,700 |
26,400 |
|
Serratia marcesens |
2.420 |
6,160 |
|
Shigella dysenteriae – Dysentery |
2,200 |
4,200 |
|
Shigella flexneri – Dysentary |
1,700 |
3,400 |
|
Shigella paradysenteriae |
1,680 |
3,400 |
|
Spirillum rubrum |
4,400 |
6,160 |
|
Staphylococcus albus |
1,840 |
5,720 |
|
Staphylococcus aureus |
2,600 |
6,600 |
|
Streptococcus hemolyticus |
2,160 |
5,500 |
|
Streptococcus lactis |
6,150 |
8,800 |
|
Streptococcus viridans |
2,000 |
3,800 |
|
Vibrio comma – Cholera |
3,375 |
6,500 |
Energy Needed for kill factor
(Microwatt seconds per square centimeter)
|
Protozoa |
90% Kill |
100% Kill |
Chlorella vulgaris
|
13,000
|
22,000
|
Nematode eggs
|
4,000
|
92,000
|
Paramecium
|
11,000
|
20,000
|
Energy Needed for kill factor
(Microwatt seconds per square centimeter)
|
Virus |
90% Kill |
100% Kill |
|
Bacteriophage (E. Coli) |
2,600 |
6,600 |
|
Infectious Hepatitis |
5,800 |
8,000 |
|
Influenza |
3,400 |
6,600 |
|
Poliomyelitis – Polio |
3,150 |
6,000 |
|
Smallpox |
|
|
|
Tobacco mosaic |
240,000 |
440,000 |
Energy Needed for kill factor
(Microwatt seconds per square centimeter)
|
Yeast |
90% Kill |
100% Kill |
|
Brewers yeast |
3,300 |
6,600 |
|
Common cake yeast |
6,000 |
13,200 |
|
Saccharamyces carevisiae |
6,000 |
13,200 |
|
Saccharamyces ellipsoideus |
6,000 |
13,200 |
|
Saccharamyces sp. |
8,000 |
17,600 |
       
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HPAC Vol. 69, No. 5. May, p28.
|
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