|
This
Article was found in the Journal Of Applied Physics
Volume
13, May, 1942
Bactericidal
Radiation
BY
A. PAULUS
Westinghouse
Electric and Manufacturing Company, Lamp Division, Pittsburgh,
Pennsylvania
Science
is gradually selecting and studying various groups of waves in
the electromagnetic spectrum. These groups are studied for the
purpose of finding their characteristics and their application
to human welfare. While the electromagnetic spectrum extends to
and beyond the infinitely short cosmic waves on one side and
through the visible and heat regions and radio waves of various
lengths on the other side, consideration is here given to a
small group of waves in the ultraviolet region (Fig. 1).
Just as visible
light is made up of different wave-lengths which are separated
by a prism into the rainbow colors, so the ultraviolet region
may be separated into groups which have widely differing
effects. Beyond the visible, the group of waves of 4000 to about
3300 angstroms are chemically active, affect photographic plates
and, in common with still shorter wave-lengths, cause
fluorescent and phosphorescent materials to glow.
A second group
measuring 3300 to 2850 angstroms are biologically effective
causing erythemal and tanning reactions, increasing vitamin D,
and preventing rickets. The effective bactericidal wave-lengths
2850 to 2000 angstroms have been receiving much attention during
the past. few years as the development of suitable generators
makes possible the practical application of bactericidal
radiations.
Below 2000
angstroms is a group of wavelengths which produce ozone in the
air. A small proportion of these ozone producing waves, together
with the more active bactericidal waves, produces the best
conditions for the destruction of bacteria and mold.
EFFECT
OF ULTRAVIOLET ON BACTERIA
In 1877 Downes
and Blunt showed that bacteria were killed by sunlight. The
lethal effect was attributed to mild and extended oxidation or
in short to heat. Later the killing effect was attributed to the
production of hydrogen peroxide in the aqueous media. Both these
early theories have been disproved by experiments in which the
lethal effect was produced independently of heating or oxygen
and where the chemical action in the media was negligible. As a
result of studies by Clark, Norton, and others and confirmed by
Bayne, Jones, and Van der Lingen, it has been generally accepted
that the bactericidal effect of selected wave-lengths of ultraviolet
is the result of direct photo-chemical action on the organisms.
Thc disintegration of
Fig. 1.
The radiant energy spectrum. The Sterilamp is an efficient
generator of radiation in the bactericidal band of the
ultraviolet spectrm.
micro-organisms
under radiations, principally of 2537 angstroms, is shown in
fig. 2, where paramecia which are single cell organisms
found in stagnant water, are exposed to the radiations from a
Sterilamp. The picture shows how the radiations cause the
organism to swell, blister, and finally disintegrate. Bacteria
are so small that the changes which take place because of
irradiation cannot he shown by an ordinary microscope, but they
are probably similar to the changes in the much larger
paramecia.
Fig.
2. Progressive stages in the killing of bacteria by
shortwave radiation. At the upper left are normal paramecia. At
the upper right are two paramecia after thirty seconds of
irradiation, showing distension just beginning. Distension
continues in the third view, followed by a change in the osmotic
tension of the cell wall, which causes the organism to swell and
break, as shown in the lower right-hand corner.
ULTRAVIOLET
GENERATORS
The sun
is considered the only natural source of ultraviolet but only a
small part of the short waive-lengths reach the earth because of
the filtering action of the atmosphere. The shortest
wave-lengths which can penetrate the atmospheric blanket measure
about 2900 angstroms, and the amount of this radiation is only
about one millionth as great as that at 3130 angstroms. The
ultraviolet radiation from the sun in the bactericidal region is
great enough to have been noted, but it is weak when compared to
the output of many artificial sources.
Practically all
artificial sources of ultraviolet sources of ultraviolet
radiation, with the exception of the carbon or iron electrode
are lamps, have one element in common -mercury vapor. Electrical
discharges through mercury vapor produce more radiation in the
ultraviolet region than similar discharges through any other
known vapor or gas. The earliest and best known source of
ultraviolet is the quartz mercury arc lamp which has an envelope
of fused quartz and at least one electrode of mercury. The
output of ultraviolet can be shifted by varying the temperature
and pressure of the mercury vapor. Increasing the temperature
and pressure produces more of the longer wave-lengths and the
high pressure mercury lamp becomes an efficient source of
visible light. If the pressure is kept low, that is, if it can
be measured by a few' millimeters of mercury column, the
radiation produced by an electric current is very largelv in the
bactericidal region with maximum output at 2537 angstrom units.
These low
temperature, low pressure conditions are inherent in the
Sterilamp, fig. 3, and make it an efficient bactericidal agent,
which is inexpensive to operate, has long life with small change
in output does not heat up the surrounding air, and gives wide
distribution of radiation. The glass tube which forms the lamp
is made of glass which cuts off the radiation slightly below
2000 angstroms thus limiting the production of ozone so
that the concentration does not rise to more than one part in
three or four million parts of air. It is also true that
radiation of 2537 angstroms breaks up ozone and thus acts as an
automatic regulator of ozone concentrations. The slight amount
of ozone developed is helpful in controlling the development of
mold and bacteria in spaces which are shaded from direct
radiation.
Fig.
3. Sterilamps
The electrical
discharge in a Sterilamp is between unheated electrodes, because
this permits simple construction of the lamp and the current
controlling transformer, permits control of the output of the
lamp by changing the primary voltage of the transformer, makes a
lamp which will operate satisfactorily under varying temperature
conditions, and will have a long effective life. Deterioration
of output comes gradually because of changes in the glass known
as solarization and to a slight extent to blackening of the tube
by vaporization of the electrode material.
MEASUREMENT
OF ULTRAVIOLET
Much of
the recent advance in the application of ultraviolet radiation
is due to improvements in the equipment and methods for
measuring the different groups of wave-lengths in the
ultraviolet spectrum. Three photo-cells developed by Dr. H. C.
Rentschler, head of Westinghouse Research Department, cover the
principal groups of wave-lengths in the ultraviolet spectrum.
For measurement of the almost complete ultraviolet region there
is a photo-cell having a thorium cathode which is responsive to
a spectral range from 3750 angstroms to about 200 angstroms,
with a maximum response at 2600 angstroms. Another photo-cell,
made up with a tantalum cathode, responds to wave-lengths from
2950 to 2000 angstroms with maximum sensitivity at about 2400
angstroms. This photo-cell is useful in measuring the
bactericidal region of ultraviolet as it is not sensitive to the
wave-lengths near the visible edge of the ultraviolet spectrum.
A third photo-cell, with a platinum coated cathode, is not
affected by radiations above 2000 angstroms and, therefore, is
capable of measuring the wave-lengths which produce ozone in the
air.
The ultraviolet
radiations in the regions affecting these photo-cells cause a
minute electric current to flow through the tube and charge a
condenser which discharges through a relay tube which in turn
causes a larger condenser to discharge through the coil of a
relay which operates a counting mechanism with an audible click.
After discharge, the condensers again -charge up and the action
is repeated as long as ultraviolet light reaches the electrodes
and current flows through the photo-cell. As each click
represents a definite amount of ultraviolet radiation for which
there is no generally accepted unit, the meter is called a click
meter (Fig. 4). Each click may represent any desired quantity of
radiation by varying the size of the condenser in the circuit of
the photo-tube. However, the standard meter is calibrated so
that one click represents 220 micro-watt seconds.
Fig.
4. The new Westinghouse portable a.c. ultraviolet meter.
Shown at the left is the photo-tube in its separate housing,
which fits into the front compartment of the carrying case when
not in use.
Another
small indicating meter is an adaptation of the foot candle
meter. A filter which cuts out about 95 percent of the visible
light but permits passage of ultraviolet is placed over the
window of the meter, and a fluorescent screen is placed next to
the filter. Ultraviolet light which strikes the screen is
converted into visible light and acts on the light sensitive
cell which operates the pointer. This meter is very convenient
for testing ultraviolet sources, such as the Sterilamp, which
give off little heat but it is not reliable for hot sources, as
the cell is sensitive to heat as well as light, and the meter
must be placed very close to or in contact with the lamp to get
a reading.
APPLICATIONS
OF STERILAMPS
The
development of an ultraviolet generator which operates at
temperatures only a few degrees above the surrounding air
requires but a small amount of power, has a fairly uniform
output and long life, opens up almost unlimited possibilities
for usefulness. Early applications of Sterilamps were for the
preservation of meat held in refrigerators. The radiations are
effective in preventing or greatly reducing the development of
mold and slime during the storage and marketing periods.
Installations of Sterilamps in retail meat stores reduce
trimming losses and, in case of failure of the refrigeration
equipment, insure against meat spoilage until repairs can be
made. Displays in refrigerated cases are protected from
contamination by air-borne organisms, and the sales appeal is
strengthened. The meat packer and wholesaler use Sterilamps to
keep their products free of organisms which cause spoilage and
thus supply the retail trade with meats having low bacterial
contamination and, consequently, of superior keeping qualities
and flavor (Fig. 5). Protection of meat in this way has made
economically possible the tenderizing of beef in a few days at
temperatures of 600F or more. Thus, only about one-tenth of the
time is required by this new and patented process compared to
the former methods of aging at about 34F.
Fig. 5. Beef
may be made uniform render and palatable in three days in this
room. Weekly capacity of meat tenderizing in this room by the
Westinghouse Tenderay Process is approximately 144,000 pounds.
The ultraviolet lamps on the ceiling bathe the meat and air in
the room with invisible rays deadly to mold and bacteria, hence
prevent spoilage. A relative humidity kept at 90 percent
prevents deydration, which causes shrinkage and loss of
juiciness in meat. A temperature of 60 F speeds up the natural
chemical reactions which break down tough tissues and bring
aboout tenderizing. Those elements, togeather with a
scientifically predetermined technique in handling, make up the
tenderay Process.
In hospital
operating rooms, Sterilamps are mounted over the operating table
(Fig. 6) to reduce the danger of infection of surgical wounds'
by organisms floating in the air or exhaled by the operating
staff. The records in more than 4000 operations in one hospital
show that in 1782 operations performed without radiation there
resulted 207 infections and six deaths, while 2463 operations of
the same types resulted in only six infections and no deaths.
Time required for healing was also reduced by more than half.
Fig. 6.
Sterilamps mounted above the operating table in this
operating room in the Duke Hospital, Durham, North Carolina,
reduce bacterial count in the air and provide a virtually
sterile environment on and around the operating site.
In wards
and nurseries the use of ultraviolet radiation has reduced
spread of infectious diseases from person to person. Evidence
has also been accumulated to show that selected radiation in
schools and offices has reduced or eliminated the spread of
colds and other diseases which have a tendency to become
epidemic.
Sterilamps have
been applied in dairy barns, poultry houses, breweries,
bottlers, bakeries, pharmaceutical houses, air conditioning
systems, and many other places. A new industry has thus been
developed that has as its primary purpose the protection of
products and people from the unseen dangers ever present in the
air.
|
