Bactericidal Radiation


This Article was found in the Journal Of Applied Physics

Volume 13, May, 1942


Bactericidal Radiation


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.


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.


 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.


 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.


 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.



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