STERILIZATION OF AIR WITH RADIANT ENERGY 167 



If air 1 meter thick at 760 mm pressure shows a negligible absorption 



o 



for wavelengths greater than 2050 A, why is it that we find a rather sharp 



o 



cut-off in transmission of sunlight at 2950 A, so that no rays lethal to 



o 



bacteria (2500 to 2800 A) reach the earth? This opacity of the atmos- 

 phere is attributable to the presence of ozone at high altitudes. Ozone 



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shows a pronounced absorption band extending from 2200 to 2900 A. 



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Below 2200 A ozone is transparent again, but the dense oxygen in the 

 lower atmosphere is relatively opaque to wavelengths 2200 to 2000 so 

 that the opacity increases rapidly from 1850 to shorter wavelengths. 



It, therefore, follows that midsummer sunlight reaches the earth 

 without containing an appreciable amount of ultraviolet energy destruc- 

 tive to bacteria. Apparently, sunlight as a means of sterilizing the 

 lower strata of air is of questionable value. For this task, therefore, one 

 must resort to other ultraviolet sources which possess pronounced 



o 



energy radiations near 2650 A. 



Sterilization of Air with Ultraviolet Radiant Energy 



The sterilization of the air in an operating room has been successfully 

 accomplished with the aid of a Westinghouse " Sterilamp " by Hart and 

 Gardner [1937]. Their work proved that the presence of staphylococci 

 in the air in the operating room was a source of wound contamination 

 rather than contamination from the skin of the patient or personnel, or 

 by other contacts. 



They showed that the transportation of pathogenic bacteria through 

 the air to the wound can be eliminated to a great extent by laying down a 

 barrage of ultraviolet radiant energy around the operative incision and 

 exposed sterile supplies. 



Barriers of ultraviolet rays have been shown to be effective in prevent- 

 ing the spread of infection in an isolation ward. Wells' work [1940] 

 shows that an organism in air is about 20 times more vulnerable to the 



o 



ultraviolet range 2000 to 3000 A than when suspended in water. Vul- 

 nerability values were found to be reduced tenfold paralleling an increase 

 in relative humidity from 46 to 91 per cent. For this effect no acceptable 

 explanation has been proposed. 



The available information indicates that bacteria are killed by about 

 the same amount of ionizing energy no matter what its wavelength. 

 The cell damage caused by ionizing radiations must be attributed 

 directly to the liberated ions and to the chemical changes that these ions 

 induce. The nature of these changes has been the subject of much 

 experimentation. Tentatively, we may state that the number of ions 

 produced by lethal doses of radiation is small compared with the enor- 

 mous number of atoms in the tissue that is radiated. Ionic recombina- 



