8 CIRCULAR 7 6 5. U. S. DEPARTMENT OF AGRICULTURE 



the nonozonized one. After four 1-hour exposures in 4 days spore 

 germination in the ozonized room was 1.6 percent as contrasted with 

 98.4 percent in the nonozonized one. Generally the exposures were 

 stopped after the fourth day because sufficient desiccation of the film 

 had occurred to cause a reduction in germination. The slides were 

 examined daily for spore germination: when growth on the non- 

 ozonized slides had advanced so far that counts could no longer be 

 made, the slides were discarded. However, in some instances spores 

 on the ozonized slides that had appeared dead up to this time germi- 

 nated later. It is therefore concluded that ozone has a definite retard- 

 ing effect upon germination as well as a lethal effect. As would be 

 expected, scattered single spores were more sensitive to ozone than 

 were heavy clumps of them. 



Similar results were obtained with PerdciUium expansum. After 

 four 1-hour exposures in 4 days, none of the ozonized spores and IS 

 percent of the nonozonized spores germinated. 



Natural Spore Load of Storage-Room Air 



After 4 months of storage a survey was made of the natural spore 

 loads of the air in the ozonized ( 3.25 p. p. m. ) and the nonozonized 

 room. To assure complete spore circulation, in addition to the regular 

 storage-room fan. a 20-inch propeller-type fan that delivered 3.400 

 cubic feet per minute was operated 1 hour before and during the 

 exposure period. Petri dishes containing nutrient agar were exposed 

 for 30 minutes. After several days at room temperature the colonies 

 were counted. There was a great reduction in the number of viable 

 spores in the ozonized room | rig. 3 ) : averages of 785 fungus colonies 

 per plate in the nonozonized room and of only 6 colonies per plate 

 in the ozonized one were obtained. Most of the fungus colonies were 

 A nicUlium. 



Surveys were also made in commercial cold-storage plants in which 

 ozone was being used. In one plant sterile petri dishes were exposed 

 for 30 minutes at various locations in egg-storage rooms in which the 

 ozone concentration averaged about 1.8 p. p. m. In these rooms the 

 air was circulated by blowers and the relative humidity at the time 

 of exposure was SO percent. This concentration of ozone was rel- 

 atively effective in reducing the number of air-borne micro-organisms. 

 Averages of 5.2 fungus colonies per plate exposed in the ozonized room 

 and of 28.4 colonies per plate in the nonozonized room were found 

 (table -2). 



In another plant in which ozone was being used in a number of 

 apple-storage rooms, its concentration was held at 0.7 to 1.0 p. p. m. 

 Forced-air circulation was not employed in this plant, and the relative 

 humidity was 85 percent. For experimental purposes ozone was gen- 

 erated for a single 24-hour period in one apple-storage room which 

 had not been previously treated with ozone. The concentration of 

 ozone obtained was comparatively low. reaching only 0.37 p. p. m. 

 Exposure of plates containing sterile nutrient agar for 30 minutes 

 revealed that there was little or no control of micro-organisms under 

 commercial com lit ions from such low concentrations of ozone (table 3) . 



