A study of the conditions which affect the bactericidal action of propylene glycol vapor on air-suspended microorganisms has been carried out. The killing process was found to be more effective when both the total number of air-borne droplets and the number of organisms in the bacterial suspension are small. A temperature below 80°F. and an atmospheric relative humidity between 45 and 70 per cent were found to constitute the most favorable conditions for the lethal action of the vapor.

Experiments were performed to test the bactericidal efficiency of propylene glycol vapor in both small and large enclosed spaces. These studies revealed that equally marked bactericidal action is obtained when propylene glycol is dispersed in an 800 cubic foot room as occurs in chambers of 2 cubic foot capacity. The susceptibility to vapor action of bacteria re-suspended in saliva was just as great as when broth was used as the suspending medium. Both partially and completely dehydrated bacteria also succumbed to the effects of the vapor. However, when unsterile dust collected from inhabited rooms was dispersed into the air, little reduction of the natural microbic population contained in this material was observed.

Data are presented showing the minimum glycol concentration necessary for effective bactericidal action on various microorganisms. Pneumococci were killed by amounts of propylene glycol as low as 1 gm. in 20 million cc. of air. Concentrations of 1 to 5 million to 1 to 10 million were required to produce the same degree of killing of streptococci and staphylococci.

The observations here reported add further support to the previously proposed conception of the mechanism of the lethal action of propylene glycol vapor, namely, that a bactericidal concentration of the glycol accumulates in the bacterial droplet as a result of contact with and absorption of glycol molecules from the surrounding atmosphere.

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