The disinfection of closed atmospheres with germicidal aerosols

Abstract

A form of ultramicroscope apparatus is described which enables the evaporation of mist droplets to be followed for long periods. A rough method of estimating the size distribution in aerosols is also given.The rates of evaporation of particles of various phenol solutions have been studied, the solutions chosen being of possible interest as aerial germicides. It is shown that the typical evaporation curve for a binary mixture consists of three parts, which in actual cases need not all be present. Sometimes the more detailed history of an evaporation can be deduced from the results.The approximate effective size distributions in mists of hexyl-resorcinol—propylene-glycol solutions have been measured and compared with the evaporation rates.It is shown that the efficiency of aerial germicides is partly a function of their volatility, and three rough classes are distinguished. But biological evidence is adduced which suggests that the solubility of the germicides in water may be of equal importance, and that the liquid in which the germicide is dissolved is not without effect.A centrifuge is described which enables the maximum particle size in a mist to be limited to a desired value.Methods of estimating quantitatively germicidal aerosols are discussed, and a method described which has been found to give reasonably accurate results in practice. Tentative experiments have been made to determine whether the materials studied are likely to cause corrosion of metals, etc., after pro-longed use.The collision theory of aerial disinfection is discussed, as is also the applicability of Langmuir's treatment of the evaporation of fine particles in mixtures.The development of the experimental technique involved in the biological investigations appertaining to air disinfection has been discussed, together with the numerous fallacies arising and the means by which these have been overcome.The construction of a special pair of experimental chambers is described, and details given, both as regards the bacteria and germicides, of the exact experimental conditions under which the most consistent results can be obtained.The requisite characteristics of germicidal mixtures which could be utilized for air disinfection are considered, and examples of suitable germicides and solvents are given. The method of recording and evaluating the results of a varied assortment of experiments is explained to facilitate inter se comparison.Examples are given of the lethal effectiveness in the air of several germicides in broth emulsions, both with and without the presence of serum, on stock cultures of selected types of bacteria. The sensitivity to the germicides of the bacteria making up the flora of the average normal saliva is compared with that of broth emulsions of bacteria.10% hexyl-resorcinol dissolved in propylene-glycol+0·05 % sulphonated lorol (“S²”) was the most effective all round germicidal mixture of those tested. Various other germicides were found equal in germicidal activity in the air, but for reasons stated have been deemed unsuited for use in the presence of man.The relationship between germicidal efficiency in the air and mist particle size and persistence has been studied by means of centrifuged mists. Particles of 0·5–1·0μ radius are shown to be the most useful in dealing with bacterial particles containing organic matter. Laboratory cultures, emulsified in broth, have been sterilized in mist dispersions by droplets of bactericides at least as small as 0·25μ radius.Tests on the penetrating abilities of bactericidal mists and bacteria through certain cloths have been performed.Experiments to demonstrate the surface sterilizing properties of aerosols have been performed, and examples of differences in effect on horizontal and vertical surfaces are noted.The degree of tolerance of man and animals (mice) for a number of phenolic substances and some organic solvents has been compared with the degree of tolerance of bacteria for the same substances.The tests on man have been confined to the detection by the special senses of the substances suspended in the air as a fine mist.The tests on animals were designed to show acute and chronic effects, the results being recorded in terms of clinical symptoms and pathological changes in the organs. A record was also kept of the findings as regards protozoal parasites of the host, and compared with the normal incidence among ordinary laboratory stock.Tests on bacteria were instituted for the purpose of gaining information on the host-parasite tolerance ratio for our germicides when in the air and when in the test-tube.