THE MECHANISM OF AERIAL DISINFECTION BY GLYCOLS AND OTHER CHEMICAL AGENTS : I. DEMONSTRATION THAT THE GERMICIDAL ACTION OCCURS THROUGH THE AGENCY OF THE VAPOR PHASE

Theoretical analysis of the mechanism of action of chemical aerial disinfectants reveals that the rapid killing action which is obtained cannot be accounted for by a collision process between germicidal aerosol particles and the air-borne bacteria. However, a mechanism involving condensation of germicide molecules in the vapor state on to the bacteria-containing droplets results in a theoretical velocity of the correct order of magnitude. Experimental tests of this theory show that pure germicide vapors free of aerosol droplets are almost instantly lethal to air-borne bacteria. Conversely, pure germicidal aerosols in the absence of vapor, had no effect on air-borne bacteria within 20 minutes or more. Therefore, it may be concluded on both theoretical and experimental grounds that rapid air sterilization requires the existence of the germicide in the vapor state.

THE MECHANISM OF AERIAL DISINFECTION BY GLYCOLS AND OTHER CHEMICAL AGENTS : II. AN ANALYSIS OF THE FACTORS GOVERNING THE EFFICIENCY OF CHEMICAL DISINFECTION OF THE AIR

The effectiveness of any compound as an aerial germicide depends upon the extent of condensation of its vapor on air-suspended bacteria, and on the rate at which the resulting concentration of germicide can produce death of the microorganisms. The properties of any compound conducive to production of the highest rate of kill of air-borne microorganisms by means of the smallest possible concentration of germicide vapor, are as folows: (a) a low vapor pressure, but not lower than 0.001 mm. Hg at 25°C.; (b) high hygroscopicity; (c) toxicity for bacterial metabolism—a high degree of potency is not necessary although the killing action will be more efficient the higher the antibacterial activity of the compound employed. For any compound the killing action is always a direct function of the concentration of its vapor in the air. The maximum amount of a hygroscopic substance which can exist in the vapor state decreases as the relative humidity increases. Hence, at high relative humidities the bactericidal efficiency is lowest. At lower relative humidities the air can contain more vapor, and hence a greater effect is possible. At any relative humidity, the killing action is greater, the more closely the germicide vapor concentration approaches the saturation point. The presence of soluble compounds in droplets containing bacteria promotes more extensive condensation of the germicide than would otherwise occur, and so enhances its effectiveness. In the absence of such soluble substances, low atmospheric humidities may cause complete desiccation of a bacterial particle. Under these conditions its surface may become resistant to the condensation of the vapor, and thus prevent effective germicidal action. The influence of temperature changes on the killing efficiency maybe correctly deduced from a consideration of the effect of a rise in temperature on the vapor pressure of the germicide and on the rate of its bactericidal action in vitro. Equations are presented for estimating quantitatively the magnitude of some of the effects discussed.

Studies in the dynamics of disinfection; the effect of lethal temperatures on standard cultures of Bact. coli; the nature of the probit-log survival-time relationship at pH 7.0

1. A study has been made of the probit-log survival-time relationship in a number of experiments in which whole standard cultures of Bact. coli were subjected to heat at temperatures ranging from 47 to 55° C.

2. It is concluded that the whole mortality up to 99·99 % could not be covered by a single straightline relationship since the graphs are curves concave upwards and to the left.

3. Between 95 and 99·99 % mortality the relationship approximated closely to linearity.

4. Several individual experiments showed ‘humps’ in their curves. These, although always occurring at the same stage in time in the disinfections, varied in their probit levels.

5. By combining the data for all experiments, an idealized probit-log survival-time curve was obtained in which the changes of slope were less marked than in the curve representing the disinfection of similar cultures by phenol.

The authors wish to express their gratitude to Messrs I.C.I. (Pharmaceuticals) Ltd. for their valuable financial assistance, which has enabled one of us (H. E. F. D.) to devote his full time to this work.

Studies in the dynamics of disinfection; the effect of lethal temperatures on standard cultures of Bact. coli; a detailed analysis of the variations of death-rate with time

1. Whole cultures of Bact. coli grown under carefully controlled conditions have been subjected to the action of heat at temperatures ranging from 47 to 55° C. and the survivor curves determined.

2. The occurrence of excessive variation between replicate plates of counts made when the mortality exceeds 95 % was very much less evident than when phenol was the lethal agent.

3. At temperatures of 35° C. and below, disinfection of the cultures was never complete, as a permanent population of cells became established. In some cases the numbers of heat-resistant cells reached nearly 0·01 % of the original population, but great fluctuations were observed.

4. The death-rate during the active part of the disinfection was not constant but, in general, increased with time. In the faster disinfections this increase was difficult to detect and the logarithmic death-rate appeared to be virtually constant, but the increase is nevertheless believed to have been present in all cases.

5. The data for all the experiments have been combined to give a composite disinfection curve from which it was concluded that the death-rate was low at first but rose to a maximum at which it remained constant until the mortality had reached at least 99·99%.

6. This finding led to the decision to treat all the experiments as if the regression of log survivors on time had been linear between mortalities of 90 and 99·99%. The standard errors of the calculated regression coefficients were small, so that this method of treating the experimental data appears to be justified.

The authors wish to express their gratitude to Messrs I.C.I. (Pharmaceuticals) Ltd. for their valuable financial assistance, which has enabled one of us (H. E. F. D.) to devote his full time to this work.

Studies in the dynamics of disinfection; the variation of the concentration exponents for hydrogen and hydroxyl ions with mortality level, using standard cultures of Bact. coli at 51 degrees C

1. Data obtained by the disinfection at 51° C. of standard cultures of Bact. coli at pH values ranging from 2·8 to 8·8 have been used for the calculation of the concentration exponents of hydrogen and hydroxyl ions. The times taken to reach various degrees of mortality from 50 to 99·99 % have been taken as measures of the activities of the ions.

2. The formula Cn×t = constant proved satisfactory for describing the data for all the mortality levels and at all the pH values tested except pH 2·8, 6·25, 6·4 and 7·0. In these instances the cultures exhibited heightened sensitivity, though the results at all, except pH 2·8, could be included when the 99·.99 % mortality times were used. The exponents increased sharply at pH values below 3·9.

3. The values of the concentration exponents varied similarly under both acid and alkaline conditions, according to the mortality level chosen. The largest value was obtained from the 90 % mortality times, there being a decrease when both higher and lower levels were employed. The changes were greater under acid conditions, when the values ranged from 0·60 to 0·25, compared with 0·60–0·47 for the alkaline reactions.

4. The mean of the pH values for maximal survival at the various mortality levels, derived from the regressions of log mortality time on pH, was ·51, but this was close to a point of greater susceptibility. Experimentally, maximal survival occurred at pH 6·65, which is very near to the neutral point of water at 51° C.

5. It is concluded that within the pH range 6·25–7.0 the hydrogen and hydroxyl ions were relatively inactive, the effects observed being due to other factors possibly including the action of undissociated water molecules.

The authors acknowledge with thanks the receipt of a grant from Messrs I.C.I. (Pharmaceuticals) Ltd. towards the expenses of this investigation.

A system of air recirculation and antibacterial surface treatment in a surgical ward

A system of surface treatment with an antibacterial agent and air recirculation through treated filters was set up in a 15-bedded surgical ward, using an adjacent 10-bedded ward as a control.Nasal, wound, and urinary cross-infection was studied in both wards for over one year.Total bacterial counts and counts of Staphylococcus pyogenes were made from settle plates and blanket sweep plates. After the installation of the system aerial contamination in the test ward was reduced appreciably more than in the control ward. The number of blankets contaminated with Staph. pyogenes was similar in both wards. There was no significant difference in wound, nasal, or urinary cross-infection between the two wards.

The epidemiology of the common cold. III. The effect of ventilation, air disinfection and room size

Mechanical ventilation at 7 air changes/hr., air disinfection using vaporized hexyl resorcinol or α-hydroxy-α-methyl butyric acid, or irradiation of the air of the room above 8 ft. with ultraviolet radiation when applied to various clerical offices did not result in any observed alteration in the numbers of colds experienced by those working in them.

There were no differences in the numbers of colds experienced by those working in rooms of different sizes. The working population in these rooms varied from, about 20 to more than 80.

We should like to express our thanks to the Ministry of Pensions and National Insurance and to the Shell Petroleum Company, to those members of their staffs and to the nurses and others who helped us in this investigation. We should also like to thank the Shell Petroleum Company for the supply of α-hydroxy-α-methyl butyric acid, and the British Thomson Houston Company of Rugby for the loan of the ultraviolet lamp fittings.

Disinfection of hospital blankets with synthetic phenolic compounds

A cheap method by which hospital blankets may be effectively disinfected (approximately 3d. per blanket) is described. A recommendation is made that blankets from the patients' beds be divided into: ;socially dirty' blankets to be laundered, possibly at infrequent intervals; and ;socially clean' blankets to be disinfected frequently. The wide range of a synthetic phenolic compound is described. This substance is effective against all the common pathogenic bacteria in the presence of organic matter, anionic, or cationic detergents. Details are given of laboratory trials with this method of disinfection and of pilot trials at the Group hospital laundry. The recommended method is simpler and takes less time than ordinary washing.

Disinfection of woollen blankets in steam at subatmospheric pressure

Blankets may be disinfected in steam at subatmospheric pressures by temperatures below boiling point inside a suitably adapted autoclave chamber. The chamber and its contents are thoroughly evacuated of air so as to allow rapid heat penetration, and steam is admitted to a pressure of 10 in. Hg below atmospheric pressure, which corresponds to a temperature of 89 degrees C. Woollen blankets treated 50 times by this process were undamaged. Vegetative organisms were destroyed but not spores. The method is suitable for large-scale disinfection of blankets and for disinfecting various other articles which would be damaged at higher temperatures.

Chemical disinfection in the prevention of infection in hospital

Much cross-infection can be prevented by using chemical disinfectants to reduce the numbers of pathogens in sources such as superficial lesions and carrier sites, and by interrupting routes of spread. Examples are given to show the value of using disinfectants in the light of epidemiological knowledge.

Studies on disinfection of clinical thermometers. II. Oral thermometers from a tuberculosis sanatorium

A study was made of the chemical disinfection of oral thermometers used by patients with active pulmonary tuberculosis. Only 10% of these thermometers were found to be contaminated with acid-fast bacilli. Because of this small number, the use of a phenol coefficient type test with Mycobacterium tuberculosis was suggested as an alternative method for evaluating tuberculocidal activity. These data, in conjunction with data from Part I of these studies dealing with disinfection of oral thermometers from a general hospital, were the criteria used to judge the efficacy of the disinfectants. It was concluded that 70% ethyl alcohol, 2% phenolic no. 1, and 3% phenolic no. 3 were reliable disinfectants for thermometers. Fifty per cent ethyl alcohol, 2% phenolic no. 2, 2% iodophor (300 ppm available iodine), and 0.1% benzalkonium chloride, aqueous or tincture, were unreliable.