CHAPTER VII

DISINFECTION BY HEAT

IT is hardly necessary to say that burning of infectious material, infected clothing, etc., is an effectual method of disposing of it. This method of disinfection is always to be recommended, when practical and consistent with a due regard for economy and the rights of individuals. As a rule, articles of little value, which have been soiled with infectious material, had better be burned; and this is especially true of old clothing and bedding. But we have other efficient methods of disinfection, which make it unnecessary to sacrifice articles of value except under unusual circumstances.

While all disease germs are readily killed by exposure for a short time to the temperature of boiling water, many of the most important pathogenic bacteria are quickly destroyed by a much lower temperature than this—that is, when exposed in a liquid or in a moist condition. When in a desiccated condition,

or exposed to the action of hot, dry air, a much higher temperature is required. This fact must constantly be kept in view in carrying out practical measures of disinfection, and for this reason the disinfection of clothing, blankets, etc., by dry heat is rarely employed. At quarantine stations and municipal disinfecting stations disinfection by steam is relied upon to a great extent, and has been proved by experience to be superior to all other methods. The disinfection of bandages, instruments, and dressings of all kinds for the "aseptic" treatment of surgical wounds is also accomplished by exposure to moist heat (steam or boiling water).

In considering the value of heat as a disinfectant, we must take account of the very great difference in the resisting power of growing bacteria and of the reproductive elements formed by some of them, which are known as "spores."

The spores of certain bacteria found in surface water and in the soil may resist the temperature of boiling water or of live steam for several hours, but fortunately the spores of known disease germs have far less resisting power. In experiments made nearly twenty years ago, I found that the spores of the anthrax bacillus did not grow after exposure to the temperature of boiling water for four minutes.

As already stated, bacteria which do not form

spores are quickly killed by a temperature considerably below that of boiling water. The exact thermal death-point of a considerable number of the most important disease germs was determined by the writer in a series of experiments made in 1885. The cholera germ and the micrococcus of pneumonia were the least resistant of all those tested, and were destroyed by ten minutes' exposure to a temperature of 130° Fahr. The typhoid bacillus was killed in the same time by a temperature of 140°. In general the statement may be made as a result of my own experiments and those of other investigators, that pathogenic bacteria which do not form spores are killed by ten minutes' exposure to a temperature of 140° Fahr. (moist heat), with the exception of the tubercle bacillus, which requires a somewhat higher temperature (160° Fahr.). The list of known disease germs which are killed by ten minutes' exposure to a temperature of 140° Fahr. (60° C.) includes the bacillus of typhoid fever, of diphtheria, of bubonic plague, of glanders, the micrococcus of pneumonia, of erysipelas and puerperal fever, of boils and abscesses, the spirillum of cholera and of relapsing fever. In addition to these known germs it has been determined that the same temperature destroys the infecting power of vaccine virus, and presumably of smallpox virus, of hydrophobia virus, and of certain

other kinds of infectious material in which the specific germ has not yet been demonstrated.

While dry hot air is, as a rule, unreliable for the destruction of disease germs, certain bacteria are quickly destroyed by desiccation. This is true of the cholera spirillum and of the micrococcus of pneumonia. On the other hand, the bacillus of typhoid fever, the bacillus of diphtheria, the bacillus of tuberculosis and the bacillus of bubonic plague may retain their vitality for weeks, or even months, when in a desiccated condition. This is true also of the virus of smallpox and of scarlet fever.

Low temperatures do not destroy bacteria. They have been exposed to a temperature of - 87° C., obtained by the evaporation of liquid carbonic acid, but when again brought under favourable conditions showed no diminution in their capacity for development. Repeated freezing and thawing has, however, a deleterious action. The typhoid bacillus may be killed in cultures which are frozen and thawed out at intervals of three days, by repeating the operation five or six times.

The facts stated in this chapter make it evident that heat constitutes the most generally useful agent for the destruction of infectious material. Any article of food or drink which has been recently brought to a temperature approaching that of the boiling

point is surely free from living disease germs dangerous to man.

All articles of clothing which have been subjected to the ordinary operations of the laundry are safely disinfected.

Vessels containing the infectious discharges of persons suffering from cholera, typhoid fever, etc., if thoroughly treated with boiling water may be disinfected, together with their contents. To make sure of this the quantity of boiling water used should be three or four times greater than the contents of the vessel, and from ten to twenty minutes should be given for the disinfecting action of the hot water.

Articles of bedding and clothing which would be injured by immersion in boiling water may be disinfected by exposure to steam in a properly constructed disinfecting chamber or "steriliser."

Clothing may also be disinfected by dry heat if freely exposed in a closed chamber to the action of hot dry air, at a temperature of 125° C. for two hours.