Sterilization by ethylene oxide.

Although sterilization by steam under pressure and by hot air are procedures which are now well understood and efficient when properly conducted, there is an increasing number of objects in medical or surgical use which are too heat labile to be so treated. It is for such items as plastic catheters, disposable plastic syringes, and prostheses that ethylene oxide has been proposed as a convenient sterilizing agent. This has been used for years for disinfesting spices and for preventing bacterial spoilage in foodstuffs, but it was only after the publications of Phillips and Kaye (1949) that the possibilities of ethylene oxide as a sterilizing agent began to be appreciated. Ethylene oxide is a colourless liquid with a boiling point of 10-7°C. At temperatures above this it is a gas with a characteristic ethereal smell. It forms an explosive mixture when more than 3% of it is present in air and it is moderately toxic; its toxicity may be compared with that of ammonia. It is very soluble in water and also dissolves in rubber and many plastics. Although the gas forms explosive mixtures with air, if it is mixed with carbon dioxide or with freons in proportions of not more than 12% by weight the mixture is not inflammable. Chemically ethylene oxide is a powerful alkylating agent and as such is much used in industry. Its bactericidal effect is probably due to this action: it is lethal to bacteria and to viruses. It is of interest that although vegetative organisms are more easily killed than are spores the difference in resistance is not very great compared with that found in sterilization by hot air or steam. The organism commonly used to test sterilization by ethylene oxide is Bacillus subtilis. var. globigii but Staphylococcus aureus can be almost as resistant. The gas penetrates readily and will pass into the lumen of small tubes, and through fabrics, paper, and many, but not all, plastics. Unlike sterilization by steam or hot air, where it is comparatively easy to lay down a safe time and temperature to guarantee a sterile load, the use of ethylene oxide is more complex because a greater number of variables need to be considered. These were defined by Phillips and Kaye (1949). As would be expected sterilization depends on time and con-