268 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ACCEPTABILITY TO TISSUES The incidence of skin sensitization is extremely low and only occasional cases have been encountered even though there has been widespread usage throughout the world for many years. There is irritation to the conjunctiva with solution strengths exceeding 0.1•o (0.05•o being the eye irrigation strength and 0.01•o the preservative strength) and the aqueous concentra- tion normally recommended for contact with mucous surfaces is 0.05•o. At this concentration there is no irritant effect on soft tissues nor is healing delayed, according to Shepherd and Kinmonth (16). The gluconate salt is frequently used at 1.0•o strength in creams, lotions and disinfectant solu- tions. Strengths of from 0.1 •o to 0.5•o of the hydrochloride salt may be used for prophylaxis in the lower regions of the nostrils, in which circumstances the gluconate creeps upwards to cause mild discomfort at the 0.5•o strength. Herein lies the particular -virtue of the hydrochloride salt. Its limited water- solubility enables it to be incorporated at strengths at which the soluble salts are ruled out on irritancy grounds. As it is less readily leached away there is more persistent action. It is dangerous to use chlorhexidine on the brain or meninges. Apart from such obvious limitations there is a wealth of medical literature which demonstrates its absolute safety at the active concentrations (17-24). The low toxicity and freedom from sensitization have been two important fac- tors accounting for its wide application and acceptance in medicine. ANTIBACTERIAL ACTION Chlorhexidine is both inhibitory and lethal to vegetative Gram-positive and Gram-negative bacteria in vitro and at relatively high dilutions accord- ing to Davies et al (2) Heat resistant spores are not susceptible to its lethal action at ambient temperature even in strong concentration. At 98-100øC, however, it destroys mesophilic spores, hence its use (0.01•o) in the steriliza- tion of certain eye drops. Laurence (25) found pathogenic fungi to be as susceptible to its inhibitory and lethal action as vegetative bacteria but Hall (10) has shown that susceptibility is in the following descending order: vegetative bacteria, fungi and yeasts, fungal spores, bacterial spores. Animal viruses are not susceptible according to Hurst (26). Its possible mode of action, referred to previously, of exerting a lethal action by combining with the cell surface, disorganizing permeability barriers and coagulating the cytoplasmic contents, was thought by Salton (14) to resemble that of

FORMULATION AND PROPERTIES OF CHLORHEXIDINE 269 the quaternary ammonium salts such as cetrimide. Activity is reduced in the presence of organic matter such as blood, serum and milk in vitro though considerable activity still remains even at dilutions down to 0.02 %. That there is adequate residual activity in the presence of blood or protein was demonstrated by Calman and Murray (17) who chose chlorhexidine as the best of nine alternative antibacterial agents for use in obstetrics, basing their recommendation on both long and short contact time tests. Activity of aqueous preparations increases over the range pH 5-8 but normally it is applied at pH 6-7 because of the risk of precipitation of base under alkaline conditions. In vive activity against both Gram-positive and Gram-negative organisms was first demonstrated in mice by Martin (27) and later by Jeffries and Price (28). These and many successive workers have given ample assurance that the high dilutions lethal to common pathogens are well beyond those likely to cause irritancy. UTILIZATION IN MEDICINE AND SURGERY It has often been said that Lister's antiseptic surgery was succeeded by aseptic surgery, but in reality this is untrue in modern times because the surgeon now relies on both methods of preventing infection. In the prepara- tions which precede surgery, heat sterilization, wet or dry, is augmented in many ways by the judicious use of antibacterial agents. Thus, formaldehyde gas may be released for gaseous sterilization of the operating theatre, while a variety of cleansing germicidal solutions such as halogenated phenols, quaternaries, formaldehyde solution, alcohols and hypochlorites are used for walls, trolleys, bowls etc. In this connection the use of a chlorhexidine- cetrimide combination (Savlon hospital concentrate) has become wide- spread. Another formulation which has diverse applications is a solution of chlorhexidine gluconate with nonionic detergent, colour and characterizing antiseptic type perfume (Hibitane 5% concentrate). Diluted to 0.5 % in 70% alcohol for the pre-operative and local disinfection of the patient's skin, it is equivalent to 1% iodine in the same strength of spirit yet it does not share the disadvantage of occasionally causing skin reactions. There have been many publications in support, for instance Lowbury, Lilly and Bull (29) tested a number of antiseptics for their ability to reduce transient skin flora and they concluded that these two solutions had approximately the same effect and were significantly more potent than other antiseptics tested, yet the prolonged usage of chlorhexidine did not cause sensitivity reactions,