THE CHOICE OF PERFUMES FOR PRESSURIZED PRODUCTS 393 potential ingredients on a variety of odorous materials under controlled conditions TM. Unfortunately, the spaces into which air fresheners are sprayed are not laboratories into which carefully dosed amounts of specific malodours have been released. The latter are complex and infinitely variable in chemical nature, and it is impossible in practice to adjust the ratio of material sprayed. Thus, whilst the theoretical background and fundamental research are taken very much into account, perfume compounds recom- mended are to a certain extent empirical in nature and result from practical testing under conditions of use. In cases where a non-perfumery chemical such as lauryl methacrylate is included in the formulation specifically to react with odorous materials in the air, it is important not only that the perfume does not react with it in the dispenser but that it does not compete for it in the atmosphere after spraying. This calls for careful co-operation between perfumery supplier and aerosol producer. Air fresheners are by no means used only where a definite malodour is to be removed, but very often to give a pleasing odour and restore agreeable conditions in rooms where there is a tendency to staleness. Thus, odour preferences cover a very wide range, with the emphasis usually on freshness. It has been stated that certain fragrances are able to exert a refreshing action by virtue of their ability to counteract the vasoconstrictive effects of stale air on the bodf •. Freshness is interpreted variously in the form of lemon, pine, lavender, cologne, etc., but this is being modified as the demand for greater suavity makes itself felt. Light bouquets with a green- ness reminiscent of the countryside are examples of progressive requirements. An increasingly important branch of aerosol air fresheners is that aimed at reducing the microbiological population of the atmosphere. The need for compatibility of perfumes with the active ingredients is self evident, and factors arising from the tendency to introduce a higher water content with efficiency in mind justifiably deserve assessment. INSECTICIDES •lasking ability for the solvents and active ingredients, possible reaction with the latter and solubility, are points which deserve special review with respect to perfumes for insecticides. Since the solvents utilised are pre- dominantly hydrocarbon in nature, solubility of perfumes is often quite unsatisfactory, and it is necessary to formulate them specifically •vith this aspect in mind. Furthermore, many active insecticidal ingredients have a very marked unpleasant odour, which is augmented by that of certain solvents. These must be masked efficiently and economically to give as pleasing and appropriate a reodorisation as possible •6.

394 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS A suspicion that perfume compounds could adversely affect the in.- secticidal power of pyrethrum based insecticides appeared in the form of a public statement in 1959". A survey made at that time concluded that there appeared "to be little or no documented foundation for any theory that perfumes destroy pyrethrins". Subsequently, a careful investigation reported by Pickthall et al •8 involving typical selections from the various chemical groups of perfumery materials, was unable to detect any significant reduction in pyrethrin potency attributable to them. Furthermore, certain materials, for example acetophenone and diphenyl methane, commonly used in perfumery have been shown to possess synergistic activity with respect to pyrethrins TM. Our own experience confirms the view that perfume compounds do not represent a hazard in so far as reduction of insecticidal effect is concerned. A specific requirement of perfumes for mothicides, which are sprayed directly on to fabrics, is that they should not promote staining. PERFUMES, COLOGNES AND TOILET WATERS These consist essentially of a solution of the perfume compound in alcohol with or without a certain proportion of water, pressurized with either dichlorodifluoromethane or dichlorotetrafluoroethane, or a mixture of both. The possibilities of the complex reactions involving propellant, alcohol and water described earlier, taking place are thus greatly reduced. The containers are normally either plastic-coated glass, internally pro- tected aluminium, or stainless steel. The presence of water reduces the hazard of reaction between alcohol and exposed aluminium. DiGiacomo 2ø has described the possible need for special cleaning in the case of stainless steel. Perfume solutions should be chilled and filtered prior to pressurization by the methods utilised for traditional products. This procedure, allied to the fact that alcohol is an excellent cosolvent in the system, reduces very significantly the limitations arising from inadequate solubility after pressur- ization. With pressurized perfume presentations of this type, the principal effect comes from the alcoholic solution deposited on the wearer, so that provided there has been no undesirable change within the pack, it is the same as that from the corresponding unpressurized product. There is, however, some impression received from the "mist" when the product is being sprayed, especially with the drier sprays, so that the points mentioned earlier regarding impact on the olfactory receptors should by no means be overlooked. In this field, where olfactory quality is rightly expected to be of the highest, the presence of slight unanticipated impurities in the per- fumery ingredients and alcohol used can be seriously detrimental, as their effect may be magnified disproportionately.