JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS started my training as a dermatologist, this statement could be regarded as a truism, but during the last decade we have gradually become aware of facts that require us at least to restrict, or even to doubt, its validity. I feel obliged to clarify the terms "irritation" and "eczema". To give a good definition of eczema is one of the more difficult things that can be asked of a dermatologist, even more so if he has only a very short time in which to discuss this question. One of the best definitions available is un- doubtedly that of Hebra: "Eczema is everything that looks like an eczema". This definition has only one drawback: it does not make us any wiser. Another difficulty is that the opinions of dermatologists differ on what should be called eczema and dermatitis. For the moment, the best we can do is say that "eczema" denotes a family of skin disorders that may, at the same time or at different times, show redness, little nodules (papules), vesicles or even blisters, and weeping or scaling. Under the microscope we see, as their pathological counterparts in the epidermis, cellular pro- liferation (acanthosis), oedema, spongiosis and vesicle formation and in the dermis- hyperaemia and cellular infiltration. To further delimit my position, I may state that this definition of eczema includes conditions which many dermatologists call dermatitis, dermatitis venenata, eczematous dermatitis, and many other names. This evening I will restrict myself to those types of eczema that are provoked by external causes, the contact eczemas. It is very important to distinguish two subspecies within this species: the eczemas caused by what we call a toxic influence, the orthoergic eczemas, and those caused by an allergic reaction to an external influence, the allergic eczemas. Both kinds of reaction are very important. When you wish to market a new product it should not possess sensitizing properties otherwise it might cause allergic reactions in some of the persons who use it. I will not go into this important subject, except to refer to the difficulties that have been caused by sensi- tization due to p-phenylenediamine hair-dyes and, more recently, by the tetrahalogenated salicylanilide disinfectants in soaps. You are undoubtedly aware of the fact that a typical characteristic of sensitization is that, as a rule, it occurs only in a few of the persons exposed to the material. Other agents may, when applied externally, cause reactions in every user. We call such reactions toxic, or irritations. As in every biological occurrence, susceptibility to toxic reactions varies between so-called normal subjects. This explains why some subjects are irritated by concentrations of a given substance that are innocuous for others. The important difference between toxic and allergic reactions lies in the fact that increased con-

THE INTERACTION OF DETERGENTS AND THE HUMAN SKIN centrations of a so-called toxic substance may cause a reaction in everyone, whereas allergic reactions depend in the first place on an immunological process, in which concentration plays only a secondary role. Another important fact to bear in mind is that although some irritations may cause eczema--a so-called ortho-ergic eczema--not every result of irritation is called "eczema". It must be admitted at once, however, that it is a difficult and often purely subjective matter to draw the line between "simple" skin irritation and an "orthoergic" or "irritative" eczema. Let me dwell for a moment on the words "toxic reaction" and "irrita- tion" In a wide sense, every adverse reaction directly caused by some substance is toxic, but in common parlance the term "toxicity" is often limited to the more serious reactions and "irritation" to the less severe everyone will agree for instance, that skin necrosis caused by a strong alkali or acid is toxic. Moreover, such a reaction is not an eczema. A little scaling or redness of the skin caused by the same substances in high dilu- tions is adequately described as irritation, and only a few would call them eczema. I hope I shall not confuse you by telling that van der Meiren and Achten (1) succeeded in provoking a condition that had all the patho- logical symptoms of eczema, by applying to the skin of normal persons a carefully chosen concentration of a potassium hydroxide solution or sul- phuric acid, both substances causing necrosis in higher concentration. After this introduction, I can more easily approach the subject of this evening: the pathogenesis of conditions caused by detergents and having the symptoms of a "simple" irritation or of an orthoergic eczema. It has already been mentioned that some doubt had arisen as to whether deter- gents cause eczema or skin irritation at all and, if so, what pathogenesis is involved. I will also report on a related subject, the results of investigations into what happens in a skin exposed to detergents irrespective of whether such exposure leads to pathological effects. May I go back to the question---do detergents irritate a normal skin or deteriorate the condition of an eczema? If the answer to this first question is in the affirmative, we shall have to consider which class of detergents is the worst offender. Then we must consider the question of how to choose from a series of new detergents those that are the least liable to give un- desired reactions. It can be easily demonstrated that positive reactions can be obtained in man by patch-testing with soaps or syndets in various con- centrations. The same holds for experimental animals. Because the con- centration of the detergents and the kind of contact in patch-testing is quite different from the conditions existing under normal use of these