Irritant contact reactions of the skin or nails, as opposed to allergic contact dermatitis, do not involve the immune system. Instead, as their name suggests, they are caused by direct irritation. For example, they may be provoked by such factors as excessive acidity or basicity (that is, too high or too low a pH) of a particular cosmetic or grooming agent. Highly alkaline toilet soaps, for example, especially if left in contact with the skin for too long a time, are well-known causes of direct irritation. Irritation is also far more likely to occur if potential irritants are applied to previously broken or diseased skin, and for this reason persons with excessively dry skin, atopic dermatitis, and psoriasis are particularly prone to such reactions.

Volatile substances that dissolve the protective oil layer of the skin and thereby expose it to greater irritation can also do it. Acetone and acetate, found in nail removers, and sodium or potassium hydroxide, which are ingredients in cuticle removers, are good examples of potential irritants that work in this fashion.

Mechanical factors that damage the skin surface and disrupt its protective barrier also predispose the skin to irritant dermatitis. Problems can result from the rubbing that may be required to apply a cosmetic or from the use of cosmetics containing abrasive particles. You are more likely to see this when soaps and cleansers that contain apricot pits or other ground abrasives are used.

Unfortunately, irritant reactions to cosmetics can sometimes be difficult to distinguish from true allergic reactions. Both may appear as itchy, reddish patches on the involved skin accompanied by blistering. As a rule, however, irritant reactions tend to appear within only a few hours of contact, whereas typical allergic reactions require at least two days or more to develop following contact with the offending cosmetic.

Treatment, like prevention, is a fairly simple matter of immediately discontinuing contact with the culprit product(s). This usually results in complete clearing within a few days. Occasionally, when the inflammation is severe, your doctor may need to prescribe a mild topical corticosteroid cream such as hydrocortisone to hasten clearing.