Ammonium, Potassium, and Sodium Persulfate are inorganic salts used as oxidizing agents in hair bleaches and hair-coloring preparations. Persulfates are contained in hair lighteners at concentrations up to 60%, in bleaches and lighteners at up to 22% and 16%, respectively, and in off-the-scalp products used to highlight hair strands at up to 25%. They are used in professional product bleaches and lighteners at similar concentrations. Much of the available safety test data are for Ammonium Persulfate, but these data are considered applicable to the other salts as well. Acute dermal, oral, and inhalation toxicity studies are available, but only the latter are remarkable, with gross lesions observed in the lungs, liver, stomach, and spleen. In short-term and subchronic feeding studies the results were mixed; some studies found no evidence of toxicity and others found local damage to the mucous membrane in the gastrointestinal tract, but no other systemic effects. Short-term inhalation toxicity was observed when rats were exposed to aerosolized Ammonium Persulfate at concentrations of 4 mg/m3 and greater. Ammonium Persulfate (as a moistened powder) was not an irritant to intact rabbit skin, but was sensitizing (in a saline solution) to the guinea pig. It was slightly irritating to rabbit eyes. Ammonium Persulfate was negative in the Ames test and the chromosomal aberration test. No significant evidence of tumor promotion or carcinogenicity was observed in studies of rats receiving topical applications of Ammonium Persulfate. The persulfates were reported to cause both delayed-type and immediate skin reactions, including irritant dermatitis, allergic eczematous dermatitis, localized contact urticaria, generalized urticaria, rhinitis, asthma, and syncope. The most common causes of allergic dermatitis in hairdressers are the active ingredients in hair dyes, and Ammonium Persulfate has been identified as a frequent allergen. A sensitization study that also examined the incidence of urticarial reactions was performed with 17.5% Ammonium, Potassium, and Sodium Persulfate under occlusive patches. At this concentration and exposure conditions, a mixture of these Persulfates was not sensitizing, and application of Ammonium, Potassium, and Sodium Persulfate did not result in an urticarial reaction. In normal use (i.e., not occluded and rinsed off), it was expected that a concentration greater than 17.5% would also be safe. Given the clinical reports of urticarial reactions, however, manufacturers and formulators should be aware of the potential for urticarial reactions at concentrations of Persulfates greater than 17.5%. Based on the available data, the Cosmetic Ingredient Review (CIR) Expert Panel concluded that Ammonium, Potassium, and Sodium Persulfate are safe as used as oxidizing agents in hair colorants and lighteners designed for brief discontinuous use followed by thorough rinsing from the hair and skin.