the skin may result in redness, burning, and irritation. This side effect is dose-dependent. Because of these possible side effects, it is recommended to start with a low concentration and build up as appropriate, as the skin gradually develops tolerance to the medication. Skin sensitivity typically resolves after a few weeks of continuous use. Irritation can also be reduced by avoiding harsh facial cleansers and wearing sunscreen prior to sun exposure. One in 500 people experience hypersensitivity to BPO and are liable to experience burning, itching, crusting, and possibly swelling. About one-third of people experience phototoxicity under exposure to ultraviolet (UVB) light. Other uses As a bleach, it has been used as a medication and a water disinfectant. In specialized contexts, the name may be abbreviated as BPO. Benzoyl peroxide is used in dentistry as a tooth whitening product. Safety Explosion hazard Benzoyl peroxide is potentially explosive like other organic peroxides, and can cause fires without external ignition. The hazard is acute for the pure material, so the compound is generally used as a solution or a paste. For example, cosmetics contain only a small percentage of benzoyl peroxide and pose no explosion risk. Toxicity Benzoyl peroxide breaks down in contact with skin, producing benzoic acid and oxygen, neither of which is very toxic. With regards to BPO's carcinogenic potential, one study concluded, "caution should be recommended in the use of this and other free radical-generating compounds". A 1999 IARC review of carcinogenicity studies found no evidence linking BPO acne medication to skin cancers in humans. However, some animal studies found that the compound could act as a carcinogen and enhance the effect of known carcinogens. Skin irritation In a 1977 study using a human maximization test, 76% of subjects acquired a contact sensitization to benzoyl peroxide. Formulations of 5% and 10% were used. The U.S. National Institute for Occupational Safety and Health has developed criteria for a recommended standard for occupational exposure to benzoyl peroxide. Cloth staining Contact with fabrics or hair, such as from still-moist acne medication, can cause permanent color dampening almost immediately. Even secondary contact can cause bleaching, for example, contact with a towel that has been used to wash off benzoyl peroxide-containing hygiene products. Use During Pregnancy The use of topical benzoyl peroxide has not been studied during breastfeeding. Because only about 5% is absorbed following topical application, it is considered a low risk to