Hydroquinone, also benzene-1,4-diol, is an aromatic organic compound which is a type of phenol, having the chemical formula C6H4(OH)2. Its chemical structure has two hydroxyl groups bonded to a benzene ring in a para position. Hydroquinone is commonly used as a biomarker for benzene exposure. The presence of hydroquinone in normal individuals stems mainly from direct dietary ingestion, catabolism of tyrosine and other substrates by gut bacteria, ingestion of arbutin containing foods, cigarette smoking, and the use of some over-the-counter medicines. Hydroquinone is a white granular solid at room temperature and pressure. The hydroxyl groups of hydroquinone are quite weakly acidic. Hydroquinone can lose an H+ from one of the hydroxyls to form a monophenolate ion or lose an H+ from both to form a diphenolate ion. Hydroquinone has a variety of uses principally associated with its action as a reducing agent which is soluble in water. It is a major component in most photographic developers where, with the compound Metol, it reduces silver halides to elemental silver. [HMDB]

Hydroquinone, also benzene-1,4-diol, is an aromatic organic compound which is a type of phenol. Hydroquinone is commonly used as a biomarker for benzene exposure. The presence of hydroquinone in normal individuals stems mainly from direct dietary ingestion, catabolism of tyrosine and other substrates by gut bacteria, ingestion of arbutin containing foods, cigarette smoking, and the use of some over-the-counter medicines. In human medicine, hydroquinone is used as a topical application in skin whitening to reduce the color of skin. In 2006, the United States Food and Drug Administration revoked its previous approval of hydroquinone and proposed a ban on all over-the-counter preparations. The FDA stated that hydroquinone cannot be ruled out as a potential carcinogen. This conclusion was reached based on the extent of absorption in humans and the incidence of neoplasms in rats in several studies where adult rats were found to have increased rates of tumours, including thyroid follicular cell hyperplasias, anisokaryosis, mononuclear cell leukemia, hepatocellular adenomas and renal tubule cell adenomas. Numerous studies have revealed that hydroquinone can cause exogenous ochronosis, a disfiguring disease in which blue-black pigments are deposited onto the skin, if taken orally; however, skin preparations containing the ingredient are administered topically. The FDA has classified hydroquinone currently as a safe product, as currently used. (Wikipedia)