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Hydroquinone is a chemical that has been used for over a century in skin care products. One molecule of hydroquinone consists of two hydroxyl compounds and the compound benzene. At room temperature in its raw state, hydroquinone has a lustrous white color and is comprised of long, thin crystals. While hydroquinone is the most common name for the chemical, it is also referred to as quinol or benzene-1,4-diol.

In nature, hydroquinone is produced in the bodies of bombardier beetles, which use the chemical as a defensive secretion. Hydroquinone is also found in plants and fungi, including the poodle-dog bush and the Agaricus hondensis mushroom. Harvesting hydroquinone from its few natural sources is costly and impractical, and as a result, the ingredient is usually manufactured in industrial settings.

The production of hydroquinone is performed through one of three common methods. Each of the methods involves combining another substance, usually phenol or benzene, with at least one other chemical. The combination of the materials causes a reaction to occur and hydroquinone to be formed along with at least one by-product. When producing skin care formulations, manufacturers use different concentrations of hydroquinone. The concentration is represented by a percentage and refers to how much of the volume of the product is made up of hydroquinone.

Hydroquinone first became widely used during the 19th century. Initially, the chemical was an ingredient in solutions used for developing photographs in early dark rooms. Presently, hydroquinone is still found in a wide variety of film photographic developers.

The effects that hydroquinone had during film developing led to further investigation into its properties and other potential benefits. One thing that was quickly discovered about the ingredient was that with repeated handling, it had the ability to lighten the skin tissue. Although the reason for this eluded scientists initially, skin-bleaching creams came onto the market containing this ingredient. Now, the mechanism of action of hydroquinone is still not fully understood. Scientists do know for certain that hydroquinone works by decreasing the amount of pigment in the skin.

The color of the skin arises from the presence of a specific pigment called melanin. The more melanin a person has in their skin, the deeper and darker its color. Because cells are continuously shed and replaced by newer cells, the body is always manufacturing melanin.

Sometimes, the body will suddenly begin to produce excessive levels of melanin in one particular area, causing that one region of skin to appear darker than the surrounding tissue. Often, this sudden increase in melanin levels is due to sun exposure. When the discoloration appears as random blotches on the complexion, it is known as age spots, while smaller dots of discoloration are commonly referred to as freckles. During pregnancy and other times when a woman's hormones fluctuate widely, a form of skin discoloration called melasma or chloasma can occur as well.

With topical application, hydroquinone works locally on the skin to reduce the amount of melanin in the area to which it was applied. As a result, the discoloration gradually fades away. Some doctors and medical scientists believe that the effectiveness of hydroquinone is due to the fact that it interferes with a chemical that is necessary for starting the production of melanin in the skin cells.

In 1982, the U.S. Food and Drug Administration (FDA) approved hydroquinone for use as a skin-whitening agent. Products with hydroquinone strengths of 2 percent or less can be purchased without a prescription. For those who do not respond to the lower strengths, a doctor can prescribe hydroquinone in concentrations of as much as 4 percent.

Hydroquinone has the potential to cause uncomfortable side effects like redness, itching and swelling. As a result many people are unable to use hydroquinone or must opt for milder formulations that contain the ingredient. Products that contain hydroquinone are not recommended for use by pregnant or nursing mothers

Since 2006 when the FDA released a statement saying that it was reviewing its previous approval of hydroquinone, concerns have mounted about whether or not the ingredient is safe. One issue being studied is if hydroquinone has the potential to cause cancer because it contains the toxin benzene. Another side effect being investigated is exogenous ochronosis, a condition where the skin develops bluish-black spots. While it has been established that ingesting hydroquinone can lead to this problem, doctors are unsure whether or not there is a similar risk with topical use of the ingredient. As of June 2012, the FDA has yet to release its final ruling on hydroquinone.

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