Hydroquinone is an active ingredient in both photographic developing solution and skin lighteners. Scary? You be the judge.
Hydroquinone is used to reduce the appearance of age spots, freckles, chloasma (after pregnancy), and melasma. The ingredient topically reduces melanin production without actually bleaching the skin and has antioxidant benefits.
The highest concentration available in the US is 4 percent, which requires a prescription. In other countries, however, hydroquinone is sometimes used as a general skin lightener, and the concentrations can get up to a harmful 30 percent. At concentrations under 4 percent, potential irritation is usually counteracted by mixing it with cortisone (an anti-inflammatory) and tretinoin. Since hydroquinone can be compromised when exposed to air or sunlight, it should always be packaged in an opaque container that minimizes air contact.
Hydroquinone is banned in France due to a perceived cancer risk, which has been generally rejected by the American dermatological community. To find out more information about hydroquinone's risks, read this.