Psoralen is a name for a group of organic chemicals found in plants of the genus Psoralea, such as celery, carrots, and turnips. The Chinese herb po gu zi, also known as bu gu zhi or Psorales corylifolia L., is also a member of the psoralen family as are many other natural flowering plants. Together, they produce chemicals that can make the skin highly reactive to ultraviolet A (UVA) light, and, for this reason, they are called PUVA compounds, and are used to treat skin conditions such as psoriasis, vitiligo, and hair loss. They are toxic chemicals, however, in larger doses, as they interfere with the production of deoxyribonucleic acid (DNA), and can, therefore, can cause skin rashes, burns, and skin cancer in healthy individuals such as agricultural workers who pick celery and parsnips that contain the compounds.
The parent compound family that psoralen is derived from is known as a furocoumarin, which is a five-carbon ring furan compound similar to benzene's six-carbon ring, combined with coumarin, which produces a fragrant odor and is known as as a benzopyrone. These compounds are produced by plants as a defense mechanism, as they are natural pesticides that are highly toxic to insects and mammals. Plants also use them as an agent against infection by molds and invasive microorganisms. The molecules of a psoralen compound are small enough to pass through a cell wall and bind to adenine and thymine bases in DNA structure, where they are inert until exposed to UVA light, when they become active and halt the process of normal cell division.
In small doses, psoralene compounds have been used in Chinese medicine as a muscle injection or topical cream to stimulate the production of melanin in the skin while under exposure to sunlight. This unique trait also caused them to be widely used as suntan activators in suntan lotions, where light levels were reduced due to climate, until they were banned for such use in Switzerland in 1987. Later evidence of their contribution to melanoma cases worldwide caused a wider ban on the chemicals for this use in 1996. Replacement compounds with ultraviolet B (UVB) blocking chemicals and psoralen were introduced as replacements in countries such as France, and promoted as safe tanning compounds for people who had a hard time obtaining a tan, but the claims to their safety remain controversial. A related compound used in tanning agents that has suspected photomutagenic qualities is umbelliferone, commonly found in carrots, coriander, and other plants.
The medical use of psoralen for skin conditions was pioneered in the 1940s in Egypt as a treatment for vitiligo, and is seen as preferable to steroid therapy, even though it has a high rate of side effects. In related skin conditions like psoriasis, it works by drying out the skin itself, which reduces skin infection rates. As of 2011, research is under way to use psoralen as an ingredient by blood banks to disinfect blood after exposing it to UVA radiation. It would have no detrimental effect on red blood cells or plasma, and render white blood cell DNA in the blood inert.