Phototoxins are toxin

Toxin

A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...

s that can cause allergic reactions in particularly susceptible individuals and which can cause dangerous photosensitivity

Photosensitivity

Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light.- Human medicine :Sensitivity of the skin to a light source can take various forms. People with particular skin types are more sensitive to sunburn...

 in a much broader range of subjects.

Phototoxins are common in:

• a variety of plants (including food plants where they may be a biological defence):
  • many citrus
    Citrus
    Citrus is a common term and genus of flowering plants in the rue family, Rutaceae. Citrus is believed to have originated in the part of Southeast Asia bordered by Northeastern India, Myanmar and the Yunnan province of China...
    
    es contain essential oil
    Essential oil
    An essential oil is a concentrated hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils or aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove...
    
    s that are photosensitisers;
  • some herbal
    Herbalism
    Herbalism is a traditional medicinal or folk medicine practice based on the use of plants and plant extracts. Herbalism is also known as botanical medicine, medical herbalism, herbal medicine, herbology, herblore, and phytotherapy...
    
     remedies (notably St John's wort
    St John's wort
    St John's wort is the plant species Hypericum perforatum, and is also known as Tipton's Weed, Chase-devil, or Klamath weed....
    
    , though incident rates for this plant are reportedly low);

• some prescribed medications (such as tetracycline antibiotics); and
• many essential oils, perfume
  Perfume
  Perfume is a mixture of fragrant essential oils and/or aroma compounds, fixatives, and solvents used to give the human body, animals, objects, and living spaces "a pleasant scent"...
  
  s and cosmetics
  Cosmetics
  Cosmetics are substances used to enhance the appearance or odor of the human body. Cosmetics include skin-care creams, lotions, powders, perfumes, lipsticks, fingernail and toe nail polish, eye and facial makeup, towelettes, permanent waves, colored contact lenses, hair colors, hair sprays and...
  
  .

Ingested medications may cause systemic photosensitivity and topically applied medications, cosmetics and essential oils may lead to local (or perhaps systemic) photosensitivity. Curiously and ironically, para-aminobenzoic acid (PABA

4-Aminobenzoic acid

4-Aminobenzoic acid is an organic compound with the formula H2NC6H4CO2H. PABA is a white grey crystalline substance that is only slightly soluble in water...

) found in some sunscreens can also cause photosensitivity.

Upon exposure to light, notably light containing ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 radiation, discolouration of the skin (whether as inflammation, lightening or darkening) or rashes may result. In extreme cases, blistering may also occur.

Uses

The marigold plant produces the phototoxin alpha-terthienyl, which functions as a nematicide

Nematicide

A nematicide is a type of chemical pesticide used to kill parasitic nematodes.One common nematicide is obtained from neem cake, the residue obtained after cold-pressing the fruit and kernels of the neem tree. Known by several names in the world, the tree was first cultivated in India in ancient...

. When exposed to near ultraviolet light, such as in sunlight, alpha-terthienyl generates the toxic singlet oxygen

Singlet oxygen

Singlet oxygen is the common name used for the diamagnetic form of molecular oxygen , which is less stable than the normal triplet oxygen. Because of its unusual properties, singlet oxygen can persist for over an hour at room temperature, depending on the environment...

. Alpha-terthienyl results in damage to the respiratory, digestive and nervous system of larvae, resulting in 100% death rates in concentrations of 33 ppb. This makes it an interesting natural insecticide.

Rose bengal

Rose bengal

Rose Bengal is a stain. Its sodium salt is commonly used in eye drops to stain damaged conjunctival and corneal cells and thereby identify damage to the eye...

 and other singlet oxygen

Singlet oxygen

Singlet oxygen is the common name used for the diamagnetic form of molecular oxygen , which is less stable than the normal triplet oxygen. Because of its unusual properties, singlet oxygen can persist for over an hour at room temperature, depending on the environment...

 generating phototoxins are also used in synthetic organic chemistry. They have also found use in photodynamic therapy

Photodynamic therapy

Photodynamic therapy is used clinically to treat a wide range of medical conditions, including malignant cancers, and is recognised as a treatment strategy which is both minimally invasive and minimally toxic...

, where the toxin is activated by intense light to destroy cancer cells.