Skin affected by photoxicity responds with what resembles an exaggerated sunburn. The involved chemical can enter into the epidermis due to topical application, or it can reach the skin via circulation following ingestion or injection. The chemical in question needs to be “photoactive”, meaning that when it absorbs light, the absorbed energy produces molecular changes that cause toxicity. Many synthetic compounds, including some drugs like retinoids, tetracyclines or fluoroquinolones, are known to cause photoirritation. Dermal contact with some such chemicals causes photodermatitis; many plants cause phytophotodermatitis, including Giant Hogweed, Common Rue, and St. John’s Wort. Phototoxicity is a common phenomenon in humans and it also occurs in other animals.

Photosensitization is a reaction to any substance applied topically to the skin that occurs only in the presence of ultraviolet light in the UVA range, and it may be either phototoxic or photo allergenic. A phototoxic reaction will be evident within minutes or hours of exposure while a photo allergenic reaction may not be observed until 1 - 3 days after the substance has come into contact with the body.

The furanocoumarins are a class of organic chemical compounds produced by a variety of plants as a defense mechanism against predators. Essential oils containing furanocoumarin react to UV light and can cause an inflammatory response in the skin. Visible reactions can be immediate but can peak up to three days after initial light exposure. Visible signs of exposure can last for weeks, and might include one or more of the following:

Severe Redness (Sunburn)
Darkening or Pigmentation of the Skin
Edema (Swelling)
Blistering

Some essential oils contain furanocoumarins and as a result, some essential oils can be photocarcinogenic. A photocarcinogen is a substance which can cause cancer following exposure to light. This effect often results from free radicals generated by the photocarcinogen. Many chemicals that are not carcinogenic can still be photocarcinogenic. Some readily available essential oils containing furanocoumarins are listed below:

Angelica Root (Angelica archangelica)
Bergamot* (Citrus bergamia, Citrus aurantium)
Cumin (Cuminum cyminum)
Grapefruit – distilled or expressed (Citrus x paradisi)
Lemon – expressed (Citrus x limon, Citrus limonum)
Lime* – expressed (Citrus x aurantifolia, Citrus x latifolia)
Mandarin Leaf (Citrus reticulata, Citrus nobilis)
Orange, Bitter (Citrus x aurantium)
Rue* (Ruta graveolens, Ruta montana)

*May lead to photocarcinogenesis
Many formulators categorize all citrus essential oils as being phototoxic, but several are safe to use, including those that are steam-distilled. Although furanocoumarins are present in expressed versions, the molecules are not volatile and are left behind during steam distillation. Some sun-safe (Non-phototoxic) essential oils include:

Bergamot – steam distilled – bergapten-free furanocoumarin-free*
       (Citrus bergamia, Citrus aurantium)

Lemon – steam distilled
      (Citrus x limon, Citrus limonum)

Lime – steam distilled
      (Citrus x aurantifolia, Citrus x latifolia)

Mandarin – expressed (Citrus reticulata)

Orange, Sweet – expressed
      (Citrus sinensis, Citrus aurantium var. sinensis)

Tangerine – expressed
      (Citrus reticulata, Citrus nobilis, Citrus tangerine)

Robert Tisserand, recognized as one of the world’s leading experts in aromatherapy, indicates that there is no phototoxic risk if essential oils are used in a product that is either not applied to the body or is a wash-off formulation, such as shampoo or soap. However, essential oils can adhere to the skin if used in a diffuser or via steam inhalation. As formulators, it is our responsibility to know and understand the mechanisms that cause photo irritation so we can work to prevent the issue and thus reduce the risk of harming the consumer.

References:

Robert Tisserand and Rodney Young, Essential Oil Safety (Second Edition. United Kingdom: Churchill Livingstone Elsevier, 2014), 85.

NAHA (National Association for Holistic Aromatherapy): Safety Information. https://naha.org/explore-aromatherapy/safety/