Why are some oils phototoxic?

When we learn aromatherapy, we should be taught about the phototoxicity of certain essential oils but what does this really mean and why do some oils react to direct sunlight when applied topically? This article intends to explain these facts as well as showing definitively which oils to avoid in the sun and what this means for the practising aromatherapist.

What is phototoxicity?

According to Dijoux et al. 2006, "Phototoxicity is a skin reaction caused by concurrent topical or systemic exposure to specific molecule and ultraviolet radiation. Most phototoxic compounds absorb energy particularly from UVA light leading to the generation of activated derivatives which can induce cellular damage". So to put this in layman's terms, we are talking about burning of and damaging the skin after applying a specific essential oil (in the context of this article), following exposure to the sun, tanning beds or heat lamps - basically exposure to any form of Ultraviolet UVA light. The effects of photosensitisation include reddening of the skin (erythema), burning and blistering and inflammation of the skin.

Of all the essential oils we learn about, Bergamot (Citrus bergamia L.) is cited as being the most phototoxic. Over 90% of the whole oil, which is distilled from the epicarp and mesocarp of the fresh fruit, consists of volatile monoterpene hydrocarbons, alcohols and esters. A non-volatile fraction consists of 4-7% coumarins and psoralens (Phytotherapy Research, 2014). Bergamot contains a furocoumarin by the name of Bergapten, which is biologically active and induces a lethal and mutagenic photosensitisation of bacteria and "lethal and clastogenic effects on mammalian cells in tissue cultures (Ashwood-Smith et al., 1980 cited by Cosento et al.). Bergamot contains about 2% coumarin-psoralens. As a result, some essential oil suppliers have created bergapten free Bergamot oil (FCF), but does this make any difference to the therapeutic effects? It is certainly important in the perfume industry but as clinical aromatherapists or essential oil therapists, we can use the whole natural plant with cautions. Bergamot oil typically is absorbed by the skin within 40-60 minutes (Schilcher 1985) so it is important to warn clients if you have used Bergamot oil that is not FCF to avoid exposing their skin to the sun for at least an hour after application, although Tisserand advises that these essential oils can produce a phototoxic reaction for up to 12-18 hours after application but does not state why - perhaps it is best to always err on the side of caution. In tests carried out, skin pigmentation at the application site with 10% dilution of Bergamot oil that had been irradiated still showed after 23 days (Marzulli and Maibach 1970). The best course of action would be to avoid using phototoxic oils altogether or to use extremely low dilutions of around 0.5 to 1% (1-2 drops in 10mls of base product). The other option is to cover the area after application to protect it from UV rays.