Photosensitization and phototoxicity can occur when certain essential oil constituents (natural chemicals found in particular essential oils), particularly furanocoumarins (FCs), react when exposed to Ultraviolet UVA light. Inflammation, blistering and reddening/burning of the skin are common.
Although an essential oil can be distilled from citrus leaves, the result has very different chemical properties from that produced from the fruit. Citrus oils will be either expressed from the peel or distilled from the whole fruit. Oils expressed from the peel are more therapeutically, but they are also most likely promote phototoxicity. On the other hand, those distilled from the whole fruit are not likely to promote phototoxicity but lack aroma and robustness in terms of their general efficacy. When purchasing a citrus essential oil, look on the label for how the oil was obtained – by steam, the distillation of the whole fruit or expressed from the peel.
There is generally no phototoxic risk if the oils are used in a product that is either not applied to the body or is washed off the skin, such as shampoo, bath preparation or soap. However, essential oils can adhere to the skin if used in a sauna or steam inhalation. There is no risk if the skin to which the oils are applied is covered in such a way as to prevent UV rays from reaching them.
The recommendation is to avoid the use of phototoxic oils on the skin that will be exposed to direct sunlight or UV rays (i.e. sunlamps, tanning bed, etc) within 12-18 hours of use.
The Complete Aromatherapy and Essential Oils Handbook for Everyday Wellness by Nerys Purchon and Lora Cantle
Essential Oil Safety: A Guide for Health Care Professionals, 2nd Edition by Robert Tisserand and Rodney Young