Phototoxicity, Photo-Irritation, and Essential Oils
Phototoxicity occurs when a drug or substance taken orally or applied to the skin is activated by exposure to UV. Unlike a skin sensitization response, which is an allergic reaction, phototoxic responses are not immune reactions, which is why they are often referred to as photo-irritation. A photo-irritant response is more like an exaggerated sunburn.
There are scores of substances known to be photo-irritating, including many antibiotics, antifungals, and antiseptics in medications, soaps, deodorants, and other cosmetics. Dyes and coal-tars in cosmetics can also be photo-irritating. In essential oils, furanocoumarins are the naturally occurring plant molecules that may cause photo-irritation.
Furanocoumarins as a compound are safe for the skin, and there is no risk of photo-irritation if the skin is covered with clothing. When furanocoumarin-containing oils are applied to the skin and then exposed to sun light, the furanocoumarin molecules store energy from UV and release the energy in a burst onto the dermis and epidermis. This micro-burst causes the sunburn-like irritation.
The degree of irritation relies on a few factors: the amount of furanocoumarin in an oil, the amount of oil applied to the skin, how soon after application the skin is exposed to the sun, and the intensity and duration of sun exposure.
Citrus and the Sun
There is some internet created confusion about citrus oils and the sun. Unscientific claims are made that citrus oils are photo-irritating because of the presence of limonene, a terpene found in most citrus oils and many non-citrus oils. This is inaccurate. Limonene (which is just one of thousands of compounds found in an essence), is non-irritating to the skin, even in the sun. In lieu of limonene, it is the furanocoumarins that are the issue.
The levels of furanocoumarins present in essential oils vary. For example, cold pressed oil from lemon peels have furanocoumarins while steam distilled lemon oil does not. Essential oils distilled from the leaves and twigs of citrus trees, known as petitgrains, along with Neroli, distilled from orange blossoms are never photo-irritants.
All Libations creations are formulated to perfection with these guiding principles. We are especially thankful to the work of Robert Tisserand and his Essential Oil Safety Data Manual.
Here is a list of essential oils by degree of photo-irritation and how they are best used:  
Strong: Dilute to use. If applied undiluted do not expose the skin to sunlight that day.
Moderate: When applied at the given or greater dilution percentage, avoid direct, extended exposure to sunlight that day. Incidental sunlight (a short jaunt outside) is safe.
Mild: If diluted to therapeutic levels at 15% or above, avoid exposing the skin to intense sun. Incidental exposure is safe for most people. When lightly diluted these oils are rarely photo-irritating to the skin.
Non-photo-irritating citrus family oils:

Nadine Artemis, the founder of Living Libations, is the author of Holistic Dental Care: The Complete Guide to Healthy Teeth and Gums, and Renegade Beauty: Reveal and Revive Your Natural Radiance, which was named one of “The Top 10 Books on Skin Care” by The Strategist of New York Magazine. She is a respected media guest and contributor, and her products have received rave reviews in the New York Times, LA Times, Elle, People, Vogue, and Hollywood Reporter. Described by Alanis Morissette as “a true-sense visionary,” Nadine crafts elegant formulations and healing creations from rare botanicals that have skin glowing around the world. Her concept of Renegade Beauty encourages effortlessness and inspires people to rethink conventional notions of beauty and wellness.