Photosensitizing essential oils are those that might react with skin when exposed to the sun causing a dark rash or dark pigmentation. Citrus Oils like Lemon, Lime, and Grapefruit are all photosensitizing and should be kept in a dark glass. The citrus-based oils have been known to react with plastic as well. Everyone's skin is different so it is best to use caution. While walking to your car may not cause any problems -- a day at the beach might be too much for you.