Specifically, it is the compound bergaptene in many of these oils that causes the photo toxic reaction.
Bergaptene is abundant in almost all the citrus essential oils, so avoid the sun when using these essential oils:
While the bergaptenes in citrus oils are the most common culprits, it's the furocoumarins (psoralens) in other plants and essential oils that can be the problem.
Common plants containing furocoumarins include:
Wild Parsnip (pictured above), however, is highly photosensitizing and quite abundant here in the Coulee Region. According to the Wisconsin Department of Natural Resources, many people are not aware of Wild Parsnip as a potential culprit for skin rashes. Wild Parsnip (Pastinaca sativa) can cause severe blistering and discomfort, and can also be a threat to pets and livestock.