There are certain essential oils that intensify the effects of sunlight, causing burns, blisters, and discoloration. Basically, phototoxic oils contain chemical constituents that absorb UV light, store it, and release it into the skin. Because reactions can occur up to 18 hours after using the oil, we need to be mindful after applying phototoxic oils. Use the recommended dilution rates for these oils to avoid a phototoxic reaction.