Photosensitive reactions are caused by light-activated compounds. The most common types are phototoxic reactions, photoallergic reactions, and polymorphous light eruptions. Each has a different pathophysiology and may be difficult to tell apart.
In phototoxic reactions, incoming light activates a compound that directly damages cell membranes and/or cellular DNA and results in a strong inflammatory response that looks like VERY severe, or exaggerated, sunburn. Phototoxic reactions tend to be limited to exposed areas and do NOT require previous exposure.
Interested in learning more about caring for your self and other in the outdoors? Take one of our wilderness medicine courses. Guides and trip-leaders should consider attending our Wilderness First Responder course.