Skin

"The skin is the largest organ of the body, with a total area of about 20 square feet. The skin protects us from microbes and the elements, helps regulate body temperature, and permits the sensations of touch, heat, and cold.
Skin has three layers: The epidermis, the outermost layer of skin, provides a waterproof barrier and creates our skin tone.
The dermis, beneath the epidermis, contains tough connective tissue, hair follicles, and sweat glands.
The deeper subcutaneous tissue (hypodermis) is made of fat and connective tissue.
The skin’s color is created by special cells called melanocytes, which produce the pigment melanin. Melanocytes are located in the epidermis."

Phototoxicity

Phototoxicity is a chemically induced skin irritation requiring light (photoirritation or photosensitivity). The skin response resembles an exaggerated sunburn. The involved chemical may enter into the skin by topical administration or it may reach the skin via systemic circulation following ingestion or parenteral administration. In any case, the chemical needs to be "photoactive", which means it is able to absorb photons and, subsequently, to turn the absorbed energy into photoreactivity. Light-induced toxicity is a common phenomenon in humans, however, it might occur also in other animals.