This photo induced skin reaction leads to a mono-morphous eruption that consists of multiple, uniform, red, papular lesions.
The eruption is unusual because it spares the face but it affects the lateral aspects of the upper arms, shoulder girdle, back, and chest.
[1] This condition's pathogenesis is not very well understood but scientists believe it an unfortunate side effect that results from a strong immune response to potentially cancer-causing cell damage.
[4][2] Patients present numerous lesions of small papules that are dull red, dome shaped, hard, and usually not more than 2–4 mm in diameter.
[4] It is also speculated that cosmetics and sun care products containing hydroxyl peroxides may trigger this condition through free nuclear radiation photo-toxic reactions.
A photo-toxic reaction pathogenesis is also possible through free radical release in sun screen oxidation by ultra violet radiation.