Photo aging of the skin is induced by dermal UV exposure. UV-A (ultraviolet A) accounts for 90% of all UV radiation. UV-B and UV-A rays, on the other hand, can harm and alter the metabolism of epidermal cells (keratinocytes, melanocytes, Langerhans, and Merkel cells).
In human keratinocytes, UV-A photons cause skin damage after a brief exposure, phosphorylating JNK via increased intracellular ROS ( reactive oxygen species) levels (HaCaT).
Other studies have shown that ROS impacts keratinocytes, showing particular external stress that might cause this cell type to generate oxygen.