Extrinsic or Photo Aging is aging that is caused by external factors such as Sun Exposure, Medications, Diet, Disease, Smoking etc. Extrinsic Aging is something that we can control to a degree.  This kind of aging happens simultaniously with Intrinsic Aging and is usually the result of Cumulative Sun Damage.  The UV solar radiation is divided into 3 sections, UVA, UVB and UVC.  UVC is the most dangerous of the UVR's and is blocked by the ozone layer and at the moment is of no threat to us.  UVB solar radiation affects the Epidermal Layers of our skin, and is absorbed by, and damages, the DNA in our cells.  This causes thickening of the uppermost layers of the skin, the stratum corneum, and can cause mutations in the cells of the epidermis and eventually leading to skin cancers such as Basal Cell Carcinomas (BCC's) and Squamous Cell Carcinomas (SCC's), UVB can be filtered by glass.  UVA Solar Radiation reaches the Dermal Layers of our skin and causes Oxidative Damage by producing Free Radicals and Reactive Oxygen Species (ROS), that damage nearby healthy cells causing cumulative damage.