L. Kligman wrote in 1983 about chronic UV radiation damage to the human dermis. Based on UV exposure to hairless mice, it was shown that repair can occur if skin is protected by sunscreens of both UVB and UVA using a broad spectrum sun protection factor (SPF) of 6 and 15. Repair occurred in situ in a novel form of sub-epidermal reconstruction zones of new connective tissue with parallel collagen bundles and a network of fine elastic fibers; though regular use of sunscreens does not provide full repair. Additional repair is possible with use of topical ingredients, like retinols and alpha hydroxyl acids, inclusive of use of sun protecting agents and moisturizer. So it is important to protect the skin from both UVB (290-320 nm) - primarily responsible for severe sunburn damage and cancer, and UVA (320-400 nm) - which penetrates deeper into skin to produce reactive oxygen species (ROS) that can cause DNA, cell, and tissue damage. Both UVB and UVA can cause sunburn, skin cancer, and premature skin aging.