The sun emits radiation in the form of ultraviolet (UV) light, which is responsible for most skin damage. There are three UV wavelengths, UVC, UVB and UVA, from shorter to longer. The amount of energy in the UV radiation decreases with increasing wavelength. Skin cells have evolved mechanisms to counteract the damage caused by UV exposure, including damage signaling pathways and repair mechanisms, but these mechanisms can be overwhelmed by the amount of damage, and the efficiency of these repair mechanisms diminish with age. UV exposure also suppresses the immune system in the skin, diminishing the body’s defense against mutated, oncogenic cells. Sun damage is cumulative, and the more damage accumulated the higher the risk of skin cancer. The accumulation of UV damage sufficient to cause cancer may intersect with immune senescence, resulting clinically in a tipping point wherein despite future avoidance of sun exposure, multiple and frequent skin cancers begin to emerge. UV exposure has both immediate and delayed effects. UV immediately induces inflammation by release of cytokines, causing the redness and swelling we know as “sunburn”. If the dose of UV is higher than the threshold for radiation injury of a cell, the cell will activate apoptosis (programmed cell death) and die.
UVA
UVA is a longer wavelength that goes deeper into the skin reaching the mid dermis. There it causes damage to the collagen and elastin responsible for photoaging in the form of wrinkles, dilated blood vessels and skin thinning. UVA has less energy, so it causes less damage to DNA, but it causes photosensitization, such as in autoimmune conditions like Lupus, and is also an important cause of photoproducts, which promote cancer, especially melanoma.
UVB
UVB is shorter higher energy wavelength responsible for sun burns and most skin cancers. UVB is the most energetic wavelength to reach our skin and is absorbed mainly in the epidermis. It is responsible for most skin carcinogenesis as well as sunburn.
UVC
UVC is mostly absorbed by the atmosphere and very little reaches earth.
