UVA activates melanin pigment already present in the upper skin cells. It creates a tan that appears quickly but is also lost quickly. Furthermore, UVA penetrates into the deeper skin layers, where connective tissue and blood vessels are affected. As a result the skin gradually loses its elasticity and starts to wrinkle. Therefore, large doses of UVA cause premature ageing. Furthermore, recent studies strongly suggest that it may enhance the development of skin cancers. The mechanisms of this UVA damage are not fully understood, but a popular hypothesis assumes that UVA increases oxidative stress in the cell.
UVB stimulates the production of new melanin, which leads to a heavy increase in the dark-colored pigment within a few days. This tan may last a relatively long time. UVB also stimulates the cells to produce a thicker epidermis. Therefore, UVB is responsible both for the darkening and thickening of the outer cell layers – these reactions are the body's defence against further UV damage.
However, higher doses of UVB cause sunburn which increases your likelihood of developing cancer. The exact mechanism of how UVB initiates or promotes cancer is not yet known. In people suffering from Xeroderma pigmentosum, a rare pigmentation disease, the ability to repair DNA damage caused by exposure to UV radiation is impaired. The much-increased rates of skin cancer in these patients suggest that direct UV damage of DNA may be the mechanism that links exposure to the development of cancer.