There are many different types of rays present in sunlight. The sun rays that are most damaging to the skin are called "ultraviolet (UV) rays." There are two basic types of ultraviolet rays that reach the earth’s surface — UVB and UVA. UVB rays are responsible for producing sunburn. The UVB rays also play the greatest role in causing skin cancers, including the deadly black mole form of skin cancer (malignant melanoma).
UVA rays also play a role in skin cancer formation. In addition, the UVA rays penetrate more deeply into the skin and play a greater role in premature skin aging changes including wrinkle formation (photoaging). There are approximately 500 times more UVA rays in sunlight than UVB rays. Therefore, in addition to protecting your skin from the effects of UVB rays, it is also very important to protect from the damaging effects of the more numerous UVA rays. Traditional chemical sunscreen products have been more successful at blocking UVB rays than UVA rays.