Which type of UV radiation is primarily responsible for skin aging?

UVA radiation is primarily responsible for skin aging because it penetrates deeper into the skin compared to other types of UV radiation. It affects the dermis, which is the layer of skin that contains collagen and elastin fibers, crucial for maintaining skin elasticity and firmness. Prolonged exposure to UVA rays can lead to the degradation of these proteins, resulting in wrinkles, sagging, and other signs of aging.

In contrast, UVB rays are primarily associated with sunburn and are responsible for stimulating the production of vitamin D in the skin. They affect the outermost layer of skin, the epidermis, and while they can also contribute to skin damage, their role in the aging process is less significant than that of UVA rays. UVC rays, on the other hand, are largely absorbed by the Earth's atmosphere and do not reach the skin, making them irrelevant in the context of skin aging. Therefore, UVA is the correct choice as it has the most direct impact on the aging of the skin through its deeper penetration and long-term effects on skin structure.