UVA rays penetrate deeply into the skin and contribute to premature skin aging and skin cancer. Our exposure to UVA is more constant than to UVB — we encounter UVA rays no matter what the season or time of day. UVB rays are the main cause of sunburn, but they also contribute significantly to premature skin aging and skin cancer. High SPF sunscreen products (those with an SPF 30) are designed primarily to absorb UVB plus the shorter UVA rays.