You probably know that sunscreen prevents sunburns. It does this by stopping dangerous ultraviolet rays from penetrating your skin. These rays also damage the DNA in your cells, raising your risk for skin cancer as well as premature signs of aging.1  The best quality evidence suggests that broad spectrum sunscreens—those that protect you from both ultraviolet A (UVA) and ultraviolet B (UVB) rays—can reduce your risk of skin cancer by 40–50%. Studies show that sunscreens block the damaging UVA and UVB rays from harming skin cells in the short term, in animals or petri dishes.2,3,4 Long-term studies that evaluate the effectiveness of wearing it every day are harder to come by. It’s difficult to get a large group of people to wear sunscreen reliably every day (or not) for years and years and wait to see if they develop cancer. However, in 1996, researchers began a study in which participants were randomized to apply sunscreen daily for 4.5 years and were then followed for another 10 years. Participants who had worn sunscreen daily during the 4.5 years were less likely to have been diagnosed with squamous cell carcinoma or melanoma during the follow-up.5,3