SPF 40 sunscreen blocks about 97.5% of ultraviolet B (UVB) rays, which are the main cause of sunburn and skin damage, when applied correctly and generously. It provides much stronger protection than lower SPF products, making it ideal for people who spend a lot of time outdoors or have sensitive skin. For best results, reapply every 2 hours, especially after swimming or sweating, and combine it with other sun-protection measures, such as wearing a hat or staying in the shade. Using SPF 40 sunscreen properly can help reduce the risk of sunburn, premature skin aging, and skin cancer. Please consult a dermatologist for personalized advice.