While sun hats provide a significant amount of sun protection, they do not block 100% of UV rays. The level of UV protection depends on several factors, such as the hat's material, weave, and brim size. Look for hats with a UPF (Ultraviolet Protection Factor) rating for enhanced sun protection. Additionally, it's important to use other sun protection measures alongside wearing a sun hat, such as applying sunscreen and seeking shade during peak sun hours.