Just like our face, our lips are also sensitive to the harmful rays of the sun. Since the skin around your mouth is thinner, it is even more vulnerable to UV damage. SPF also known as sun protection factor is a relative measure of how long a cosmetic will protect you from UVA and UVB rays. SPF 15 blocks 93% of UVB rays, SPF 30 blocks 97% of UVB rays and SPF 50 blocks 98% of UVB rays. Dermatologists recommend using a SPF15 or SPF30 sunscreen.