When hair is exposed to 200 hours of ultraviolet (UV) radiation in sunlight, the cuticle edges begin to fuse as the cuticle “shrinks” dramatically. After 400 hours, porosity continues to increase with further cuticle damage, and after 1200 hours, the cuticle becomes rigid, brittle, and may crack, leading to even further increases in porosity. Very long hair accumulates the most UV damage at the ends. So make the most of hats, shade, and scarves to keep your hair healthy year-round. Give hair as much after-sun care as you give your skin.