Adult Bike Helmets New One Piece Mountain Helmet Open Face BMX Breathable Secure Unisex Helmets Materials: EPS + PC One size adjustable, head circumference is 54-62cm.Internal length is 20cm,internal width is 18cm. Weight: 210-220G Number of holes: 21.Windy hole design help you keep fresh and cool hair. It also offers protection from wind,dust and other possible hurts for riders. Helmet liner using Velcro design, convenient and easy to dismantle, more security.