Two reinforced layers: strong ABS shell resists impact and high quality EPS foam provides shock-absorbing. Complies with U.S. Consumer Product Safety Commission Safety Standard for Bicycle Helmets for Persons Age 5 and Older 3 sizes available. Small: 18.9 -21.3 (48-54 cm), suitable for kids 2-8 years old. Medium: 21.3 -22.4 (54-57 cm), suitable for youth 8-14 years old. Large: 22.4 -24 (57-61 cm), suitable for people aged above 14 Suitable for various outdoor sports such as biking, skateboarding, skating, scooter, longboard, hoverboard and so on 11 vents provide sufficient air flow, reduce sweating, keep user cool and comfortable Fully adjustable for a custom fit, provided by the crank adjustment dial, 2 sets of pads with different thickness, and length-adjustable chin straps