A full 18 vents! That's about as cool as you can get in a bike helmet. The Bell Influx is rigged for off road use. . Microshell is bonded not taped on to its EPS foam layer to provide a sturdier, more solid helmet. One hand dial microadjustment is easy to reach while riding. Visor comes off in a snap if you are looking to put in some on road miles. Specially designed ventilation channels bring cool air in through the front, pass it over the head, and flush warm air out of the rear ports. Adjustable cam lock chin straps simply open the cam, position comfortably, and close the lever. Removable, washable pads an extra pad set is provided for a variable fit. Weight: 11 ounces Medium. S 20 21.75 inches head circumference, M 21.75 23.25 inches, L 23.25 24.75 inches.