A helmet for use by an operator or rider of a motorized vehicle, such as a motorcycle or snowmobile, includes a ventilation system with an air intake subsystem, an air diffusion subsystem, and an air exhaust subsystem. The air intake subsystem includes a plurality of air intake vents located in the outer shell of the helmet, as well as a plurality of air intake holes located within the foam liner of the helmet. The air diffusion subsystem includes a plenum located between an upper portion and a lower portion of the foam liner, which can act as a pressure chamber to forcefully direct air onto the user's head. The air exhaust subsystem includes one or more exhaust ports that create a vacuum near the back of the helmet to draw large volumes of airflow through the helmet as it travels forward.