Unless it is terribly cold, I always run a small crack at low speeds. My helmet has a notch that is just a half inch of crack. At high speeds the vents pull air

Unless it is terribly cold, I always run a small crack at low speeds. My helmet has a notch that is just a half inch of crack. At high speeds the vents pull air through. It is funny because I never thought about CO2, just that it is too freaking hot.