5.5 breaths/minute, 1L per breath, through the nose.
So what is prescribed by the author of “Breath”? The “optimum” air intake is 5.5L/minute, taken at roughly 5.5 breaths/minute. Exhaling fully and completely before inhaling again. All through the nose. There is a certain % of CO_2 in our exhalations, and when that % dips, we experience negative effects.