Nitric oxide in air constitutes what is called ‘ambient NO’. Most of the inhaled ambient NO enters the pulmonary blood by diffusion and binds to hemoglobin.The residual NO becomes part of the exhaled breath that only appears in the initial wash-out phase of a typical 11-second exhaled breath. Ambient NO is a function of the location, time of day etc. and can vary between 0 ppb and upwards of 200 ppb.