Nitrous oxide (N2O) N2O having low blood solubility rapidly diffuses into alveoli and dilutes the alveolar air, and PP of oxygen in alveoli is reduced. The resulting hypoxia, called diffusion hypoxia, is not of much consequence if cardiopulmonary reserve is normal, but may be dangerous if it is low. Diffusion hypoxia can be prevented by continuing 100% O2 inhalation for a few minutes after discontinuing N2O, instead of straight away switching over to air. Diffusion hypoxia is not significant with other , anaesthetics because they are administered at low concentrations (0.2-4%) and cannot dilute alveolar air by more than 1-2%. Ref:- kd tripathi; pg num:-376