Hypoxaemic hypoxia (airway obstruction) is more damaging to cells than anaemic or stagnant hypoxia. In order to fully understand the classification of hypoxia, it is useful to consider the example of carbon monoxide poisoning. It is known that hypoxaemic hypoxia is of particular importance in the development of cellular hypoxia and it goes without saying that, in the context of the difficult airway, the principal cause of hypoxaemia is airway obstruction. It is important to understand the mechanisms by which hypoxaemia develops, and the factors which determine the rate of this process. Causes of hypoxaemia occurring during anaesthesia can be divided into the following three categories: problems with O2 supply, problems with O2 delivery from lips to lung, and problems with O2 transfer from lung to blood. Pre-oxygenation aims to increase body O2 stores to their maximum, so that periods of apnoea are tolerated for longer before critical desaturation occurs.