The toxicity of oxygen is a problem when (mostly ill) lungs are exposed to high concentrations of oxygen> 50% for a long time (days). Oxygen radicals are supposed to do structural damage to the alveolar membranes. Another problem of breathing of 100 % oxygen are atelectasis (collapsed areas of the lung due to complete resorption of oxygen in absence of nitrogen). A continous flow, providing an positive endspiratory pressure, solves the problem.Conclusion: If an pilot is alone in the cockpit in dangerous hights, he would be wise to wear a mask, breathing 40 % of oxygen, thus changing his hypoxia tolerance from seconds to minutes. Hygenical problems shouldn`t be a problem in the times of single use devices. By the way- every fire brigade can show you how to keep reusable masks hygienically clean.