Last week, there was tragedy on Southwest flight 1380, when an engine explosion led to a broken window in the fuselage and the death of a passenger.
While the pilot, Captain Tammie Jo Shultz, was able to land quickly to prevent further injuries, the sudden depressurization could have led to many more injuries. Especially as photo evidence shows that, despite clear instructions at the beginning of the flight, many passengers were not covering both their nose and mouth with the oxygen mask
The problem, in my view, is that there is nothing obvious about the design of the mask that indicates it should go over both the nose and mouth. Its circular opening reassembles a cup.
In order to prevent misuse, misuse has to be more difficult to achieve that proper use. In this case, perhaps a more triangular shape would better indicate an orientation that would cover the nose as well.
Emergency situations, especially when life and death are involved, are very stressful. Stress makes people more likely to lose focus and make mistakes. Designing in a way that actively prevents error can save lives.