How are passengers instructed to use oxygen masks?

The instruction for passengers to pull down the mask, place it over the nose and mouth, and secure the straps is critical for ensuring the effective use of oxygen masks in emergency situations. This process is designed to provide immediate oxygen supply in case of cabin depressurization or other emergencies where supplemental oxygen is needed.

By pulling the mask down, passengers have direct access to the oxygen supply quickly. Placing it over the nose and mouth is essential so that the mask can deliver oxygen effectively. Securing the straps ensures that the mask stays in place, allowing passengers to breathe normally without worrying about the mask slipping away, which is vital in high-stress situations.

The other options do not fully encompass the necessary actions for proper usage of oxygen masks. Simply holding the mask over the mouth without securing it may not allow for an adequate seal, preventing the effective delivery of oxygen. Fastening the mask around the head without placing it over the face does not guarantee proper usage either since the oxygen would not be effectively provided. Lastly, waiting for crew assistance contradicts the protocol of acting swiftly during emergencies, as passengers are trained to use the masks independently to ensure their safety.