What is an essential precaution when using an oxygen mask during flight?

Ensuring that an oxygen mask fits snugly before requiring use is crucial for effective operation and safety during flight. A proper fit enhances the mask's ability to seal around the face, preventing outside air from entering and ensuring that the supplied oxygen is delivered effectively. This is particularly important during emergency situations, such as a drop in cabin pressure, where adequate oxygen supply is essential for the crew and passengers. An improperly fitting mask may lead to insufficient oxygen intake, which could compromise the safety and performance of the aircrew.

The other options present precautions that do not address the vital need for a secure and effective seal when using the mask. For example, keeping the mask stored until necessary does not prepare the aircrew for immediate use, and sharing the mask poses a contamination risk. Additionally, the idea of using the mask only above 10,000 feet ignores scenarios where an oxygen supply might be needed at lower altitudes, especially if there’s an emergency. Hence, a snug fit is the primary concern when preparing to use an oxygen mask in flight.