Before the plane takes off, the crew demonstrates how to wear an oxygen mask, telling the passengers how to cope with an emergency. The crew recommends that the adult wear it first, and then the child. When an emergency strikes, parents instinctively try to put an oxygen mask on their child first. And helping children, who are the weak, be safe first is a social principle. Then, why should adults wear the oxygen mask first?
When a plane loses cabin pressure due to an error in the pressure control system during flight, people may lose consciousness within thirty seconds due to lack of oxygen. The time to wear an oxygen mask is very tight. While an adult is trying to put an oxygen mask on a child first, the situation may get worse if the adult faints; because it is not easy for the child to put an oxygen mask on the adult. Therefore, in order for both adult and child to be safe, the adult must wear the mask first and then put it on the child.
If you have someone to protect and take care of, not only on board, you need to check your own condition first. When you are ready to help, your helping hand will be more useful.