How do you breathe in an airplane?

Modern aircrafts are pressurized so that people can breathe onboard.

It would be better for the structure and the aircraft if it was not pressurized, that way there would be no stress on the material.

However, the aircraft needs to be pressurized, for people to breath. Normal pressure on earth is around 14,7 PSI (ISA Standard) or 1 Bar (ISA Standard). Aircrafts are pressurized to a height of 8,000 ft.

Sensors are installed to determine if the cabin altitude exceeds 10,000 ft. At this point the crew will be warned, and if the altitude should increase to more than 14,000 ft the oxygen masks will automaticly deploy.

Humidity in the air is created by recirculation of the air in the cabin.