Flexi Says: Generally speaking, the air pressure inside your body is the same as that outside of your body. Also, the air pressure is contacting all of your body at the same time, not just the top of your head and shoulders. Simply put, the net force or pressure is canceled out so you don't get crushed by the pressure. However, dramatic changes in air pressure will eventually become noticeable and, if the pressure becomes great enough, become crushing.
