The “vertical limit” is defined as the highest altitude at which humans can survive. This altitude is generally understood to be approximately 18,000 feet above sea level. If humans spend time above this altitude, their health gradually declines due to a lack of oxygen and lower barometric pressure.his altitude is generally understood to be approximately 18,000 feet above sea level. If humans spend time above this altitude, their health gradually declines due to a lack of oxygen and lower barometric pressure.