The initial conditioning of the inhaled gases occurs in the upper respiratory tract. The conditioned gases are at approximately 32 degrees C and more than 95% relative humidity as they enter the lower respiratory tract. This level of heat and humidity will preserve the body's defense mechanisms. When the upper airway is bypassed the gases being delivered to the lower airways need to be as close to physiologic levels as possible based on the length of time the endotracheal tube will be in place. The delivery of gases above or below normal physiologic levels has been shown to produce clinical problems.