Respiratory mucosal and lung structures and functions may be severely impaired in mechanically ventilated patients when delivered gases are not adequately conditioned. Although under- and over-humidification of respiratory gases have not been defined clearly, a safe range of temperature and humidity may be suggested. During mechanical ventilation, gas entering the trachea should reach at least physiologic conditions (32 degrees C-34 degrees C and 100%relative humidity) to keep the ISB at its normal location. Clinicians must keep in mind that relative humidity is more important than absolute humidity: the warmer the gas, the higher the risk of tracheal mucosa dehydration and proximal airway obstruction. Practical assessment of the adequacy of the humidification system in use is not easy. The consistency (thin, moderate, or thick) of the patient's sputum should be evaluated regularly [47]. Full saturation of inspiratory gases is likely when water condensation is observed in the flex tube [91,92]. Nevertheless, no clinical parameter is accurate enough to detect all the effects of inadequate conditioning [45]. When mechanical ventilation is extended beyond several days, adequate conditioning of respiratory gases becomes increasingly crucial to prevent retention of secretions and to maximize mucociliary function; a requirement that respiratory gases reach at least physiologic conditions is appropriate.