Humidification devices and techniques can expose the airway mucosa to a wide range of gas temperatures and humidities, some of which are excessive and may cause injury. Humidified gas is a carrier of both water and energy. The volume of water in the gas stream depends on whether the water is in a molecular form (vapor), particulate form (aerosol), or bulk form (liquid). The energy content of gas stream is the sum of the sensible heat (temperature) of the air and any water droplets in it and the heat of vaporization (latent energy) of any water vapor present. Latent heat energy is much larger than sensible heat energy, so saturated air contains much more energy than dry air. Thus every breath contains a water volume and energy (thermal) challenge to the airway mucosa. When the challenge exceeds the homeostatic mechanisms airway dysfunction begins, starting at the cellular and secretion level and progressing to whole airway function. A large challenge will result in quick progression of dysfunction. Early dysfunction is generally reversible, however, so large challenges with short exposure times may not cause irreversible injury. The mechanisms of airway injury owing to excess water are not well studied. The observation of its effects lends itself to some general conclusions, however. Alterations in the ventilation-perfusion ratio, decrease in vital capacity and compilance, and atelectasis are suggestive of partial or full occlusion of small airways. Changes in surface tension and alveolar-arterial oxygen gradient are consistent with flooding of alveoli. There also may be osmotic challenges to mucosal cell function as evidenced by the different reaction rates with hyper- and hypotonic saline. The reaction to nonisotonic saline also may partly explain increases in specific airway resistance. Aerosolized water and instilled water may be hazardous because of their demonstrated potential for delivering excessive water to the airway. Their use for airway humidification or toilet should be eliminated or minimized. Water vapor is the best form of humidification because it is unlikely to deliver sufficient water to cause pulmonary injury. The mechanisms of thermal injury in epidermal cells have been well studied, although specific observations of injury mechanisms in the airway are sparse. The findings of the epidermal studies can readily be applied to airway mucosal cells, however. This work demonstrates that it is prudent to avoid raising the average tracheal mucosal temperature above approximately 43 degrees C to 45 degrees C. Thus respiratory gases that arrive at the tracheal end of the endotracheal tube should average less than 43 degrees C to 45 degrees C and 100% RH. It should be noted that to deliver temperatures of this magnitude in the trachea would require higher gas temperatures at the circuit wye. These temperatures are much greater than the upper temperature limits imposed on humidifiers by international standards. Additionally, the reports to date of pulmonary thermal injury associated with humidifiers have been solely as the result of equipment malfunction or misuse--a situation that is increasingly less likely to occur with the control and monitoring features of modern devices. In summary, to avoid the injurious effects of excess heat and water in the airway, inspiratory gases should be delivered to the patient's airway at core temperature and 100% RH. This gas condition is the only one that is neutral to the airway mucosa and poses no water volume and heat energy challenge. Humidifiers, however, do not measure the gas temperature at the patient airway but only at the circuit wye. To compensate for any cooling of the gas as it passes from the wye to the patient the gas temperature at the wye must be set higher than core temperature. To safely avoid the risk that this higher temperature may accidentally reach the patient and cause an injury, the average gas temperature at the wye should restricted to less than 43 degrees