Humidity is the amount of water vapour present in a gaseous environment. Normally, the air travelling through the airways is warmed, moistened and filtered by cells in the nasopharynx and down the airways, so that by the time it reaches the alveoli it is fully saturated at a temperature of 37ºC.
This humidification is necessary to compensate for the normal loss of water from the respiratory tract. If the humidification apparatus of the body is impaired (as with disease and/or dehydration), alternative methods of humidification may need to be considered.
Oxygen therapy will further compound these problems, causing further dehydration of mucous membranes and pulmonary secretions. As a rule of thumb oxygen usually requires humidifying if delivered at or above 40 per cent or if therapy is prolonged, that is, for more than four hours. If in doubt seek advice.
External humidification is, however, essential when oxygen therapy is being delivered to a patient whose physiological humidification has been bypassed by an endotracheal tube or tracheostomy tube.
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