A heat and moisture exchanger (HME) performs the function of the upper airways in conditioning the inspired air and filtering out dust and large airborne particles. It works passively by retaining heat and moisture from the expired air and returns it to the patient in the next inspired breath (Doctor et al. [67], Wong et al. [289]). HMEs may contain a bacterial filter and consist of spun, pleated, highly thermal conductive material. There are several types of HMEs; they can be attached directly onto a tracheostomy tube (Figure 12.11) or within a laryngectomy stoma (Figure 12.12), or used in a ventilator breathing circuit (Figure 12.13) for patients who are intubated and mechanically ventilated. HMEs are portable, simple to use and a cost‐effective means of providing humidification (Ari et al. [11], McNulty and Eyre [154], NTSP [206]).

A cold water bubble humidifier delivers gas, which is bubbled through water at room temperature and delivers partially humidified oxygen at about 50% relative humidity. They are no longer advised as there is no evidence to suggest they are of clinical benefit to patients, and they may be an infection risk (O'Driscoll et al. [209]).

In a water humidification chamber, inspired gas is forced over a heated reservoir of water, actively humidifying the oxygen or air. Hot water bath humidifiers are more efficient for providing humidification in patients who are mechanically ventilated or who are on high‐flow oxygen therapy (McNulty and Eyre [154], NTSP [206]). However, they are associated with higher costs and potential hazards (Gaffney and Dalton [87]).

While they are not true humidification devices, aerosol generators create highly saturated micro‐droplets of water, which are suspended in the gas to be delivered, through an aerosol device (McNulty and Eyre [154]). These devices are particularly useful for spontaneously breathing patients with COPD to aid expectoration of viscous secretions or to deliver bronchodilatory drugs. If an electrical compressor or ultrasonic nebulizer is not available, a jet nebulizer can be given via compressed air. Care must be taken to ensure the nebulizer is not delivered using oxygen in patients who are at risk of hypercapnic respiratory failure.