A heat and moisture exchanger (filter), also known as the HME (filter), is used to replicate the warming, humidifying, and filtering functions of the upper airways when they are bypassing during anesthesia. During surgery under anesthesia, the heat and moisture exchanger (filter) conserves some of the heat and moisture exhaled by the patient, and conditions the inspired gas by warming and humidifying it. Aside from the commonly used HME and HMEF, tHME and nHME filters are also available and have a broader range of applications.