3. What are the AARC guidelines? The AARC guidelines for humidification are based on the ANSI Z79 standards for heated humidifiers, which postulate a moisture return of 30 mg H2O to the patient. The flow rate at which this water amount should be achieved is not specified in the guidelines. Many practitioners believe that the more humidity is being returned the better it is for the patient. However, the guidelines talk about 30 mg H2O as the desirable amount; i.e. any more or less could be equally undesirable. Another fact that needs to be considered is the difference from one patient to another. If the patient does not generate 30 mg H2O, no heat and moisture exchanger will be able to return this amount, since they are based on passive humidification return. The clinician will have to decide if alternate methods of humidification, such as heated water bath humidifiers will be an appropriate treatment alternative. Other factors influencing moisture return are the location of the patient's endotracheal tube, outside room temperature and any pre-existing pulmonary diseases.