Patients with artificial airways frequently need aerosol or humidity therapy. For this study, we used aerosols generated from a heated Puritan all-purpose nebulizer and humidity generated from a heated Bennett cascade humidifier to determine the effects of these therapies in spontaneously breathing neurosurgical patients with nasal endotracheal tubes and normal lungs. Crossover comparison of oxygenation status before and after aerosol or humidity therapy was done by analyzing P(A-a)O2 in these patients. We found that aerosols have a detrimental effect on the patient's oxygenation status, suggesting that humidity is preferable in maintaining adequate oxygenation in a patient with a normal lung plus artificial airway. The arterial pH and PaCO2 were not affected by aerosol therapy. Care should be exercised with regard to the adverse hypoxemic effect of bland aerosols when a large-reservoir jet nebulizer is used in an intubated patient who already has impaired cardiopulmonary function or borderline PaO2.