Of particular concern to facilities engineers and healthcare workers is the evaluation of air flow patterns in or near a respiratory isolation room and negative pressure. The Centers for Disease Control and Prevention recommended the use of smoke tubes for this purpose in the second edition of Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Facilities, 1994. This article provides an assessment of the potential for patient and healthcare worker exposure to hazardous levels of acid mist when smoke tubes are used to monitor negative pressure. Three brands of smoke tubes (Sensidyne, MSA, and Drager) were initially investigated. In a benchtop experiment, air was pushed in 30-ml increments through a smoke tube and into 10 ml of deionized water in a scintillation vial. The hydrogen ion concentration of the solution was measured and the generation rate of acid mist was estimated to be 3.3 mg per 30-ml squeeze. The quantity of hydrochloric acid (9.8 mg) from three 30-ml puffs through Sensidyne smoke tubes would need to be diluted by an air volume of at least 1.4 m3 to be below the Occupational Safety and Health Administration ceiling limit of 7 mg/m3. Because of the equivalency of acid mist generation rate by the three brands of smoke tubes, only Sensidyne air flow indicator tubes (0501) were used in the subsequent study to evaluate the dispersion of hydrochloric acid mist into an experimental room. Three smoke generation rates were evaluated in duplicate. Smoke was generated by pushing air either through one or three Sensidyne smoke tubes using a personal sampling pump at a nominal flow rate of 50 ml/min or through one Sensidyne smoke tube using six 30-ml puffs of the aspirator. The acid mist was directed under the door to the experimental room. Air samples were collected at ten locations. When six 30-ml puffs of air were pushed through a Sensidyne smoke tube and directed under the door to the experimental room, the mass concentration of hydrochloric acid averaged 0.34 mg/m3 (0.23 ppm) at a distance of 0.68m (2.2 ft) from the door. Caution should be exercised to prevent inhalation of concentrated acid mist because particles in this size range (0.38 to 0.63 microm) will penetrate deep into the lungs. Also, because of its irritant properties, the amount of smoke needed to verify negative pressure or air flow patterns should be minimized. While the acid mist dissipated extremely fast under these experimental conditions, consideration of the smoke's irritant properties and a hospital patient's respiratory condition must be weighed. In all cases, whenever smoke tubes are used in the indoor environment, dilution of the mist must be provided.