Background and Objective: Acute vasoreactivity testing using inhaled nitric oxide (NO) is performed to determine the treatment strategy in patients with pulmonary hypertension. Operators of the test are exposed to NO and nitrogen dioxide (NO₂) to the same extent during the test procedure, but the safety zone for operators in the catheterization laboratory is unclear. This study aimed to clarify the safety zone for operators during the NO inhalation test. Methods: The mask was placed on the examination table in the catheterization laboratory. NO was added using the INOvent^® (INO-Therapeutics, LLC), which was attached to an oxygen line and delivered NO to the mask. During simulated NO inhalation tests (n = 10) in a catheterization laboratory, NO and NO₂ concentrations were measured at nine specified locations: at the mask outflow port (E0); 25 cm (E25) and 50 cm (E50) from the mask outflow port; heights of 25 cm (A25) and 50 cm (A50) above E0 and E25; and a height of 25 cm (B25) below E0 and E25. During NO inhalation testing conducted in 5 patients, concentrations of NO and NO₂ were measured at E0 and E25A50. The Steel-Dwass and Student t-tests were used to perform intergroup analyses. Results: NO-E0A25 (0.00 ± 0.00 ppm), NO-E25A25 (0.01 ± 0.10 ppm), NO-E0A50 (0.00 ± 0.00 ppm), and NO-E25A50 (0.00 ± 0.00 ppm) were significantly lower than NO-E0 (17.01 ± 0.22 ppm), NO-E25 (4.06 ± 0.47 ppm), and NO-E50 (1.90 ± 0.39 ppm) (p < 0.01, all comparisons). NO₂ concentrations were less than 0.1 ppm at all nine locations. NO and NO₂ concentrations were not affected by room temperature (20, 24, or 28℃) or the flow rate of oxygen (15 or 10 L/min). During NO inhalation testing in patients, NO-E25A50 (0.20 ± 0.45 ppm) was significantly lower than NO-E0 (7.20 ± 2.39 ppm, p < 0.01); however, no significant difference in the NO₂ concentration was seen between NO₂-E0 (0.220 ± 0.179 ppm) and NO₂-E25A50 (0.180 ± 0.205 ppm). Conclusions: An operator safety zone in NO vasoreactivity testing is a zone >25 cm in all directions from the mask for NO inhalation. The concentrations of NO and NO₂ to which an operator is exposed in the clinical setting are within the permissible exposure limits defined by the National Institute for Occupational Safety and Health.