INTRODUCTION: Nitric and hydrofluoric acids are commonly used in the commercial cleaning industry. We are unaware of reports of nitric acid inhalation forming methaemoglobin. Additionally, methaemoglobinaemia and treatment with methylthioninium chloride (methylene blue) may precipitate clinical uncertainty due to similar wavelengths of absorbance in pulse oximetry. CASES: We report two patients with respiratory distress from symptomatic methaemoglobinaemia following a prolonged, inhaled occupational exposure to nitric acid in the context of industrial cleaning. Their methaemoglobinaemia was successfully treated with methylthioninium chloride, per remote toxicology advice. However transient oxygen desaturation as reported by pulse oximetry resulted in concern from the treating team. DISCUSSION: The liberation of oxides of nitrogen from nitric acid bypasses the upper airway without irritation and dissolves in the mucoid lower respiratory tract, oxidising haemoglobin to methaemoglobin. Prolonged undetected exposure with filter saturation, and impaired ventilation is the proposed cause of methaemoglobinaemia in the cases presented. Additionally, methylthioninium chloride absorbs light at the 660 nm wavelength interfering with pulse oximeter interpretation, precipitating the appearance of rapid, severe oxygen desaturation. CONCLUSION: Lack of upper airway irritation can lead to unrecognised prolonged nitric acid fume exposure causing methaemoglobinaemia. Remote toxicology advice should include pulse oximeter interference expectations in the presence of methaemoglobinaemia and when administering methylthioninium chloride.