SARS-CoV-2: severe acute respiratory distress syndrome coronavirus 2, ROS: reacting oxygen species, eNOS: endothelial nitric oxide synthase, COPD: chronic obstructive pulmonary disorder, IDO: indoleamine 2,3-dioxygenase, RCTs: randomized controlled trials, ppm: parts per million, α7nAChR: α7 nicotinic acetylcholine receptors, iNO: inhaled nitric oxide. Here, IDO2 is linked to chronic obstructive pulmonary disease.