Free radicals and their metabolites, also called reactive oxygen species (ROS), have been implicated in the pathogenesis of many diseases. Because of its continuous exposure to toxic pollutants in the ambient air, such as cigarette smoke, air pollution, and mineral dusts, the lung is very vulnerable to ROS-induced injury. In this review, the role of ROS in the pathogenesis of obstructive lung diseases is reviewed. A central theme in this review is the pivotal role of transition metals such as iron, vanadium, and nickel in ROS-induced cell damage, not only in exposure to mineral dusts but also in cigarette smoke and air pollution.