The most common benign pneumoconioses are siderosis (iron), stannosis (tin), and baritosis (barium). Chest x-rays of workers with benign pneumoconioses show small opacities caused by lung retention of one of these radiopaque metals. Workers are asymptomatic and have normal pulmonary function studies. The x-ray changes gradually resolve after the workers are removed from exposure. Tin and barium miners may also be exposed to silica. [Rom, p. 526, 537; Rosenstock, p. 410] Other metals with high radiodensity that can potentially produce benign pneumoconioses are titanium, vanadium, zirconium, antimony, hafnium, and the rare earth metals. [Harber, p. 502] See "RARE EARTH METALS."