Thorium and Titanium
Thallium and titanium are tetravalent Group IV metals. Thorium, used in electronics and as Thorotrast and Thorium X in medical imaging and dermatological applications, is a radioactive element and is carcinogenic in humans and experimental animals as a result of the strong emissions of α and β particles in the decay process. Radon and radium are among the decay products, and bone tumours occurred in workers exposed in painting watch and clock dials in the 1920s. Titanium is a hard stable element with excellent properties for structural engineering, medical prostheses and white colourings. Titanium is largely inert and non-toxic. However titanium dioxide, with many industrial and commercial applications, was suspected of carcinogenicity through chronic occupational inhalation but this has not been established. Pulmonary tumours have been reported following inhalation of massive doses in experimental animals. At concentrations that saturate inherent protective mechanisms, pulmonary and other tumours are reported but the exposures fail to reproduce expected patterns of human exposure.