This book is an update of the WHO environmental Health criteria monograph on barium (IPCS 1990), which evaluates the risks to human health and the environment posed by exposures to barium. Barite, the mineral from which barium sulfate is produced, is used as a constituent in drilling muds in the oil industry. It is also used as a filler in a range of industrial coatings, as a dense filler in some plastics and rubber products, in brake linings, and in some sealants and adhesives. Oral intake from drinking-water and food is the most prevalent route of exposure to soluble barium compounds for the general population. Insoluble barium sulfate has been extensively used at large doses (450 g) as an oral radiocontrast medium, but no adverse systemic effects have been reported.