Barium is a dense alkaline earth metal that occurs in nature as a divalent cation in combination with other elements. Physiologically, it exists as an ion in the body. In addition to its natural presence in the Earth's crust, and therefore its natural occurrence in most surface waters, barium is also released to the environment via industrial emissions. The residence time of barium in the atmosphere may be up to several days. Barium sulfate exists as a white orthorhombic powder or crystals. Barite, the mineral from which barium sulfate is produced, is a moderately soft crystalline white opaque to transparent mineral. The most important impurities are iron(III) oxide, aluminium oxide, silica, and strontium sulfate.Barium sulfate has a low toxicity and relatively high density of about 4.5 g·cm-3 (and thus opacity to X-rays). For this reason it is used as a radiocontrast agent in X-ray imaging of the digestive system ("barium meals" and "barium enemas"). Lithopone, a pigment that contains barium sulfate and zinc sulfide, is a permanent white that has good covering power, and does not darken when exposed to sulfides. (Wikipedia). Barium hydroxide is strongly alkaline and therefore corrosive. Barium nitrate caused mild skin irritation and severe eye irritation in rabbits. The lack of reports of skin or eye irritation in humans, despite its widespread use, suggests that barium sulfate, often used as a contrast medium, is not a strong irritant. Useful information on the sensitization potential of barium compounds was not identified. Oral intake from drinking water and food is the most prevalent route of exposure to barium compounds for the general population. For the occupational environment, data from industry in the United Kingdom and predictions made using the Estimation and Assessment of Substance Exposure (EASE) model suggest that exposures can be controlled to less than 10 mg/m3 8 hours time weighted average (total inhalable dust). In some situations, control will be to levels significantly below this value. Short term exposures may be higher than 10 mg/m3 for some tasks.The critical end points in humans for toxicity resulting from exposure to barium and barium compounds appear to be hypertension and renal function. Using a no observed adverse effect level (NOAEL) in humans of 0.21 mg barium/kg body weight per day, a tolerable intake value of 0.02 mg/kg body weight per day for barium and barium compounds has been developed in this document.Dissolved barium in aquatic environments may represent a risk to aquatic organisms such as daphnids, but it is apparently of lesser risk to fish and aquatic plants, although data are limited. No adverse effects have been reported in ecological assessments of terrestrial plants or wildlife, although some plants are known to bioaccumulate barium from the soil.(Concise international chemical assessment document 33; http://www.inchem.org/documents/cicads/cicads/cicad33.htm).