2. Relevance to Public Health 2.1 Background and Environmental Exposures to Cesium in the United States

Naturally-occurring cesium exists as the stable isotope (Cs) in the earth’s crust at an average concentration of about 1 ppm in granites and 4 ppm in sedimentary rocks. The most important source of commercial cesium is the mineral pollucite, which usually contains about 5–32% Cs2O by weight. The largest deposits of pollucite are located in Manitoba, Canada and account for about two-thirds of the world’s known supply. Cesium has very low mobility in soil surfaces. Clay minerals and soils rich in exchangeable potassium adsorb cesium by binding the cations to interlayer positions of the clay particles. The low hydration energy of cesium cations is primarily responsible for their selective sorption and fixation by clays, which can result in limited uptake of cesium by grass and plant material. Continental dust and soil erosion are the main emission sources of naturally-occurring cesium that is present in the environment. As a result of human activities, however, cesium is released into the environment globally in small amounts. Cesium has also been detected in the fly ash of hazardous waste incinerators and coal burning power plants. Cesium is deposited on plants and trees by wet and dry deposition and can be absorbed into the flora through its foliage. The deposited cesium can make its way to soil through litter decomposition. Due to its low relative abundance, limited use in industry, and relatively low level of toxicity observed in animal studies, exposure to stable cesium is not considered to be a significant public health concern.