Cesium-137 is one of the more well-known byproducts of nuclear fission. Emits beta and relatively strong gamma radiation. Half-life is 30 years. It is one of most common isotopes used in industry (moisture-density gauges, well-logging devices, thickness gauges, etc.)
Liquid at room temperature, combines easily with chlorides to form a powder. Exposure can be through ingestion, inhalation, or just being near it (since it emits gamma radiation). In humans, is flushed from the body fairly quickly.
EPA has established a maximum contaminant level of 4 millirem per year for beta particle and photon radioactivity from man-made radionuclide's in drinking water. Cesium-137 would be covered under this MCL. The average concentration of cesium-137 which is assumed to yield 4 mrem/year is 200 picocuries per liter. If other radionuclide's which emit beta particles and photon radioactivity are present, the sum of the annual dose from all the radionuclide's must not exceed 4 mrem/year.