http://www.epa.gov/radiation/radionuclides/cesium.htmHow can cesium-137 affect people's health?
Like all radionuclides, exposure to radiation from cesium-137 results in increased risk of cancer. Everyone is exposed to very small amounts of cesium-137 in soil and water as a result of atmospheric fallout. Exposure to waste materials, from contaminated sites, or from nuclear accidents can result in cancer risks much higher than typical environmental exposures. Great Britain's National Radiological Protection Board predicts that there will be up to 1,000 additional cancers over the next 70 years among the population of Western Europe exposed to fallout from the nuclear accident at Chernobyl, in part due to cesium-137.
If exposures are very high,serious burns, and even death, can result. Instances of such exposure are very rare. One example of a high-exposure situation would be the mishandling a strong industrial cesium-137 source. The magnitude of the health risk depends on exposure conditions. These include such factors as strength of the source, length of exposure, distance from the source, and whether there was shielding between you and the source (such as metal plating).