CDC:

Radioisotope Brief: Cesium-137 (Cs-137)

How can it hurt me?

External exposure to large amounts of Cs-137 can cause burns, acute radiation sickness, and even death. Exposure to Cs-137 can increase the risk for cancer because of exposure to high-energy gamma radiation. Internal exposure to Cs-137, through ingestion or inhalation, allows the radioactive material to be distributed in the soft tissues, especially muscle tissue, exposing these tissues to the beta particles and gamma radiation and increasing cancer risk.

Read full fact sheet at: http://emergency.cdc.gov/radiation/isotopes/cesium.asp