Exposure to gamma radiation can damage organs in the human body. Most fission products emit both gamma and beta radiation. Beta radiation can be blocked by wood or aluminum. Beta radiation causes the most damage if its particles are ingested or inhaled. Tritium, and Strontium-90, which seeks the bones in the human body, emit beta radiation.
Alpha radiation is the least pentrating of the three.
It can be blocked by a sheet of paper and cannot penetrate human skin. But it's very harmful if ingested or inhaled.