Definition of Terms. More precise definitions of these terms are given in the references. Absorbed radiation dose - R's, grays, etc. Roughly speaking, absorbed dose is a measure of the total energy deposited by radiation in water or human tissue. 100 R's Dosimeters measure dose.
Activity measures the number of radioactive decays in a sample per second.
A typical source activity is one mCi. Personal Radiation Tolerance. Consult the references above to inform yourself on the biological effects of radiation, and on the major sources of radiation that everyone is exposed to. With any dose over 500 R, you are dead within a week from lack of white blood cells, always. But if 5000 people get 1 R each, one of them is likely to die of cancer, as a result of that radiation dose.