1) Potassium is a naturally radioactive element. 40K accounts for 0.012 % of the total amount of potassium. Its half-life is 1.3 * 109 years and the atomic weight is 39.10 g/mol. A given person weights 70 kg and contains 140 g potassium.
2) The department of health in Norway has set a limit for the content of radioactive Cs in meat from sheep and reindeer to 600 Bq per kg after the Tjernobyl accident. Assume 137Cs is the only radioisotope in the deposition. It has a half-life of 30 years. The biological half-life is set to 110 days and the excretion is assumed to have an exponential progression. A person eats 1.0 kg of reindeer meat containing 6000 Bq 137Cs. All of the Cs is absorbed in the body and evenly distributed.
3) Working with radioactive material requires different degrees of care relative to the nuclide at hand. There exists and international list over nuclides and their dangers.