The professor who taught radiation protection lab I took had this way of putting things in perspective (we were quite uneasy about the souces littering the benches all around): he placed a bag of white, crystaline powder in detection chamber; the counter went wild. What was it? Potasium chloride, containing approximately amount of potasium a person's body (mostly skeleton) contains. What was counted was decay of K-40, a natural isotope of potasium (long-lived beta emiter). The activity in an average person's body is about 5 kBq (not much, but that is within us).