Classification according to radiotoxicity
The radioactive isotopes are classified into 4 groups according to their toxicity. The first group consists of alpha emitters: plutonium, neptunium, americium, curium as well as the natural isotopes polonium 210 and lead 210. Among the beta emitters, the radioactive isotopes of iodine which affect the thyroid and strontium 90 (which attaches itself to bone) have a high toxicity, whereas cesium 137 (which spreads among the muscle mass) is less dangerous. At the lowest end of the scale we find tritium (whose energy deposit is particularly low), krypton 85 (a noble gas which occurs as a by-product in nuclear industry) and metastable technetium 99 (frequently used in medical scans).
IN2P3/ICRP