Are radioactive materials all man-made?
No. Over 60 radionuclides can be found in nature and many are present all around us at relatively low levels. Some radionuclides are being generated in the upper atmosphere all the time by the action of cosmic radiations from outer space. Other radionuclides have been present in the rocks since the Earth was formed. In total there are approximately 1,500 known radionuclides and most of these are produced under artificial conditions. Most of the radioactive materials used for medical treatment are man-made.
