Although they are "natural" in the sense that radioactive atoms naturally decay and radioactive elements are a part of nature, all radioactive emissions are dangerous to living things. Alpha particles, beta particles, neutrons, gamma rays and cosmic rays are all known as ionizing radiation, meaning that when these rays interact with an atom they can knock off an orbital electron. The loss of an electron can cause problems, including everything from cell death to genetic mutations (leading to cancer), in any living thing.
Because alpha particles are large, they cannot penetrate very far into matter. They cannot penetrate a sheet of paper, for example, so when they are outside the body they have no effect on people. If you eat or inhale atoms that emit alpha particles, however, the alpha particles can cause quite a bit of damage inside your body.
Beta particles penetrate a bit more deeply, but again are only dangerous if eaten or inhaled; beta particles can be stopped by a sheet of aluminum foil or Plexiglas. Gamma rays, like X-rays, are stopped by lead.
Neutrons, because they lack charge, penetrate very deeply, and are best stopped by extremely thick layers of concrete or liquids like water or fuel oil. Gamma rays and neutrons, because they are so penetrating, can have severe effects on the cells of humans and other animals. You may have heard at some point of a nuclear device called a neutron bomb. The whole idea of this bomb is to optimize the production of neutrons and gamma rays so that the bomb has its maximum effect on living things.
As we have seen, radioactivity is "natural," and we all contain things like radioactive carbon-14. There are also a number of man-made nuclear elements in the environment that are harmful. Nuclear radiation has powerful benefits, such as nuclear power to generate electricity and nuclear medicine to detect and treat disease, as well as significant dangers.
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