There are 92 different chemical elements, made up of protons, neutrons, and electrons. If there are extra neutrons in the atom, the neutron(s) will tend to leave that atom, in the form of radioactive decay. Every element, such as carbon, hydrogen, or uranium has a small fraction that is radioactive. These nuclei are unstable, and when the extra matter is released, an electrical charge is given off. IF the charge is strong enough, it will have ionizing effects on chemical bonds, disrupting those bonds. These chemical changes can have biological effects, causing injury to the cell nucleus and its DNA.
Different elements have different behaviors. When Uranium 238 decays, it gives off an alpha wave. Alpha waves consist of a Helium atom, two neutrons and two protons, that have lost its electrons. They are regarded as having little penetrating ability in the skin, but can be damaged if inhaled. If a hydrogen atom has two extra neutrons, it is called tritium. When tritium decays, it gives off a beta wave. Beta waves are related to electrons. When potassium or radon decay, they give off gamma rays. Gamma waves can also come from outer space, and lightning. Gamma waves are related to neutrons. Geiger counters measure these ionizing events, notably the discharge of alpha, beta, and gamma rays. Since Geiger counters only measure events and since each element gives off a different amount of energy when it undergoes nuclear decay, they cannot tell how much or what kind of energy is sensed. However, the ions are active on biological tissues, destroying chemical bonds and injuring DNA. These injuries can lead to cancers, and birth defects. Many of the atomic elements which give off ionizing radiation are biologically active and have enhanced effects in terms of uptake and distribution. Moreover, there is often a distinction made between whether the material is external, applied to the skin, or internal applied, ingested or inhaled. These distinctions may be artificial. Consider being in a dusty environment. Dust will be on the clothes, in the air, on the food. If the substance is in the air, it will become internalized by breathing or injesting. There is also a distinction between biologically active atoms and inert gases. Consider this report about Three Mile Island (Makhijani and Saleska, from MIT, in The Nuclear Power Deception, p.96) : there was 13-17 curies of Iodine released, and between 2.4 to 11 million curies of noble gases released. There is the suggestion that the release of Iodine is the most important, but it is clear that inert gases can be damaging. The gas radon is clearly a carcinogen according to the EPA.