Radionuclides, also referred to as radioactive isotopes, are atoms with unstable nucleus and are able to emit excess energy either by emission of g-rays or subatomic particles (a-, b-particles). Number of radionuclides is uncertain, as number of very short lived radionuclides is very large. Currently, ≈3000 radionuclides are known, whereas ≈2400 have lifetime <60 days. Radionuclides have no nutritional value but food can serves as a major exposure route to radionuclides, depending on radionuclides. Plants are able to absorb radionuclides from soil depending on the geochemical characteristics of the soil and even accumulate in the edible portion. This exposure route applies to e.g. uranium and thorium. Contamination of drinking water is another route of human exposure, either directly or via life-stock exposed to contaminated water. Building material is another exposure route for radionuclides such as radon where the human exposure is via inhalation.
Deliberate human exposure to radionuclides is e.g. for medication purpose such as in cancer treatment.
Effects of human exposure to radionuclides depend on radionuclide and radiation type (a-, b- or g-). Radiation is known to induce DNA damage, causing eventual mutation. Other known effects include damage to the immune system and induction of neurological disorder by 137Cs. 210Po is known to accumulate in the ovaries and, even at low dosage, kills primary oocytes.