Natural radioactivity includes cosmic radiation, earth radiation, and radiation from materials in the human body. Cosmic radiation creates subatomic particles and radioactive isotopes, including carbon-14 used in radiocarbon dating. Terrestrial radiation comes from isotopes of carbon, potassium, thorium, and uranium found in soil, rocks, and water. The human body also contains radioactive isotopes from terrestrial sources. The amount of natural radioactivity varies based on geographic location and occupation.
The term “natural radioactivity” refers to any source of radioactivity that is not man-made. The major sources of natural radioactivity include cosmic radiation, earth radiation, and radiation from material in the human body. While the high levels of radiation produced by man-made devices kill human cells and can cause cancer, the naturally occurring low levels have not been found to have adverse health effects. Each human receives an average of 2.4 millisieverts (mSv) of natural radiation per year, although this amount varies based on geographic location and occupation.
Cosmic radiation consists of subatomic particles from outer space, mainly protons and hydrogen nuclei. The sun also emits radiation during solar flares. As these charged particles enter Earth’s atmosphere, they collide with atmospheric atoms and molecules to create other types of subatomic particles and radioactive isotopes, including carbon-14.
The isotopes of a given element will have the same number of protons, but the number of neutrons will be different. Carbon-14 has a nucleus that contains 6 protons and 8 neutrons, for a total of 14 nuclear particles. This isotope is radioactive, which means it undergoes spontaneous decay and emits particles. Carbon-14 emits an electron to decay into the stable isotope nitrogen-14 over a set period of time. Materials containing carbon-14 can be placed in geological time using a process known as radiocarbon dating, in which the amount of carbon-14 in the material is used to determine its age.
Terrestrial radiation is the second major source of natural radioactivity. This radiation comes from isotopes of carbon and potassium, as well as thorium and uranium, which can be found in soil, rocks or water. The last two isotopes decay into radon and radium, which are extremely radioactive, although rare. Their decay rate is also quite long: for example, uranium-238 has a half-life of 4.5 billion years, which means that it takes 4.5 billion years for a given amount of substance to be reduced by half through decay . Uranium’s long half-life makes its effect on humans negligible.
In addition to terrestrial and cosmic sources of natural radioactivity, substances in the human body also produce radiation. The series of radioactive isotopes found in the human body have a terrestrial source, as they have been ingested through food, water or air. They include carbon-14, potassium-40, uranium, thorium, radium and some others. The concentrations of these substances are mostly quite low, the highest being those of carbon and potassium.
The amount of natural radioactivity a person receives depends on their geographic location. Some areas contain soils enriched with a particular isotope due to mineral deposits or organic processes. For example, wetlands may contain more uranium due to the decay of organic material containing this element. Areas of higher elevation tend to receive more cosmic radiation, as they are higher in the atmosphere. Astronauts and pilots receive more cosmic radiation every day than the average person for the same reason.
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