The term "natural radioactivity" refers to any source of radioactivity that is not man-made. Major sources of natural radioactivity include cosmic radiation, terrestrial radiation, and radiation from material in the human body. While the high levels of radiation produced by man-made devices kill human cells and may cause cancer, the low levels that occur naturally have not been found to have any adverse health effects. Every human being receives an average of 2.4 millisievert (mSv) of natural radiation per year, though this amount varies based on geographical location and occupation.
Cosmic radiation consists of subatomic particles from outer space, mostly protons and hydrogen nuclei. The sun also emits radiation during solar flares. When 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.
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A given element's isotopes will have protons of the same number, but the number of neutrons will be different. Carbon-14 has a nucleus that contains 6 protons and 8 neutrons, making a total of 14 nuclear particles. This isotope is radioactive, meaning that it spontaneously undergoes decay and emits particles. Carbon-14 emits an electron to decay into the stable isotope nitrogen-14 over a fixed 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 may be found in soil, rocks, or water. The latter two isotopes decay into radon and radium, which are extremely radioactive, though rare. Their decay rate is also quite long — for example, uranium-238 has a half-life of 4.5 billion years, meaning that it takes 4.5 billion years for a given amount of the substance to be reduced through decay by half. The long half-life of uranium makes its effect on human beings negligible.
In addition to terrestrial and cosmic sources of natural radioactivity, substances in the human body also produce radiation. The array of radioactive isotopes found in the human body have a terrestrial source, since they have been ingested through food, water, or air. They include carbon-14, potassium-40, uranium, thorium, radium, and some others. Concentrations of these substances are for the most part quite low, with the highest being those of carbon and potassium.
The amount of natural radioactivity a person receives depends on geographical location. Certain 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, since they are higher in the atmosphere. Astronauts and pilots receive more cosmic radiation on a daily basis than the average person for the same reason.