The world is constantly bathed in radiation, much of which originates from natural sources present since the Earth’s formation. This natural radioactivity results from the spontaneous decomposition of unstable atoms found naturally in the air, soil, water, and even within our own bodies. Radioactivity is the process by which an unstable atomic nucleus seeks a more stable state by emitting excess energy or matter.
Defining Natural Radioactivity
Radioactivity is a physical phenomenon rooted in the instability of an atom’s nucleus. While many atoms are stable, some naturally occurring isotopes, or radionuclides, have an excess of internal energy. This imbalance causes the nucleus to spontaneously transform, a process known as radioactive decay, until it reaches a stable configuration.
During this decay process, the atom emits energy in the form of particles or high-energy waves, which are broadly categorized into three types. Alpha particles are the largest, consisting of two protons and two neutrons, identical to a helium nucleus. Beta particles are much smaller, being high-energy electrons ejected from the nucleus when a neutron converts into a proton.
Gamma rays are not particles but are pure, high-frequency electromagnetic energy. The three types differ in their ability to penetrate matter. Alpha particles have the least penetrating power and can be stopped by a sheet of paper or the outer layer of dead skin. Beta particles are more penetrating, stopped by a thin sheet of aluminum or clothing. Gamma rays possess the greatest penetrating power, requiring dense materials like several inches of lead or feet of concrete to be significantly blocked.
Primary Sources of Natural Exposure
Natural background radiation exposure is divided into three distinct categories: cosmic, terrestrial, and internal. Cosmic radiation originates from outer space, with charged particles from the sun and distant stars continually bombarding the Earth’s atmosphere. The atmosphere and the planet’s magnetic field offer some shielding from this source. The dose received varies significantly with altitude, as less atmosphere is available to attenuate the incoming particles at higher elevations. For instance, people living in cities at a mile high, such as Denver, receive a substantially higher annual dose than those living at sea level.
Terrestrial radiation is emitted from naturally occurring radioactive materials found within the Earth’s crust, including deposits of uranium, thorium, and potassium. These elements are present in soil, rocks, and building materials like granite, brick, and concrete. They expose people both outdoors and indoors.
The most significant single contributor to the average person’s natural background dose is the gas radon. Radon is an odorless, colorless radioactive gas produced by the natural decay of uranium found in soil and rock. Because it is an inert gas, it can easily move up through the ground and accumulate inside houses and other buildings, especially those that are tightly sealed.
The final source is internal radiation, which comes from radionuclides naturally present inside the human body. These radioactive isotopes are acquired through the food we eat and the water we drink. The body contains trace amounts of elements like Carbon-14 and Potassium-40, which are incorporated into tissues and cells.
Measuring and Contextualizing Exposure
The amount of natural radioactivity is quantified using specific units to describe both the rate of decay and the resulting biological effect. The Becquerel (Bq) measures activity, representing the number of nuclear disintegrations occurring per second in a given material. This unit indicates the amount of radioactive material present but does not directly convey the risk to a person. To measure the health-related impact, the Sievert (Sv) quantifies the effective dose, accounting for the type of radiation and the sensitivity of the exposed tissue. Doses are commonly expressed in millisieverts (mSv), which are one-thousandth of a Sievert.
The average annual radiation dose from all sources for a person in the United States is approximately 6.2 millisieverts, with roughly half coming from natural background sources. The worldwide average dose from natural background radiation alone is estimated to be around 2.4 millisieverts per year. For comparison, a single chest X-ray typically delivers a dose of about 0.01 to 0.02 millisieverts. A longer flight exposes a person to approximately 0.01 millisieverts of cosmic radiation per 1,000 miles traveled, while a full-body computed tomography (CT) scan can deliver 1 to 20 millisieverts.