Extremely small liquid or solid particles suspended in air.

Alpha particle:

An energetic nucleus of a helium atom, consisting of two protons and two neutrons, that is emitted spontaneously from nuclei in decay of some radionuclides; also called alpha radiation and sometimes shortened to alpha (for example, “alpha-emitting radionuclide”). Alpha particles are weakly penetrating and can be stopped by a sheet of paper or the outer dead layer of skin.

Atmospheric testing:

Detonation of nuclear weapons or devices in the atmosphere or close to the earth’s surface as part of the nuclear-weapons testing program.


The smallest particle of a chemical element that cannot be divided or broken up by chemical means. An atom consists of a central nucleus of protons and neutrons, and orbital electrons surrounding the nucleus.

Atomic bomb:

A nuclear weapon that relies on fission only, in contrast with a thermonuclear (“hydrogen”) bomb that uses fission and fusion.

Atomic Energy Commission (AEC):

The agency of the US government that became the Department of Energy and the Nuclear Regulatory Commission.

Attributable risk percent:

The percentage of disease that could be eliminated if a particular exposure were stopped.

Autoimmune disease:

A disease caused by one’s immune system’s attacking the cells of one’s own body rather than attacking foreign cells, such as germs.

Autoimmune hypothyroidism:

An autoimmune disease that prevents the thyroid from producing enough thyroid hormone.

Autoimmune thyroiditis:

Damage to the thyroid caused when the body’s immune system attacks and destroys cells in the thyroid. It can be radiation-induced. If the damage is substantial enough, a person may develop signs and symptoms due to hypothyroidism. If there are no signs or symptoms of hypothyroidism, autoimmune thyroiditis is generally not a cause for concern about producing enough thyroid hormone.

Background radiation:

Ionizing radiation that occurs naturally in the environment, including cosmic radiation; radiation emitted by naturally occurring radionuclides in air, water, soil, and rock; radiation emitted by naturally occurring radionuclides in tissues of humans and other organisms; and radiation emitted by human-made materials containing incidental amounts of naturally occurring radionuclides (such as building materials). Background radiation may also include radiation emitted by residual fallout from nuclear-weapons tests that has been dispersed throughout the world. The average annual effective dose due to natural background radiation in the United States is about 0.1 rem, excluding the dose due to indoor radon, and the average annual effective dose due to indoor radon is about 0.2 rem.

Badged dose:

An estimate of a person’s external radiation dose, specifically the deep equivalent dose from external exposure to photons, as derived from readings of exposure by one or more film badges assigned to the person.

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