There are many radiation units, so many in fact that we will explain only the few units most likely to be used in the media or at a local town meeting on radiation-related issues. If you would like to know the definition of other radiation terms, visit the Radiation Terms section on the Health Physics Society Web site (

Current law in the United States requires the use of standard units, while the rest of the world uses an international system (SI) of units—very similar to the United States using inches and feet while centimeters and meters are used by other countries.

absorbed dose: Absorbed dose is used for purposes of radiation protection and assessing dose or risk to humans in general terms. Absorbed dose is the amount of radiation absorbed in an organ or tissue (i.e., the amount of radiation energy that has been left in cells, tissues, or organs). Absorbed dose is usually defined as energy deposited (joule) per unit of mass (kilogram). See gray and rad.

alpha particle: An alpha particle is a particle with weight (it is made up of two protons and two neutrons) and charge (it is positively charged because protons have a positive charge). Alpha particles do not travel very far and are not considered an exposure hazard unless the radioactive material that emits them gets inside the body.

as low as reasonably achievable (ALARA): ALARA is a principle of radiation protection that calls for practical efforts to be taken to keep exposures to ionizing radiation as low as reasonably achievable, economic and social factors being taken into account.

background radiation: Background radiation includes radiation from cosmic sources, naturally occurring radioactive materials (including radon), and global fallout (from the testing of nuclear explosive devices). The typically quoted average individual exposure from background radiation is 0.36 rem per year.

becquerel: Becquerel (Bq) is the unit in the International System of Units to replace the curie (see curie).

beta particle: A beta particle is an energetic electron given off by atoms when the atoms have too much energy. Beta particles do not travel very far, but travel farther than alpha particles. Beta particles are typically stopped by a few millimeters or about an eighth (1/8) of an inch of tissue; higher-energy beta particles will be stopped by approximately a centimeter or about a third (1/3) of an inch of tissue. Beta particles do have a charge—those that are negatively charged are essentially electrons and those that are positively charged are called positrons.

contamination: Contamination is the presence of radioactive material where it is not wanted—most commonly by depositing on surfaces or inside structures, objects, or people.

cosmic radiation: Cosmic radiation is penetrating ionizing radiation comprised of particles and electromagnetic energy that comes from outer space. Cosmic radiation accounts for about half of the natural background radiation we receive each year.

curie: The curie (Ci) is the original term used to describe the amount of radioactive material present or strength of the source. It is based upon the radioactive decay rate of the radionuclide. One curie is equal to 3.7 x 10 10disintegrations (37 trillion decays) per second (dps); one becquerel is equal to 1 dps. The most common activity levels used in laboratories are the millicurie (mCi) and microcurie (µCi). A millicurie (mCi) is 1/1,000 th of a curie and a microcurie (uCi) is 1/1,000,000 th of a curie. In the International System of Units, the becquerel (Bq) describes the amount of radioactive material present. One curie is equal to 3.7 x 10 10 Bq.

decontamination: Decontamination is a process through which the presence of radioactive materials (or other hazardous materials) are removed from surfaces (e.g., laboratory working areas) through cleaning and washing.

detector: A detector is a device or instrument designed to measure whether radioactive material is present. This can be followed up by using a properly calibrated instrument to estimate how much radioactive material is present.

dose: Dose is a general term used to express (quantify) how much radiation exposure something (a person or other material) has received. The exposure can subsequently be expressed in terms of the absorbed, equivalent, committed, and/or effective dose based on the amount of energy absorbed and in what tissues.

effective dose: Radiation exposures to the human body, whether from external or internal sources, can involve all or a portion of the body. The health effects of one unit of dose to the entire body are more harmful than the same dose to only a portion of the body, for example the hand or the foot. To enable radiation protection specialists to express partial-body exposures (and the accompanying doses) to portions of the body in terms of an equal dose to the whole body, the concept of effective dose was developed. Effective dose, then, is the dose to the whole body that carries with it the same risk as a higher dose to a portion of the body. As an example, 8 rem to the lungs is roughly the same potential detriment as 1 rem to the whole body based on this idea.

electromagnetic spectrum: The electromagnetic spectrum is a description of the broad range of radiation that includes radiowaves; microwaves; infrared, visible, and ultraviolet light; and x and gamma rays.

equivalent dose: Equivalent dose is a dose quantity used for radiation protection purposes that takes into account the chance that a type of radiation will cause an effect. Different types of radiation (alpha, beta, gamma) interact with human tissues differently, with some leaving a lot of energy in the tissue and others leaving very little energy in the tissue, and the energy that is left is what partially determines whether an effect will occur or not. Because of this, different types of radiation are assigned numbers based on how effective that type of radiation is at leaving its energy in the tissue, thus having more potential to cause an effect. By using equivalent dose we are provided an indication of the potential for biological effects. From this, risk comparisons can be made between the different types of radiation.

exposure: Exposure is commonly used to refer to being around a radiation source, e.g., if you have a chest x ray, you are exposed to radiation. By definition, exposure is a measure of the amount of ionizations produced in air by photon radiation.

exposure rate: Exposure rate is the amount of exposure you are receiving per unit time (e.g., 1 mR/hour).  Dose rate is the amount of dose you are receiving per unit time (e.g., 1 mrem/hour).

gamma rays: Gamma rays are high-energy electromagnetic radiation (photons) emitted in an attempt by the radionuclide to become stable, i.e., radioactive decay. Gamma rays have moderate-to-high penetrating power, are often able to penetrate deep into the body, and generally require some form of shielding, such as lead or concrete. Visible light is also in the form of photons. Gamma photons behave similarly to light, but they are invisible.

gray: Gray (Gy) is the unit in the International System of Units to replace the rad (see rad).

half-life: Also called radiological or physical half-life, this is the amount of time it takes for half of the radioactivity in a material to be gone or to decay. The half-life of a radionuclide can be fractions of a second or up to millions of years. As an example, the half-life of iodine-131 is eight days. If we start with 10 radioactivity units of iodine-131, after eight days we have five radioactivity units, or half the amount we started with. After eight more days (16 total), we would have 2½ radioactivity units left, or a fourth of what we started with. Physical or radioactive half-life refers to reduction of radioactivity by radioactive decay, biological half-life refers to elimination of internal radioactivity by biological processes, and effective half-life is a combination of radioactive decay and biological elimination.

health effect: See observable health effect.

ionization: Ionization is the process by which a neutral atom (an atom with no charge) gains a positive or negative charge.

irradiate: To irradiate is the act of exposing someone or something to radiation. Irradiated food means food that was exposed to radiation.

monitoring: Monitoring is the act of using instruments to look for radiation and is also the measurement of radiation exposure levels or radionuclide quantities.

neutron: A neutron is an elementary particle that is electrically neutral and found in the nucleus of every atom except hydrogen-1. Neutrons have no electric charge and are usually highly penetrating.

observable health effect: An observable health effect is a change in physical health that can be detected medically. Observable health effects may include changes in blood cell counts, skin reddening, cataracts, etc. Whether or not it is an observable harmful health effect depends on whether damage to the body has occurred and whether that damage impairs how the body is able to function.

personal radiation dosimeter: More widely known as a radiation “badge,” a personal radiation dosimeter is a device worn by an individual to determine the radiation dose received by the individual.

rad: Rad is the term used to describe absorbed radiation dose. It describes a specific amount of energy absorbed in a medium (human tissue, for example). In the International System of Units, the gray (Gy) describes absorbed radiation dose. One gray is equal to 100 rad.

radiation: Radiation is a term commonly used to describe ionizing radiation (i.e., x and gamma rays, alpha and beta particles, neutrons). Ionizing radiation is radiation that is capable of producing ions by passing through matter.

radiation-measuring instrument: A radiation-measuring instrument is a device or system used to tell if radiation is present and/or to tell how much and what type of radiation is present. Common instruments are Geiger-Mueller detectors (GM), scintillation detectors (such as sodium iodide detectors), and ion chambers.

radiation weighting factor: Radiation weighting factors are dimensionless factors developed for radiation protection and to assess health risks from radiation doses that take into account the biological effectiveness of different types of radiation.

radioactive decay: Radioactive decay is the spontaneous transformation of one nuclide into another nuclide or different energy state; in the transformation process, radiation is emitted. The process by which radionuclides lose that radiation and, thus, the amount of radioactivity they have is called radioactive decay.

radioactive material: Radioactive material is material that contains radioactivity and thus emits ionizing radiation. It may be material that contains natural radioactivity from the environment or a material that may have been made radioactive (see radioactivity).

radioactivity: Radioactivity is the property of a nucleus in unstable atoms that causes them to spontaneously release energy in the form of photons (e.g., gamma rays) or subatomic particles (e.g., alpha or beta particles).

radiological: Radiological is a general term pertaining to radiation and radioactive materials.

radionuclide: A radionuclide is a radioactive element, man-made or from natural sources, with a specific atomic weight.

range (of an instrument): The range of an instrument describes all values lying between the detection limits of the instrument. The lower detection limit is the minimum statistically quantifiable instrument response or reading. The upper detection limit is the maximum level at which the instrument meets the required accuracy.

rem: Rem is the term used to describe equivalent or effective radiation dose. In the International System of Units, the sievert (Sv) describes equivalent or effective radiation dose. One Sievert is equal to 100 rem. It is a unit that is the product of energy absorbed in human tissues and the quality of the radiation being absorbed (the ability of the radiation to cause damage).

risk: Risk is defined in most health-related fields as the probability or odds of incurring injury, disease, or death.

roentgen (R): The roentgen (R) is the term used to describe radiation exposure. This term for exposure only describes the amount of ionization in air. In the International System of Units, the coulomb/kilogram (c/kg) describes radiation exposure. One roentgen is equal to 2.58 x 10 -4c/kg.

safe: Safe, as it is being used in the information on this Web site, is defined as an activity that is generally considered acceptable to us. This is not to say there is absolutely no risk with an activity that is considered safe; there may be a risk from the activity or the exposure to radiation, but it is the same or lower than the risks from everyday actions. At a level of radiation that is considered safe, an effect is either nonexistent or too small to observe.

sealed source: A sealed source is a radioactive source sealed in a container (e.g., a tightly welded metal capsule) that does not allow the radioactive substance (contamination) to escape.

sievert: Sievert (Sv) is the unit in the International System of Units to replace the rem (see rem).

stochastic effects: Stochastic effects are chance effects—like rolling the dice. Radiation, like many other things, has the ability to cause cancer in cells and the chance of cancer increases as radiation exposure increases.

terrorism: Terrorism is the unlawful use of force against individuals or property to intimidate a government, the civilian population, or any segment of the population in the furtherance of political objectives.

therapeutic: Therapeutic describes the medical treatment of disease or disorders. With respect to radiation therapy, therapeutic doses (e.g., external beam treatments for tumors, radioiodine treatment for thyroid disorders) are significantly greater than those received from diagnostic procedures (e.g., chest x rays, CT scans, nuclear medicine procedures, etc).

threshold: Threshold is the point at which radiation first produces an observable effect (response) from acute exposures.

x rays: X rays are electromagnetic radiation (photons) that can be emitted from radionuclides or from certain types of devices. Generally, x rays have lower energies than gamma rays, but like gamma rays, x rays can penetrate into the body. Sometimes lead or concrete may be used as a shielding material to reduce the penetration of x rays.