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Radiation Basics

© Centers for Disease Control & Prevention

What Is Radiation?

Radiation is energy that comes from a source and travels through space and may be able to penetrate various materials. Light, radio, and microwaves are types of radiation that are called nonionizing radiation. The kind of radiation discussed in this document is called ionizing radiation because it can produce charged particles (ions) in matter.

Ionizing radiation is produced by unstable atoms. Unstable atoms differ from stable atoms because unstable atoms have an excess of energy or mass or both. Radiation can also be produced by high-voltage devices (e.g., x-ray machines).

Unstable atoms are said to be radioactive. In order to reach stability, these atoms give off, or emit, the excess energy or mass. These emissions are called radiation. The kinds of radiation are electromagnetic (like light) and particulate (i.e., mass given off with the energy of motion). Gamma radiation and x rays are examples of electromagnetic radiation. Gamma radiation originates in the nucleus while x rays come from the electronic part of the atom. Beta and alpha radiation are examples of particulate radiation.

Interestingly, there is a "background" of natural radiation everywhere in our environment. It comes from space (i.e., cosmic rays) and from naturally occurring radioactive materials contained in the earth and in living things.

Radiation Exposure From Various Sources

Source Exposure
External Background Radiation 0.60 mSv y-1, U.S. Average
Natural K-40 and Other Radioactivity in Body 0.4 mSv y-1
Air Travel Round Trip (NY-LA) 0.05 mSv
Chest X-Ray Effective Dose 0.10 mSv per view
Radon in the Home 2.00 mSv y-1 (variable)
Man-Made (medical x rays, etc.) 0.60 mSv y-1 (average)

What Types of Radiation Are There?

The radiation one typically encounters is one of four types: alpha radiation, beta radiation, gamma radiation, and x radiation. Neutron radiation is also encountered in nuclear power plants and high-altitude flight and is emitted from some industrial radioactive sources.

  • Alpha Radiation
    Alpha radiation is a heavy, very short-range particle and is actually an ejected helium nucleus. Some characteristics of alpha radiation are:
    1. Most alpha radiation is not able to penetrate human skin.
    2. Alpha-emitting materials can be harmful to humans if the materials are inhaled, swallowed, or absorbed through open wounds.
    3. A variety of instruments has been designed to measure alpha radiation. Special training in the use of these instruments is essential for making accurate measurements.
    4. A thin-window Geiger-Mueller (GM) probe can detect the presence of alpha radiation.
    5. Instruments cannot detect alpha radiation through even a thin layer of water, dust, paper, or other material, because alpha radiation is not penetrating.
    6. Alpha radiation travels only a short distance (a few inches) in air, but is not an external hazard.
    7. Alpha radiation is not able to penetrate clothing.
    Examples of some alpha emitters: radium, radon, uranium, thorium.

  • Beta Radiation
    Beta radiation is a light, short-range particle and is actually an ejected electron. Some characteristics of beta radiation are:
    1. Beta radiation may travel several feet in air and is moderately penetrating.
    2. Beta radiation can penetrate human skin to the "germinal layer," where new skin cells are produced. If high levels of beta-emitting contaminants are allowed to remain on the skin for a prolonged period of time, they may cause skin injury.
    3. Beta-emitting contaminants may be harmful if deposited internally.
    4. Most beta emitters can be detected with a survey instrument and a thin-window G-M probe (e.g., "pancake" type). Some beta emitters, however, produce very low-energy, poorly penetrating radiation that may be difficult or impossible to detect. Examples of these difficult-to-detect beta emitters are hydrogen-3 (tritium), carbon-14, and sulfur-35.
    5. Clothing provides some protection against beta radiation.
    Examples of some pure beta emitters: strontium-90, carbon-14, tritium, and sulfur-35.

  • Gamma and X Radiation
    Gamma radiation and x rays are highly penetrating electromagnetic radiation. Some characteristics of these radiations are:
    1. Gamma radiation or x rays are able to travel many feet in air and many inches in human tissue. They readily penetrate most materials and are sometimes called "penetrating" radiation.
    2. X rays are like gamma rays. X rays, too, are penetrating radiation. Sealed radioactive sources and machines that emit gamma radiation and x rays, respectively, constitute mainly an external hazard to humans.
    3. Gamma radiation and x rays are electromagnetic radiation like visible light, radiowaves, and ultraviolet light. These electromagnetic radiations differ only in the amount of energy they have. Gamma rays and x rays are the most energetic of these.
    4. Dense materials are needed for shielding from gamma radiation. Clothing provides little shielding from penetrating radiation, but will prevent contamination of the skin by gamma-emitting materials.
    5. Gamma radiation is easily detected by survey meters with a sodium iodide detector probe.
    6. Gamma radiation and/or characteristic x rays frequently accompany the emission of alpha and beta radiation during radioactive decay.
    Examples of some gamma emitters: iodine-131, cesium-137, cobalt-60, radium-226, and technetium-99m.

The International System of Units (SI) for radiation measurement is now the official system of measurement and uses the "gray" (Gy) and "sievert" (Sv) for absorbed dose and equivalent dose, respectively.

In the United States, radiation absorbed dose, dose equivalent, and exposure used to be measured and stated in traditional units called rad, rem, or roentgen (R), respectively.

For practical purposes with gamma and x rays, these units of measure for exposure or dose are considered equal. Exposure can be from an external source irradiating the whole body, an extremity, or other organ or tissue resulting in an external radiation dose. Alternately, internally deposited radioactive material may cause an internal radiation dose to the whole body or other organ or tissue.

Smaller fractions of these measured quantities often have a prefix, e.g., milli (m) means 1/1,000. For example, 1 Sv = 1,000 mSv. Micro (µ) means 1/1,000,000. So, 1 Sv = 1,000,000 µSv.

Conversions are as follows:

  • 1 Gy = 100 rad
  • 1 mGy = 100 mrad
  • 1 Sv = 100 rem
  • 1 mSv = 100 mrem

How Much Radioactive Material Is Present?

The size or weight of a quantity of material does not indicate how much radioactivity is present. A large quantity of material can contain a very small amount of radioactivity, or a very small amount of material can have a lot of radioactivity.

For example, uranium-238, with a 4.5-billion-year half-life, has only 5.5 MBq of activity per pound, while cobalt-60, with a 5.3-year half-life, has nearly 19,000 TBq of activity per pound. This "specific activity," or curies per unit mass, of a radioisotope depends on the unique radioactive half-life and dictates the time it takes for half the radioactive atoms to decay.

The SI system uses the unit of becquerel (Bq) as its unit of radioactivity. The older, traditional unit previously used in the United States is the curie (Ci).

Common multiples of the becquerel are the megabecquerel (1 MBq = 1,000,000 Bq) and the gigabecquerel (1 GBq = 1,000,000,000 Bq).

One curie is 37 billion Bq. Since the Bq represents such a small amount, one is likely to see a prefix noting a large multiplier used with the Bq as follows:

  • e
  • 1 MBq = 1 million Bq = ~ 27 microcuries
  • 1 GBq = 1 billion Bq = ~ 27 millicuries
  • 1 TBq = 1 trillion Bq = ~ 27 curies

What Is Radioactive Contamination?

If radioactive material is not in a sealed source container, it might be spread onto other objects. Contamination occurs when material that contains radioactive atoms is deposited on materials, skin, clothing, or any place where it is not desired. It is important to remember that radiation does not spread or get "on" or "in" people; rather, it is radioactive contamination that can be spread. A person contaminated with radioactive material will receive radiation exposure until the source of radiation (the radioactive material) is removed.

  • A person is externally contaminated if radioactive material is on the skin or clothing.
  • A person is internally contaminated if radioactive material is breathed in, swallowed, or absorbed through wounds.
  • The environment is contaminated if radioactive material is spread about or is unconfined.

Is It Safe to Be Around Sources of Radiation?

A single high-level radiation exposure (i.e., greater than 100 mSv) delivered to the whole body over a very short period of time may have potential health risks. From follow-up of the atomic bomb survivors, we know acutely delivered very high radiation doses can increase the occurrence of certain kinds of disease (e.g., cancer) and possibly negative genetic effects. To protect the public and radiation workers (and environment) from the potential effects of chronic low-level exposure (i.e., less than 100 mSv), the current radiation safety practice is to prudently assume similar adverse effects are possible with low-level protracted exposure to radiation. Thus, the risks associated with low-level medical, occupational, and environmental radiation exposure are conservatively calculated to be proportional to those observed with high-level exposure. These calculated risks are compared to other known occupational and environmental hazards, and appropriate safety standards and policies have been established by international and national radiation protection organizations (e.g., International Commission on Radiological Protection and National Council on Radiation Protection and Measurements) to control and limit potential harmful radiation effects.

Both public and occupational regulatory dose limits are set by federal agencies (i.e., Environmental Protection Agency, Nuclear Regulatory Commission, and Department of Energy) and state agencies (e.g., agreement states) to limit cancer risk. Other radiation dose limits are applied to limit other potential biological effects with workers' skin and lens of the eye.

Annual Radiation Dose Limits Agency
Radiation Worker - 50 mSv (NRC, "occupationally" exposed)
General Public - 1 mSv (NRC, member of the public)
General Public - 0.25 mSv (NRC, D&D all pathways)
General Public - 0.10 mSv (EPA, air pathway)
General Public - 0.04 mSv (EPA, drinking-water pathway)

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