Many units have been used to express different amounts of radiation. The units often used follow:

• roentgen (R)
• rad
• gray (Gy)
• rem
• sievert (Sv)

These units relate to radiation exposure, radiation dose, or radiation protection. Other units (curie, becquerel) relate to radioactivity.

The rad, rem, roentgen, and curie are special radiation units. These units are older than the gray, sievert, and becquerel.

The International System of Units (or SI units) includes the gray, sievert, and becquerel:

• The gray is the SI unit used for absorbed dose (see Section 2.2).
• The sievert is the SI unit used for the dose equivalent and for the effective dose equivalent (see Section 2.4).
• The becquerel is the SI unit used for radioactivity (see Section 2.3).

The roentgen describes the amount of x-rays or gamma rays to which a target (e.g., fly, mouse, rat, dog, human, cow, elephant, etc.) is exposed. The roentgen relates to the ability of x-rays and gamma rays to remove electrons from atoms in air. One roentgen corresponds to 2.58 x 10^-4 coulombs per kilogram of air.

The radiation absorbed dose is important for describing radiation effects. The absorbed dose relates to how much radiation energy gets put into a given target mass (e.g., lung, eye, thyroid gland).

The absorbed dose has units of energy divided by mass (e.g., ergs per gram or joules per kilogram). It better measures harm to organs and tissue in the body than the exposure in R.

Different absorbed doses can arise in different organs or tissue of the body for the same exposure in R.  Thus, if a person were exposed to 10 R of gamma rays, the eye, the thyroid, and the lung would have different absorbed doses. Special computer programs can calculate such doses.

Units of absorbed dose often used are the rad and gray (an SI unit).

The rad is a relatively old unit of absorbed dose.  One rad corresponds to 100 ergs of radiation energy per gram of target substance. An exposure of a fly to 1 roentgen of gamma rays results in an absorbed dose of about one rad to the total body of the fly.

The gray unit represents 1 joule of radiation energy put into a kilogram mass. Thus, 1 gray equals 1 joule per kilogram.

The gray and rad apply to all types of ionizing radiation, unlike the roentgen unit, which only applies to x-rays and gamma rays.

Some useful conversion factors that relate to absorbed dose follow:

Radioactivity arises from the disintegration of unstable atoms and is expressed in units like the becquerel (Bq) and curie (Ci).

One curie represents 37,000,000,000 (i.e., 3.7 x 10¹⁰) disintegrations per second. A curie is a very large amount of radioactivity.

• 1 megacurie (MCi) = 10⁶ curies (Ci).
• 1 kilocurie (kCi) = 10³ curies (Ci).
• 1 millicurie (mCi) = 10^-3 curies (Ci).
• 1 microcurie (µCi) = 10^-6 curies (Ci).
• 1 nanocurie (nCi) = 10^-9 curies (Ci).
• 1 picocurie (pCi) = 10^-12 curies (Ci).
• 1 femtocurie (fCi) = 10^-15 curies (Ci).
• 1 microcurie (µCi) = 37,000 becquerels (Bq)
• 1 nanocurie (nCi) = 37 becquerels (Bq)
• 1 picocurie (pCi) = 0.037 becquerels (Bq)

Special dosimetric units are used in radiation protection to limit radiation exposure of nuclear workers and the public. These units include the rem and sievert (SI unit), which apply to single and mixed radiations and are measures of potential harm to humans.

One rem of alpha radiation would be expected to pose the same risk of harm as 1 rem of gamma rays or as 1 rem of combined exposure to neutrons and gamma rays.

The rem and Sv were developed to account for different efficiencies of different types of radiation in producing harm.  Because these units apply to single and mixed radiations, it follows that:

Law of the rem (introduced here for the first time to help understand its use):

Law of the sievert (introduced here for the first time to help understand its use):

Using exposure limits in rem or sievert for the public and for nuclear workers has protected many lives around the world for many years. Exposure limits are much lower for the public than for nuclear workers.

• dose equivalent (applies to single organ)
• committed dose equivalent (applies to single organ)
• effective dose equivalent (applies to total body)
• committed effective dose equivalent (applies to total body)

The dose equivalent is a quantity that accounts for the different efficiencies of different external radiations in causing harm to a given organ or tissue. Special weighting factors (e.g., quality factor) are used to account for differences in radiation quality. The term "external radiation" simply means that radiation originates from outside the body. Examples are gamma rays from contaminated soil or from a radioactive cloud.

Radiation quality is a measure of the potential for causing harm.   The larger the quality factor is, the greater the chance for harm from a given absorbed radiation dose.

The committed dose equivalent is similar to the dose equivalent but applies to doses from radionuclides taken inside the body. Quality factors are used to account for different efficiencies of different radiations in producing biological damage internally. Committed doses are evaluated to some future time (e.g., to 50 years) after intake of radionuclides.

Effective dose equivalents account for different biological sensitivities of different organs and tissue and apply to the total body. They also account for different doses (dose equivalents) to different organs.  Effective dose equivalents specifically apply to external radiation sources (i.e., sources outside the body). 

Committed effective dose equivalents also account for different biological sensitivities of different organs and tissue but apply only to radionuclides that enter the body (e.g., via inhalation or ingestion). A committed dose equivalent of 1 Sv from inhaled alpha particle emitters would represent the theoretical committed dose of gamma rays to the total body that would yield the same risk of harm from stochastic effects such as cancer. Committed dose equivalents are evaluated to a fixed time in the future (e.g., 50 years) after intake of radionuclides.

Recently, the dose equivalent has been replaced by a closely related concept called the equivalent dose. These doses should not be used for evaluating risks for any biological effects that have threshold doses.  A threshold dose is one below which no harm occurs.

The equivalent dose is obtained by multiplying the absorbed dose by special factors called radiation weighting factors (W_R) that are intended to account for different efficiencies of the different radiations in producing biological damage. Like its predecessor the quality factor, the weighting factor W_R was designed to protect against radiation-induced harm (mainly from cancer induction). The factors W_R were not designed to evaluate risks for nuclear-weapons-related or radiological-weapons-related scenarios that involve threshold-type biological effects.

Radiation weighting factors, W_R, currently used for alpha radiation, beta radiation, gamma rays, and x-rays follow:

For neutrons, W_R depends on neutron energy. The following formula can be used where the neutron energy, E, is in millions of electron volts.

What is the equivalent dose for combined exposure of the cornea (of the eye) to 0.1 Gy of alpha radiation plus 0.2 Gy of gamma rays? The answer follows:

The committed equivalent dose is also based on the use of radiation weighting factors W_R (unlike the committed dose equivalent that is based on quality factors). It applies to radionuclides that enter the body by inhalation, ingestion, or through wounds. The radionuclide can continue to irradiate the body so that the radiation dose is committed over time.

The committed equivalent dose usually is evaluated for a fixed period (e.g., 50 years). Computer programs or dose conversions factors are used to obtain the committed equivalent dose.

Dose conversion factors are used to convert radioactivity taken into the body into a radiation dose (e.g., committed equivalent dose) for a given organ or tissue.

The effective dose (which replaces the effective dose equivalent) is obtained by multiplying the organ/tissue-specific equivalent dose with additional weighting factors that account for different sensitivities of different organs/tissue to cancer induction. These factors are called tissue weighting factors (W_T). The products are then added to get a single effective dose for the total body. This single dose in theory corresponds to the uniform dose of gamma rays to the total body that would incur the same risk for stochastic effects (mainly cancer).

Table 2.1  Effective radiation doses during air travel

U.S. astronauts in Earth orbit or on Moon missions received modest effective doses. This can be seen from estimated effective doses for Apollo Missions provided in Table 2.2 based on a report of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) entitled Ionizing Radiation: Sources and Biological Effects.

Table 2.2.  Effective doses (estimates) received by Astronauts on Apollo Missions.

Neither the effective dose equivalent nor effective dose should be used in evaluating the risk for threshold-type effects (e.g., death from destruction of bone marrow).