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their concentrations in air, food, and water. The primary dose to persons exposed
to these concentrations results from ingestion and inhalation of the radionuclides.
Absorbed Dose
A useful quantity in radiation physics is the energy actually deposited in a
certain amount (mass) of tissue. This unit is referred to as absorbed dose. The unit
of absorbed dose is the gray (Gy), formerly the rad; the gray is equivalent to the
absorption of one Joule of energy per kilogram. One gray equals 100 rad; 1
milligray (mGy) equals 100 millirad (mrad). However, the amount of energy
deposited in tissue does not account for differences in the biological effects of
different radiation types.
Equivalent Dose
The dosimetric quantity that accounts for the differences in biological
effectiveness of various types of radiation and that allows doses from different
radiations to be combined, through expressing their health effects on a common
basis, is called the equivalent dose. It is calculated by multiplying the absorbed
dose by the appropriate radiation weighting factor, "wR". For example, the factor
for alpha particles is 20 and that for gamma and beta radiation is 1, indicating that
it requires the absorption of about 20 times more energy from gamma or beta
radiation than alpha radiation to cause a given biological effect. These weighting
factors are approximate and the true value for a given type of radiation, radiation
effect, or specific population can vary by up to an order of magnitude. The unit of
equivalent dose is the sievert (Sv), formerly the rem. One sievert equals 100 rem; 1
millisievert (mSv) equals 100 millirem (mrem).
Just as different radiation types are more or less effective in damaging tissue,
different tissue types have various sensitivities to that damage. For a given
equivalent dose of radiation, the more sensitive tissues show a larger increase in

cancer and leukemia rates than do less sensitive tissues. For radiation protection
purposes, the International Commission on Radiological Protection (ICRP) has
developed weighting factors for tissues (called "wT") that describe the relative
sensitivities of different tissues to long-term effects. Tissue weighting factors
facilitate the combination of doses to allow a quantitative comparison of the long-
term risk from partial body exposure to that from total body exposure. Tissues that
are very sensitive to long-term effects from radiation have high weighting factors
(e.g., bone marrow wT= 0.12), whereas less sensitive tissues have lower weighting
factors (e.g., skin wT = 0.01).
The effective dose (that is, the dose to the whole body that represents an
equivalent risk) is estimated by multiplying the equivalent dose in each tissue type
by its corresponding tissue weighting factor and summing these weighted
equivalent tissue doses. This composite dose is proportional to the increased risk
from cancer and genetic effects. Like the equivalent dose, the effective dose is
expressed in units of sievert or millisievert. Dose limits set for occupational
exposures are expressed as effective dose and include the sum of the internal and
external doses.

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