w T = tissue weighing factor for the relevant organ or tissue T
D T = absorbed dose in the tissue or organ of interest
Q T = the quality factor averaged over the tissue or organ of interest.
The weighing factors are given in Table 6.2, which is taken from 10 CFR 835
(DOE, 1998a). Effective dose equivalent has the benefit that it is additive, and
internal and external radiations can be added numerically to drive an overall
estimate of risk.
Table 6.2. Tissue Weighing Factors
Tissue or Organ
Tissue Weighing Factor, WT
Bone marrow (red)
(a) Remainder means the five other organs or tissues with the highest dose (e.g.,
liver, kidney, spleen, thymus, adrenal, pancreas, stomach, small intestine, and
upper large intestine). The weighing factor for each remaining organ is 0.06.
(b) For the case of uniform external irradiation of the whole body, a weighing factor
equal to 1 may be used in determining the effective dose equivalent.
The methodology of ICRP Publication 26 (ICRP, 1977) has been incorporated
into 10 CFR 835 (DOE, 1998a), and into the standard, Radiological Control, Ch.
1. (DOE, 2004). The annual radiation dose limits for DOE and its contractors are
presented in Table 6.3.
However, DOE contractors usually establish lower annual administrative control
levels, typically 500 mrem/year.
In practice, it is very difficult to measure the effective dose equivalents specified
in Table 6.3 because it is necessary to know not only the type of radiation but
also its energy and direction. If the flux, energy, and direction of incidence are
known, it is possible to calculate effective dose equivalent using fluence to
effective dose equivalent conversion coefficients presented in ICRP Publication
51 (ICRP, 1987), which presents the effective dose equivalent as a function of
energy for various irradiation geometries. Conversion coefficients for
monodirectional beams of neutrons can be found in an article by Stewart et al.
(1994). Conversion coefficients for photons in various irradiation geometries,
including planar sources, can be found in a report by the Zankl et al. (1994). This
will provide more accurate values of effective dose equivalent as opposed to
numerically setting the value of effective dose equivalent equal to dose