Title 40
PART 197 APPENDIX A
where
DT,R is the absorbed dose in rads (one gray, an SI unit, equals 100
rads) averaged over the tissue or organ, T, due to radiation type,
R, and wR is the radiation weighting factor which is given in Table
A.1 below. The unit of equivalent dose is the rem (sievert, in SI
units).
| Radiation type and energy range 2 | wR value |
|---|---|
| Photons, all energies | 1 |
| Electrons and muons, all energies | 1 |
| Neutrons, energy | |
| <10 keV | 5 |
| 10 keV to 100 keV | 10 |
| >100 keV to 2 MeV | 20 |
| >2 MeV to 20 MeV | 10 |
| >20 MeV | 5 |
| Protons, other than recoil protons, >2 MeV | 5 |
| Alpha particles, fission fragments, heavy nuclei | 20 |
1 All values relate to the radiation incident on the body or, for internal sources, emitted from the source.
2 See paragraph A14 in ICRP Publication 60 for the choice of values for other radiation types and energies not in the table.
The next step is the calculation of the effective dose equivalent, E. The probability of occurrence of a stochastic effect in a tissue or organ is assumed to be proportional to the equivalent dose in the tissue or organ. The constant of proportionality differs for the various tissues of the body, but in assessing health detriment the total risk is required. This is taken into account using the tissue weighting factors, wT in Table A.2, which represent the proportion of the stochastic risk resulting from irradiation of the tissue or organ to the total risk when the whole body is irradiated uniformly and HT is the equivalent dose in the tissue or organ, T, in the equation:
Table A.2 - Tissue weighting factors, wT
| Tissue or organ | wT value |
|---|---|
| Gonads | 0.20 |
| Bone marrow (red) | 0.12 |
| Colon | 0.12 |
| Lung | 0.12 |
| Stomach | 0.12 |
| Bladder | 0.05 |
| Breast | 0.05 |
| Liver | 0.05 |
| Esophagus | 0.05 |
| Thyroid | 0.05 |
| Skin | 0.01 |
| Bone surface | 0.01 |
| Remainder | a b 0.05 |
a Remainder is composed of the following tissues: adrenals, brain, extrathoracic airways, small intestine, kidneys, muscle, pancreas, spleen, thymus, and uterus.
b The value 0.05 is applied to the mass-weighted average dose to the Remainder tissues group, except when the following “splitting rule” applies: If a tissue of Remainder receives a dose in excess of that received by any of the 12 tissues for which weighting factors are specified, a weighting factor of 0.025 (half of Remainder) is applied to that tissue or organ and 0.025 to the mass-averaged committed equivalent dose equivalent in the rest of the Remainder tissues.
For internal irradiation from incorporated radionuclides, the total absorbed dose will be spread out in time, being gradually delivered as the radionuclide decays. The time distribution of the absorbed dose rate will vary with the radionuclide, its form, the mode of intake and the tissue within which it is incorporated. To take account of this distribution the quantity committed equivalent dose, HT(τ) where τ is the integration time in years following an intake over any particular year, is used and is the integral over time of the equivalent dose rate in a particular tissue or organ that will be received by an individual following an intake of radioactive material into the body:
for a
single intake of activity at time t0 where HT(τ) is the relevant
equivalent-dose rate in a tissue or organ at time t. For the
purposes of this rule, the previously mentioned single intake may
be considered to be an annual intake. IV. Internal Component of the
Annual Committed Effective Dose Equivalent
If the annual committed equivalent doses to the individual tissues or organs resulting from an annual intake are multiplied by the appropriate weighting factors, wT, from table A.2, and then summed, the result will be the internal component of the annual committed effective dose equivalent E(τ):
[73 FR
61288, Oct. 15, 2008]