December 23, 2004
Notes on the ICRP 2005 Draft Recommendations
There seem to be potential for confusion in the ICRP 2005 Draft Recommendations
1. Potential sources of confusion
• ICRP 2005, like previous ICRP reports, assumes that the dose-effect relationship is linear without thresholds (LNT). Available evidence of non-linear and threshold responses should be discussed with an open mind.
• The overall radiobiological effect is a function of the total dose of radiation (natural, medical, NORM, enhanced NORM, etc.), not of the controllable dose alone. The relative contribution of “controllable” exposures to the overall theoretical radiological risk should be made explicit.
o Suggestion: Recommend that the contribution of “controllable” doses to the total dose of radiation received by individual or populations be indicated in risk estimation. Give examples of the relative contribution of all-inclusive natural radiation levels to the overall theoretical radiation risk.
2. Difficulties with the LNT
a. “Black box” at low doses.
In his presentation in Ottawa (1 November 2005), Dr. L-E. Holm indicated that the ICRP retains the notion of LNT, with a “black box” at the lowest dose level, within which the shape of the dose response is unknown. Although not mentioned in ICRP 2005, the “black box” notion is a source of confusion. If the dose response departs from linearity, there is necessarily a portion of the dose-response curve in the black box for which the slope is greater than that of the explicit LNT line outside of the “black box”. LNT does not apply in the black box, at lowest cumulative doses.
b. With LNT, inevitably, individual and collective risks will be calculated based on the LNT hypothesis and risk coefficients (risk/Sv). In fact, optimization and dose reduction require such calculations to be justified. This leads to a logical difficulty: one side of the equation (dose) is real; the other (risk) is hypothetical and is derived from the controllable dose, which is often a small fraction of the total dose, which is responsible for the overall observed or hypothetical risk.
1. With mounting biological indications of non-linearity and thresholds at low doses and mostly at low dose rates, the ICRP should recommend a range of risk coefficients (risk/Sv) as a function of dose (and dose rate) that starts at zero. Risk estimates calculated from a range of risk coefficients would provide a more realistic range of risk estimates and would provide the public and concerned groups with a more realistic perspective of radiation health effects.
2. ICRP 2005 should also include tables and geographical maps of all-inclusive natural radiation dose rates, from the lowest to the highest, in order to provide an unequivocal perspective of where the recommended dose limits lie in comparison with the natural environment.
3. Protection of the biota
Although is a good intention to protect non-human species, risk estimates in that domain must be based on the local radiological conditions that prevail at concerned sites. Assessment of the potential effect of uranium mine and mill effluents on local biota should be based on ambient radiation levels, in particular on radionuclide content in discharge areas. For example, it is known that some lake sediments, in mining sectors, have uranium content near commercial grade. The current health of local species should be studied carefully before applying discharge constraints, which may be unjustifiable by pre-existing health status of local biota. Furthermore, assessment of the well being of non-human biota should be balanced with that of humans. This is always done, implicitly or explicitly, in all types of food production, transportation (road construction, for example) and virtually in all human activities.