Radiation doses to the human body
Widely varying radon levels exist in different regions related to geological circumstances. New concern regarding radon exposures is traceable to the discovery that there are more houses with high radon levels than previously realised and to the use of a new method of expressing and summing doses from partial body exposures, such as the lung dose from radon daughters.
This method of expressing dose was promulgated by the ICRP and the NCRP based on defined weighting factors which make it possible to sum partial body doses and thereby estimate a total body dose which would have a quantifiable risk.
This quantity is defined as the Effective Dose (ED). Thus, the previously estimated partial body environmental radon dose to the tracheobronchial epithelium (TBE) (2500 mrem/year) was not included in whole body dose calculations because that exposure was limited to a small fraction of the body. The new method of calculation multiplies the 25 mSv/year dose to the TBE by a weighting factor (WF) which allows the dose to the TBE to be included in the Effective Dose from environmental radiation exposure.
Different WFs have been proposed, including 0.12 (EPA), 0.08 (NCRP) and (NAS-NRC BEIR V), and 0.06 (ICRP). These WFs raise the radon contribution to the whole body from 0 mSv to 3, 2, and 1.5 mSv respectively. NCRP quotes an uncertainty of +/- 50% in these numbers. Based on these estimates, radon in equilibrium with its daughters delivers 2 times more dose than previously accepted as the total dose received from all sources of natural background exposure (approx. 1 mSv/year on average in the United Kingdom).
Thus, it is not surprising that adoption of the effective dose notion by many radiation protection groups (including the NRPB, NCRP and the EPA in the United States), has led to increased concern regarding the potential health effects of radon. It should be noted that lung cancer risk coefficients from radon have not increased. There are no new cases of lung cancer that led to the increased dose estimate. In fact, the new estimates of radiation dose, imply a lower risk coefficient. That is, when the same number of lung cancer cases that occur are attributed to the higher doses (ED), the risk per unit exposure is decreased.