Artificial ƒ¿-emitters: Presumably the ICRP will tighten this definition as it currently excludes some naturally occurring isotopes of elements such as: plutonium, uranium, protoactinium, thorium, actinium, radon, radium, polonium, bismuth, gadolinium, samarium, neodymium, thallium, and lead. The inclusion of plutonium in this list as a naturally occurring nuclide naturally needs further expansion. The Oklo natural reactor found in Gabon, West Africa, achieved nuclear criticality approximately 1.7 billion years ago and operated over a million years. Analysis of the area indicates that 239Pu was produced in measurable quantities showing that uranium is not the heaviest naturally occurring element known on the Earth (Cowan 1976).
Artificial ƒÀ/ƒÁ emitters: Similarly this definition could be taken to ignore the beta emitters of the uranium and thorium chains. Both 3H and 14C are produced in the upper atmosphere and this could be argued as a natural process adding these nuclides not covered by the values in Table S2. Other naturally occurring beta emitters include isotopes of rubidium, protoactinium, bismuth, lead, thallium, francium, thorium, actinium and radium.
Head of chain activity level, 238U,232Th: The exclusion level is set at 100 times the artificially produced ƒ¿-emitters implying that alpha particles from these nuclides are less harmful by a similar scaling factor.
40K: Potassium is the 7th most abundant element on the earthfs surface and plays an integral part in life. Reference Man (ICRP 1975) contains 140 g of potassium of which 0.0117% is the radioactive 40K, which means there is about 17 mg of 40K. The activity of 17 mg 40K is approximately 4.4 kBq (or 0.06 Bq g-1) which gives an annual dose of ~0.2 mSv. Comparing these values with Table S2 one sees that the 40K in humans is well below the proposed exclusion level of 10 Bq g-1 but is 20 times large than the IAEA's 10 ƒÊSv exclusion dose. If one assumes that the exclusion dose is harmless, then is a dose that is 20 times larger still harmless? It appears so as we live with this normally.
Potassium chloride can be found in large quantities in stores selling materials for water treatment. The potassium content is about 500 g kg-1. Typically, the material is sold in 20 kg bags so each bag contains ~600 kBq of 40K giving a concentration of 30 Bq g-1. This is well above the exclusion level yet the material is handled as non-radioactive. The external dose rate in close proximity to a typical display in these types of shops would be about 150 ƒÊSv hr-1. A worker would only need to be near the pile for about 7 hours to exceed the public dose limit of 1 mSv.
Contrast 40K with the other exclusion level for ƒÀ/ƒÁ emitters (i.e., 0.1 Bq g-1). When one considers that foods like bananas and potatoes exceed this level (~0.16 Bq g-1) it becomes clearer why the ICRP had to make a special case for 40K (the IAEA exempted 40K within the human body from consideration). The question remains why the emissions of 40K (ƒÀ: 1.31 MeV max and 0.51 MeV ave; ƒÁ: 1.46 MeV) can be considered to be less harmful than other beta emitters by a factor of a 100?
Risk Estimates: Assuming an acceptable risk of death due to a practice to be one in a million then one can rate various activities common to our society.
œ Smoking 1.4 cigarettes (lung cancer).
œ Eating 40 tablespoons of peanut butter.
œ Spending 2 days in New York City (air pollution).
œ Driving 40 miles in a car (accident).
œ Flying 2500 miles in a jet (accident).
œ Canoeing for 6 minutes.
œ Receiving 30 ƒÊSv of radiation (cancer) from chest X ray (Hall 1994).
The data can be expressed in terms of life expectance lost for different causes (Cohen 1991):
œ Smoking 20 cigarettes a day - 2,200 days
œ Overweight (15%) - 730 days
œ Alcohol (US Ave) - 365 days
œ All Accidents - 207 days
œ All Natural Hazards - 7 days
œ All Industries - 60 days
œ Agriculture - 320 days
œ Construction - 227 days
œ Mining and quarrying - 167 days
œ Manufacturing - 40 days
œ Public dose limit (chronic 1 mSv yr-1) - 5 days
œ Occupational dose limit (chronic 20 mSv yr-1) - 102 days
œ Exclusion limit (chronic 10ƒÊSv yr-1) - 0.05 days
The exclusion level appears to be connected to the one in a million risk estimates although the reference above (Hall 1994) suggests that this value is 30 ƒÊSv. This can be explained as follows: UNSCEAR, using the same approach taken in their 1994 report (UNSCEAR 1994), namely an age-at exposure model applied to a Japanese population of all ages, determined that the lifetime risk of exposure-induced death from all solid cancers combined following an acute dose of 1 Sv was estimated to be about 9% for men, 13% for women and 11% averaged over genders.
Assuming the dose response to be linear over the entire range (zero to several Sv), then a one in a million risk (i.e., 0.0001%) corresponds to 9.1 ƒÊSv; however, this assumption continues to be challenged. For example, the Academy of Medicine of France in its December 2001 statement denounced the utilisation of the linear approach to within a few mSv. Similarly, epidemiological studies reported so far in areas of high natural background radiation have pointed to a lack of increased incidence of cancers and related genetic disorders (Nambi 1994, Sugahara et al 1992, Rose 1982, Gillespie 1991, Sjernfeldt et al 1987, Nambi 1987, and Taylor 1992) indicating that people residing in these areas have either an enhanced resistance (inherent or acquired) to radiation damage or an increased capacity for DNA repair or, more likely, the dose response is not linear as suggested by the French Academy of Medicine.
Conclusions: The exclusion values recommended by the ICRP are too restrictive and too generalised. As a more balanced approach, the IAEA has created exclusion limits that are isotope specific with values that range from 0.1 Bq g-1 to 10,000 Bq g-1 but a discussion of these values is beyond the scope of these comments.
The assumption that any radiation dose can be fatal seems unfounded and when one compares small risks and translates these into radiation doses. The assumption seems to breakdown when one approaches small and protracted dose values. Therefore, exclusion levels based on 10 ƒÊSv, appear too conservative when put in perspective with other risks encountered in life.
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