Most of the ionizing radiation to which people are exposed comes from sources which are natural features of the environment. These sources include radon gas and its decay products in the atmosphere (originating from natural uranium in soil and rocks), gamma rays from the ground, cosmic rays from outer space, naturally-occurring radioactivity in foodstuffs and drinking water, derived from radionuclides in the soil, as well as inhalation of respirable airborne dust. The total radiation dose received by an individual, from these natural sources, is typically about 2,4 mSv/a (millisieverts per annum), but geological and geographical factors can cause doses from any one of such sources to be elevated by a factor of 10 in high-background regions [3].
In addition to radiation from natural sources, man is exposed to radiation during medical treatment (X-rays, radiotherapy and nuclear medicine). Internationally, average doses to individuals from all medical sources range from 0,07 mSv/a to 1,8 mSv/a [3].
Thus, a typical member of the public will receive, as a matter of course, a radiation dose of between 2,5 and 4,2 mSv/a. In regions with high natural background, doses of 10 mSv/a are not uncommon.
Exposure of humans, to ionizing radiation, may occur via various routes or ‘pathways’ that can be grouped simply as:
- exposures to penetrating radiation from sources external to the body, and
- exposures to both penetrating and non-penetrating radiation from radioactive substances taken into the body by ingestion, inhalation, or absorption through the skin.
Exposures from water containing radioactive contaminants essentially occur internally through ingestion, either by direct consumption or indirectly by consumption of animal or vegetable products that have themselves taken up the water.
A detailed study of the potential major ingestion pathways, relevant to the Mooi River catchment, revealed only two pathways with potential for giving rise to significant exposures ( Appendix 7).
direct ingestion resulting from regular and continuous use of the water for drinking purposes, and
regular consumption of fish obtained from contaminated water bodies.
With respect to the latter, there is very little information on the bioaccumulation rates of radionuclides in local fish species, and international experience shows that bioaccumulation can vary by as much as three orders of magnitude. The fish pathway therefore requires more research, and could not be addressed in the present study. Accordingly, the decision was taken to address only the drinking water pathway in this study.
The process of ionization changes atoms and molecules. In cells, such changes may result in damage which, if not adequately repaired, may:
- prevent the cell from surviving or reproducing, or
- result in a viable but modified cell.
The two outcomes have profoundly different implications for the organism as a whole.
In the case of the former, the loss of large numbers of cells in a tissue can result in a loss in tissue function. Such effects are known as deterministic effects, and are characterized by a dose threshold above which the probability of causing harm increases steeply from zero to 100%. Above the threshold, the severity of harm also increases with dose. Threshold doses are generally two or three orders of magnitude above background doses, and deterministic effects are thus only now seen in the case of accidents or as a side effect of medical radiation therapy.
The outcome is very different if the irradiated cell is modified rather than killed. It may then be able to produce a clone of modified daughter cells which, in spite of the highly effective defence mechanisms within the body, may cause, after a prolonged and variable delay, a malignant condition - a cancer. The probability, but not the severity, of the cancer increases with dose. This effect is called stochastic (meaning of random or statistical nature).
Epidemiological studies have shown, with good statistical significance, that this dose-response relationship is linear for accumulated doses of more than about 200 mSv. It is widely assumed that this linear relationship, with certain corrections, holds true also at lower doses, all the way down to zero - that is, there is no dose threshold for stochastic effects. This linear relationship yields, for low doses and dose rates, a nominal probability of fatal cancer induction of 5 x 10-5 per mSv. Due to the high incidence of cancer induced by other carcinogens, it will be difficult, if not impossible, to obtain conclusive epidemiological evidence supporting this linear relationship at low doses. Some evidence suggests the opposite, in that there is actually a beneficial effect.
Stochastic effects can also take the form of hereditary effects which may be of many different kinds and severity, and are expressed in the progeny of the exposed person. Although the existence of hereditary effects in man is not in doubt, the risk estimates appear to be so small that it is not surprising that epidemiology has not yet detected hereditary effects of radiation in humans with a statistically significant degree of confidence.
Notwithstanding the fact that there is no evidence of statistically significant health effects associated with exposure to low levels of radiation, the internationally accepted principle is to keep radiation exposures as low as reasonably achievable.
Internationally a system of radiation protection has been agreed upon, based on the health effects described in section 3.3. This system has been recommended by the International Commission on Radiological Protection (ICRP), which is a non-governmental scientific organization that has been publishing this and related recommendations for over half a century. Different countries evaluate and implement the recommendations in a manner that is appropriate to their circumstances.
The following recommendations of the ICRP [4] are based on the assumption that there is indeed a linear non-threshold relationship between radiation dose and the probability of contracting cancer. Central to the system of radiation protection for proposed and continuing human activities that increase exposure to radiation are the following general principles:
No activity, which results in the exposure of persons to radiation, should be adopted unless the activity produces a net positive benefit.
All radiation doses should be kept as low as reasonably achievable (ALARA), taking economic and social factors into account.
The radiation doses should not exceed limits recommended by the ICRP.
For situations where the sources of exposure are already in place and radiation protection has to be considered retrospectively, remedial action to reduce the exposures should be based on the following general principles:
The remedial action should be justified in the sense that the costs, including social costs, should be more than offset by the reduction in radiation dose likely to be achieved.
The form, scale and duration of the remedial action should be optimized so that the net benefit to society is maximized.
To apply the above principles to, for instance, radioactivity in water, it is necessary to calculate the radiation doses which result from the use of the water.
The annual radiation dose from any given radionuclide and for any given age group is expressed as:
Annual dose (mSv/a) |
= |
Activity concentration (Bq/l ) |
X |
Annual consumption (l /a) |
X |
Dose Conversion Factor (mSv/Bq) |
The total radiation dose for that age group is, then, the sum of the doses from individual radionuclides. This implies that the activity concentration of every radionuclide must be known. However, it was not feasible to measure every radionuclide, and this had to be taken into account in the calculation of age group specific doses. The method used to calculate lifetime average doses doses in this report is given in Appendix 8.
Two methods (IWQS and AEC) are presented in the Appendices for calculating the dose. Both need to address the problem that fewer nuclides were measured in the first phase than in the second. The so-called IWQS method handled this problem in two ways :-
The so called AEC method handled this problem by regressing, for the period of Phase II, those nuclides measured in Phase I onto the dose calculated from all the nuclides measured in Phase II. This regression was used to predict the dose for Phase I. The IWQS and AEC methods differed in the assumptions used of how to deal with unmeasured nuclides.
The second problem that needed to be addressed by both methods was the fact that even in Phase II, not all the nuclides in the decay chains were measured. The so-called IWQS method took a simpler approach to this for the purpose of estimating the uncertainty in the dose arising from not measuring these nuclides. The IWQS method simply assumed that all the unmeasured nuclides had the same value. This implied that the uncertainty remaining in the dose due to the unmeasured nuclides was about 0,003 mSv/a. The AEC method had a more advanced model, based on which nuclide was related to which other via a decay chain of the shortest half-life.
Each of the radionuclides in the three decay chains of interest has its own ‘dose conversion factor’ (DCF) for the ingestion pathway, relating the dose received, in mSv, to the amount of radioactivity ingested, in Bq (becquerels, or number of nuclear disintegrations per second). The DCFs used are those published by the International Atomic Energy Agency (IAEA) [5]. The IAEA gives different dose conversion factors for the various age groups. There are various ways in which the exposure dose per year for the various age groups can be combined. Investigation into the possible ways in which to combine the age groups specific doses showed that differences for the various ways of determining lifetime exposure were in fact trivial, and a "lifetime average" method was adopted for the purposes of this study.
In many solid materials such as rocks and soil, the mobility of the elements in the decay chains is limited, even over long periods of time, and the mixture of radionuclides is therefore relatively undisturbed. In such cases, the radionuclides may be said to be in secular equilibrium, meaning that all the radionuclides in a given decay chain have similar activity concentrations.
In water systems, however, the dissolution and precipitation characteristics of the various decay chain elements may differ significantly, leading to a high degree of disequilibrium. Assumptions of equilibrium are, therefore, no longer valid. On the other hand, measurement of the activity concentration of every single radionuclide is neither economically feasible nor necessary in order to obtain a reasonable estimate of the ingestion dose. Certain radionuclides will contribute very little to the overall radiation dose because they have very small DCFs and / or their parents may be present only at very low activity concentrations.
In the first phase of the study, the parent radionuclides of the three decay series, plus the three radium isotopes radium-226, radium-223 and radium-224 that occur near the mid-points of each series, were measured. In addition, uranium-234 was assumed to be in equilibrium with uranium-238 on the basis of results from other studies [6]. From the results of this first phase, it was established that only three radionuclides of major importance remained unaccounted for: thorium-230, lead-210 and polonium-210. These were measured in phase 2 of the study, together with three radionuclides of lesser importance: actinium-227, protactinium-231 and radium-228, and, therefore, made it possible to calculate the estimated annual dose with a high degree of certainty.
Consideration was initially given to the use of gross alpha measurements for estimating the dose contributions from the radionuclides that were not individually measured. In practice, however, the uncertainties inherent in the determination of gross alpha activity, typically around 20% to 30%, lead to unacceptably large uncertainties in the final dose determination.
The use of gross beta measurements for estimating the contributions of beta emitters to the total radiation dose could not be considered, because the measurements were deemed to be unreliable owing to elevation of the beta measurements caused by water chemistry. The AEC concurred that the well-established gross beta measurement techniques used by them could not be regarded as suitable for the determination of the very low beta levels in the waters characteristic of those sampled in this study. It was accordingly decided not to use the gross beta data in dose calculation, but rather to directly measure the more important beta emitters, with the highest dose conversion factors during the second phase of the study.