Drinking Water and Health, Volume 1 (1977) / Chapter Skim
Currently Skimming:
VII RADIOACTIVITY IN DRINKING WATER
VII RADIOACTIVITY IN DRINKING WATER
Pages 857-904
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 857... ...
. BACKGROUND RADIATION The natural ionizing radiation, to which all people are exposed, includes cosmic rays and products of the decay of radioactive elements in the 857
|
From page 858... ...
higher than that in Louisiana (BEIR Committee, 1972~. Mankind has always lived with such radiation, to which, however, the radionuclides in drinking water contribute but a small share.
|
From page 859... ...
Consumption of 2 liters per day of such water might contribute as much as 8 psi per day, but this is a negligible fraction of the total daily intake of 2,300 psi of a nuclide that is the largest natural contributor to total body somatic and genetic dose. SOURCES OF HIGH-LET RADIATION Radionuclides that are produced by the decay of uranium-238 and thorium-232 are widely distributed throughout the earth's crust.
|
From page 860... ...
The natural alpha-emitters that occur in drinking water appear to be bone seekers. Of these' radium-226 and its daughters and the daughters of radium-228 probably have the greatest potential for producing radiation doses of some consequence to man.
|
From page 861... ...
35~. Furthermore, consumption of 2 liters per day of water containing 1 nCi/liter of radon222 would deliver an annual stomach dose of about 12 mrad.
|
From page 862... ...
Unfortunately, these consist, for the most part, in counting only the gross beta and gross alpha activity in the water. In addition, there is a considerable body of data on the temporal patterns and regional concentrations of the fission products strontium-90 and cesium- 137, the physical half-lives of which are about 30 yr.
|
From page 863... ...
Radioactivity in Drinking Water 863 1000 Q 800 In o In > 600 o n In ~ 400 o 200 o o O / / O / O o - 0/ // 8o8 O / 1 1 1 1 1 0 10 20 RADIOACTIVITY (psi/Liter) 30 40 50 FIGURE Vll-l Relationship between total dissolved solids and radioactivity of California domestic water.
|
From page 864... ...
These are lower, by a factor of about a million, than the current maximum permissible concentration (6 ,uCi/liter; NRC, 1976) of technetium-99m in water for the general population.
|
From page 865... ...
Peak activities in water have been reported to be about 0.01 psi/liter, which would correspond to an annual thyroid dose of about 0.05 mrem to an adult. RADIATION DOSE CALCULATIONS Estimates of the radiation doses expected to be produced by radionuclides ingested in water were calculated by means of the methods and parameters given in NCRP Report 22 (NBS Handbook 69, 1963 revision)
|
From page 866... ...
The committee pointed out, however, that there was good evidence that retention of radium-226 and other bone-seeking radionuclides is best represented by a power function model (Norris et al., 1958~. In the case of radium-226, the calculated body burden from intake at constant daily rate for 50 yr is about a factor of 10 smaller by the power function model than by the ICRP exponential model.
|
From page 867... ...
Radioactivity in Drinking Water 867 _ >, C C eD tO P)
|
From page 868... ...
It should be noted that the power function, in the form given here, deals only with the total body burden; it is silent on the question of organ distribution. Since 1959, additional data have tended to support a power-function model for bone seekers.
|
From page 869... ...
For strontium-90, the calculated body burden from ingestion of 2 psi/day for 50 yr is 229 psi, of which 222 psi are in bone; for strontium89, the corresponding values are 7.6 psi and 5.0 psi. The elective absorbed energies of the strontium isotopes in bone and total body given in ICRP 1959 were used to calculate the dose factors shown in Table VII
|
From page 870... ...
from all interstate carrier water supplies for which figures were reported for gross beta, gross alpha, strontium-90, and radium-226 activity was 3.1 psi/liter. The gross alpha activities for the same samples were less than 2 psi/liter in all but one case.
|
From page 871... ...
It was assumed that the radium-228 concentration is approximately equal to that of radium226, and for this hypothetical example a value of 0.2 psi/liter was adopted (Stehney and Krause, 1960~. Finally, potassium-40, which is ubiquitous in water supplies, is also variable; in the absence of any nationwide surveys of its concentration, it was concluded, from the range of concentrations in California (Goldberg, 1976)
|
From page 872... ...
Most of the available data suggest an apparent threshold at about 10 red of acute exposure and a sigmoid dose-response relation for production of lethality and developmental abnormalities (BEIR Committee, 1972; Brent and Gorson, 1972~. It is possible, however, that this value may be overestimated since there are relatively few observations at low doses and there is a lack of satisfactory methods for detecting minimal defects.
|
From page 873... ...
Because of the apparent existence of thresholds, nonlinearity, and critical periods, risk estimates calculated in terms of events per rem are not meaningful at low doses and dose rates. Because of this and because of the available data, it may be expected that low doses of radiation delivered at low dose rates would have a small likelihood of being embr,Yotoxic (embr,Yocidal or teratogenic)
|
From page 874... ...
On the basis of the foregoing, and the calculations of potential radiation doses, we anticipate that no measurable developmental and teratogenic ejects of radionuclides in drinking water will be found at the levels studied. Genetic Effects Many national and international groups (BEIR, NCRP, ICRP)
|
From page 875... ...
For instance, studies indicate that approximately 9OYOof organic compounds that are carcinogens are also mutagens (McCann et al., 1975; McCann and Ames, 1976~. Mutations fall into three major categories of genetic alteration: gene mutations, chromosome aberrations, and changes in chromosome number.
|
From page 876... ...
Males have only one X-chromosome, whereas females have two; thus, the phenotype resulting from a recessive gene on the male's X can be expressed, whereas, in females, unless both X-chromosomes contain the recessive, it cannot. Some gene mutations are caused by the deletion of a single base pair within the gene.
|
From page 877... ...
Present evidence indicates that natural background radiation levels (from cosmic rays and natural terrestrial radioactivity) are able to account for only part of the spontaneous incidence.
|
From page 878... ...
The BEIR Committee report is the major basis for the risk estimates reached here. In addition, because of evidence that has become available
|
From page 879... ...
The BEIR Committee based its estimates of the number of radiation induced chromosome anomalies resulting from unbalanced chromosome rearrangements on the frequency of balanced translocations recorded as semisterility in the offspring of male mice whose spermatogonia had been irradiated. For chronic irradiation at low LET and low dose rates, this frequency was taken to be 1.5 x 1O-5 per rem.
|
From page 880... ...
For induced aneuploidy the BEIR Committee relied exclusively on the rate of X-chromosome loss obtained from female mice (6 X 10-6 losses per gamete per red of chronic low-LET radiation)
|
From page 881... ...
4. Risk in Terms of III-Health The BEIR Committee assumed that an independent measure of risk could be obtained from the component of ill health that results from mutationally dependent genetic disorders.
|
From page 882... ...
The BEIR Committee's previous analysis used a rate of 3 x 1O-5 (VIZ. 1.5 x 1O-5 X 2, the correction factor for humans)
|
From page 883... ...
Only four types of chromosomal aneusomy have been found to be viable in humans, which leads to an estimate of 130 cases per million instead of the 5 originally given by the BEIR Committee. A final area of reassessment involves the induced mutation rate estimates obtained from mouse studies.
|
From page 884... ...
Although the use of these figures can lead to a somewhat higher estimate of the genetic risk from radiation than will the use of the BEIR Committee's figures, in view of the uncertainties involved, the use of the figures in Table VII-6 seems to be a responsible and prudent way to establish risks. Somatic Effects The somatic ejects of concern at the low dose rates associated with natural background radiation are those that might conceivably result from alterations in individual cells, singly or in small numbers, in the
|
From page 885... ...
RADIOBIOLOGICAL BASIS FOR EVALUATION OF CANCER RISK As has been discussed extensively elsewhere (ICRP, 1969; BEIR Committee, 1972; UNSCEAR, 1972) , there is no conclusive evidence that ionizing radiation exerts carcinogenic ejects at the low dose rates commensurate with natural background.
|
From page 886... ...
The relationship diners quantitatively, however, from one type of cancer to another, and in no instance are the parameters known well enough to enable confident prediction of the carcinogenic ejects to be expected at the low dose rates associated with background radiation levels. These limitations notwithstanding, the pattern of relationships in general implies that any simple linear interpolation on dose from observations at higher doses and dose rates, without allowance for the influence of the aforementioned variables, is likely to overestimate the risks of low-level, low-LET radiation (BEIR Committee, 1972; UNSCEAR, 1972; NCRP, 1976)
|
From page 887... ...
These include the quality factor (Qj, which has long been used to adjust for differences in linear energy transfer, or LET (ICRP, 1963; NCRP, 1967~. More recently, other dose-e~ectiveness factors have been introduced to adjust for the reduced electiveness of low-LET radiation at low doses and low dose rates, both with respect to genetic elects (BEIR Committee, 1972; UNSCEAR, 1972)
|
From page 888... ...
For estimating the risk of carcinogenesis in any one organ, it may be more appropriate to use other dose-response functions and weighting factors, depending on whether they may be indicated by the available data (NRC, 1975; NCRP, 1976~. For example, evidence has been presented elsewhere that the dose-incidence relations for cancers of the breast and thyroid may be influenced less by dose rate (BEIR Committee, 1972; NRC, 1975; NCRP, 1976)
|
From page 889... ...
They assumed a linear nonthreshold doseincidence relationship fitted to the observed human data and interpolated to pass through the control incidence at the intercept (Table VII-9~. In presenting the estimates tabulated, the BEIR Committee qualified the values on the basis of the following sources of uncertainty: 1.
|
From page 890... ...
4. The killing of susceptible cells at high doses and high dose rates can be expected to counteract the carcinogenic ejects of radiation to some extent, with the result that linear extrapolation based on ejects observed under these circumstances may, conceivably, underestimate the risk of irradiation at lower doses and dose rates (BEIR Committee, 1972~.
|
From page 891... ...
were small fractions of these values, derived by the use of dose-e~ectiveness factors (Table VII-8) intended to correct the estimates for the influence of repair at low doses and low dose rates of low-LET radiation.
|
From page 892... ...
population at a rate approximating natural background radiation levels (100 mrem per year) ranges from 0 to roughly 9,000 depending on the method used to arrive at the risk estimate.
|
From page 893... ...
893 an C
|
From page 894... ...
Under average conditions, the annual dose to bone from radium amounts to approximately 6.4 mrem/yr, which represents about 6% of the total dose to the skeleton from all sources of natural background radiation (roughly 100
|
From page 895... ...
Although the value yielded by the BEIR Committee's absolute risk model (0.2 fatal cancers per million per year per rem) is not greatly different from the value yielded by the BEIR Committee's relative risk model (since 9 out of the 1,704 fatal cancers per million per year are bone *
|
From page 896... ...
Given the uncertainties in present knowledge, the BEIR Committee's absolute risk model as used in the foregoing would seem to provide an acceptably conservative approach for the purposes at hand. SUMMARY RADIOACTIVITY IN DRINKING WATER Everyone is exposed to some natural radiation that comes from both cosmic rays and terrestrial sources.
|
From page 897... ...
Genetic Effects It has been estimated that there are about 94,400 genetic diseases per million live births in the United States. The maximum permissible dose of man-made radiation for the general population (170 mrem/yr)
|
From page 898... ...
The average carcinogenic risk associated with skeletal irradiation by radium in a population with a typical distribution of ages is estimated to approximate 0.2 fatal cases of bone cancer per million persons per year per rem. Therefore, over a period from 10 to 40 yr after the beginning of skeletal irradiation, the average risk attributable to natural background radiation is estimated to range from 0.6 per million persons per year, under typical conditions, to as much as 4.2 per million per year, in regions where 25 psi/liter of radium-226 are found in the drinking water.
|
From page 899... ...
Rem. Unit of radiation dose equivalence.
|
From page 900... ...
BEIR Committee.
|
From page 901... ...
1974. Research Needs for Estimating the Biological Hazards of Low Doses of Ionizing Radiations.
|
From page 902... ...
1975. Natural Background Radiation in the United States.
|
From page 903... ...
1972. Ionizing Radiation: Levels and Effects.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.