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EDITORIAL
Year : 2016  |  Volume : 39  |  Issue : 1  |  Page : 1-2  

Computation of excess lifetime cancer risk for environmental exposures: Is it needed? - An opinion


Associate Editor, RPE, Internal Dosimetry Section, Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Date of Web Publication1-Jul-2016

Correspondence Address:
D D Rao
Associate Editor, RPE, Internal Dosimetry Section, Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.185136

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How to cite this article:
Rao D D. Computation of excess lifetime cancer risk for environmental exposures: Is it needed? - An opinion . Radiat Prot Environ 2016;39:1-2

How to cite this URL:
Rao D D. Computation of excess lifetime cancer risk for environmental exposures: Is it needed? - An opinion . Radiat Prot Environ [serial online] 2016 [cited 2020 Oct 27];39:1-2. Available from: https://www.rpe.org.in/text.asp?2016/39/1/1/185136

Environmental radiation exposure comes primarily from the distribution of natural radionuclides in the soil, air, water, and other matrices, in addition to cosmic ray exposure. The average ambient radiation exposure is observed to be around 100 nGy/h with about ± 50% of natural variations from place to place. In high background radiation areas (HBRAs), the absorbed dose can be as high as 10 times that of normal background areas. Apart from the natural radiation exposure, there can also be extremely small radiation exposure to the members of public due to the operation of nuclear facilities (anthropogenic sources). It is also a well-known fact that the member of public receives an average radiation exposure of 2.4 mSv per annum with a range of 1-13 mSv/a.

Researchers all over the world, who make measurements of radioactivity in environmental matrices, particularly in soil, invariably evaluate several parameters. They include Ra equivalent, external hazard index (H ex ), internal hazard index (H in ), annual effective dose, activity concentration index, and the excess lifetime cancer risk (ELCR). The radioactivity in soil is in the range of 238 U: 10-30 Bq/kg, 232 Th: 20-40 Bq/kg, and for 40 K: 100-600 Bq/kg, leading to an annual effective dose of 1-2 mSv. This can at best be compared with the annual average background doses worldwide or the range.

Many researchers extend their calculations further and evaluate ELCR using the International Commission on Radiological Protection (ICRP) detriment-adjusted nominal risk coefficients (ICRP-103). I had been thinking for some time to analyze the usage of the term "excess" in ELCR calculations and also the basis on which the risk coefficients were computed. What is that "excess" mean for researchers?

  1. Is it over the normal background (base levels) at a given place? If so, generally no such calculations are elicited by the researchers.
  2. When artificially produced nuclides (anthropogenic sources) are concerned, that part of "excess" may be understandable as it is over the natural background.
  3. Is it over the risks from other cancer-causing chemical substances? If so, generally no mention of it is given?
  4. Is it simply the lifetime risk of cancer due to that radiation exposure? If so, why then the word "excess" be used?


Most researchers estimate the ELCR as the product of annual effective dose, life span (~70 years), and the nominal risk coefficient of ICRP. It is presumed that, in using the ICRP nominal risk coefficients, all the researchers are agreeing indirectly with the linear non threshold (LNT) hypothesis. The application of LNT model for trivial ("extremely") doses is highly exaggerated, particularly at the time when there are diverging views on LNT hypothesis. It may be, to some extent, understandable that the application of these risk coefficients for occupational exposures exceeding or around the limit of an annual effective dose of 20 mSv.

ICRP nominal risk coefficients are computed based on the cancer incidence risk estimates for radiation-associated cancers employing excess relative risk (ERR) and excess absolute risk (EAR) models. ERR is the ratio of the rate of disease in an exposed population to that of rate of disease in an unexposed population, minus 1. EAR is the difference in the rate of disease incidence in an exposed population and that of rate of disease in an unexposed population. Thus, the term "excess" has already been embedded in the risk coefficients, but caution should be adhered to when using for environmental exposures. The risk coefficients are estimated primarily from the data of atomic bomb survivors, indicating that the radiation exposures used were more than 100 mSv and further use of dose and dose rate effectiveness factor for application in lower exposure scenarios. This assumes the application of LNT hypothesis.

It is now clear that the rate of incidence of the disease for an unexposed (radiation) population in absolute terms is nonexistent. The unexposed population could be from the well-known normal background regions, implying that nominal risk coefficients may be applied for at least distinctly identifiable high background regions. They can also be used for radiation exposures from anthropogenic radiation sources. It may also be safely assumed that the application of nominal risk coefficients for low-level exposures will carry vely high uncertainties.

It is a fact that members of public living in HBRA continue to receive annual exposures of at least 10-20 times higher than those living in normal background areas. If one calculates ELCRs for these regions, they would be certainly high that many times, if corrections are not applied particularly, for adaptation and also uncertainties at low-level exposures are unaccounted. Is there any evidence that regions of the HBRA have cancer incidence/mortality rates of 10-20 times higher assuming that LNT model holds good? Definitely not, there are no such evidences; regions of HBRA also have the same cancer incidence rates as that of normal background areas. Several other factors are also in play, in high background areas, namely, the adaptation of persons to those areas, repair mechanism, individual sensitivity, age, and so on. Researchers computing ELCRs must include the adjustment factors for the adaptation mechanisms, age-related factors, natural background radiation doses of that area (base levels), and so on, or at least efforts should be made in that direction.

In radiation measurement parlance, if any measurement gives below detection limit value for radioactivity, the subsequent effects are assumed to be zero. For example, if 90 Sr is not detected in an environmental matrix, with best of equipment and analytical techniques, it is taken as zero dose to the members of public. Similarly, when there are no evidences of any "excess" effects, particularly for any assessment of low-level exposures, why not it be taken as zero?

Researchers, therefore, may desist (or apply extreme caution) from computing ELCRs for low-level environmental exposures leading to trivial effective doses and also for normal ambient background areas, unless they carry out some extensive studies on the cause and effects for that region. Otherwise, they can leave there with the computation of annual effective dose, other indices, and compare them with the world average and also range.

For this particular journal (RPE), I as a reviewer, on several occasions, had suggested authors to remove ELCR reporting and revise the manuscript accordingly, and all have readily agreed to the suggestion, implying that they may not be very serious of its implication at the first attempt. There is also an article in this issue involving the computation of ELCRs for both normal and slightly elevated background radiation area (Kollam). The formula remains the same, and there are no baseline levels over which the ELCRs are estimated. Although the opinion of this editorial is different, the article is published in this issue as the reviewers have independently accepted the manuscript, indicating that journal respects the peer reviews and recommendations of independent reviewers.

I wish to thank Dr. Pushparaja, Outgoing Editor, RPE for giving me the opportunity to compile and edit this issue as Guest Editor. I also wish to express my gratitude to him in guiding me for the last six years in handling some aspects of RPE journal.




 

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