Year : 2016 | Volume
: 39 | Issue : 2 | Page : 51--52
Effective doses from terrestrial radiation and their comparison with reference levels
Associate Editor, RPE, Internal Dosimetry Section, Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
D D Rao
Associate Editor, RPE, Internal Dosimetry Section, Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra
|How to cite this article:|
Rao D D. Effective doses from terrestrial radiation and their comparison with reference levels.Radiat Prot Environ 2016;39:51-52
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Rao D D. Effective doses from terrestrial radiation and their comparison with reference levels. Radiat Prot Environ [serial online] 2016 [cited 2023 May 30 ];39:51-52
Available from: https://www.rpe.org.in/text.asp?2016/39/2/51/190397
Terrestrial environment is the earth's land area including surface and subsurface features and its interaction with the atmosphere and oceans. The subcomponents of terrestrial environment primarily consist of soil, vegetation, rocks, and mineral deposits, from which some of the building materials are derived. Terrestrial systems, particularly, the soil and mineral deposits, are the main sources of natural radioactivity due to primordial radionuclides, from the time of earth's origin, leading to the exposure of human and other living species. The natural radioactivity mainly comes from 238 U series,232 Th series, and 40 K along with cosmic radiation that constitute gamma radiation exposure. In this particular editorial, it is planned to devote much of the focus to soil radioactivity and the resultant effective doses (EDs) as quite a large number of research material is published in various national and international journals on this topic. The general levels of radioactivity in soil due to 238 U,232 Th, and 40 K are around 30, 40, and 300 Bq/kg, respectively. These values vary significantly from place to place, even in normal soil environment, and can be in multiples of general levels in high background radiation regions.
Researchers commonly evaluate radioactivity in soil and building materials using gamma ray spectrometry and compute the absorbed dose and ED using default dose conversion factors and occupancy factor. The absorbed dose conversion factors for soil radioactivity, as per the UNSCEAR, at 1 m above the ground are as follows: For 238 U series - 0.462 (nGy/h)/(Bq/kg), for 232 Th series - 0.64 (nGy/h)/(Bq/kg), and for 40 K - 0.0417 (nGy/h)/(Bq/kg). The outdoor and indoor external EDs are evaluated using the number of hours spent outside and inside a house in a year and the Gy to Sv conversion factor. Similar methods are also adopted for building materials, assuming a standard model of room for dose conversion factors.
A few researchers, after computing the EDs from natural radioactivity, compare them with one of the different reference levels such as 1 mSv/a (dose limit of International Commission on Radiological Protection [ICRP] for member of public), 20 mSv/a (dose limit of ICRP for occupational workers), 2.4 mSv/a (the annual average ED from natural background), and 0.48 mSv/a (the average annual dose from external terrestrial radiation).,,,
Broadly, it may be reasonable to compare ED from the soil due to the natural radioactivity with 2.4 mSv/a although the latter is the sum of doses from cosmic, terrestrial, and also inhalation and ingestion components. The UNSCEAR (2008) gives the annual world average ED of 2.4 mSv/a, with a range of 1.0–13 mSv/a. The average ED due to the exposure of public from individual components are as follows: Cosmic radiation: 0.39 mSv/a (range 0.3–1.0); external terrestrial radiation: 0.48 mSv/a (range 0.3–1.0); inhalation of 222 Rn,220 Rn, and U/Th series: 1.26 mSv/a (range 0.2–10); and ingestion of 40 K and U/Th series: 0.29 mSv/a (0.2–1.0).
Therefore, it is clear that the comparison of ED from soil radioactivity to public exposure should be compared truly only with the external radiation exposure of 0.48 mSv/a and more precisely with that of outdoor external ED of 0.07 mSv/a, while the indoor external ED being 0.41 mSv/a. The comparison with 1 mSv/a, ICRP dose limit for exposure to member of public, is not appropriate and not a consistent comparison as it is well known that the limit of 1 mSv/a is for the anthropogenic/natural radioactive sources/man-made radionuclides produced due to the releases from operation of nuclear facilities and explicitly above the natural background doses prevailing around that site. Generally, the ED from natural background sources at a given nuclear facility is determined under preoperational survey programs, much before the nuclear facility goes operational.
In this issue, there are two articles on this topic of radioactivity in soil and phosphate rocks and both the authors have evaluated EDs and tried to compare them with the 1 and 20 mSv/a respectively. The same has been changed to appropriate reference world averages of the UNSCEAR. It is once again stressed here that many authors tend to compare the ED from soil and other building materials with the ICRP dose limit of 1 mSv/a. In the past, the editorial board had asked the authors to change their comparison of the EDs from the soil with ICRP dose limit of 1 mSv/a to an average dose of 0.48 mSv/a as per the UNSCEAR. They all have readily agreed and revised their manuscripts accordingly, and the editor would like to express thanks to the authors for agreeing to the changes. The European standards do give reference levels of ED for using building materials, as 1 mSv/a or a range of 0.3–1 mSv/a for their use as construction materials, again above the background radiation doses.
Similarly, radioactivity is also determined in the dietary items such as vegetables, milk, cereals, and pulses which accumulate (though to a very small fraction of soil radioactivity) through transfer of radioactivity from the soil. The EDs estimated from the intake of these natural radionuclides through dietary components should be compared to the world average dose from ingestion of 0.29 mSv/a.
Comparison of estimated EDs from natural radioactivity from fertilizers is another example, where ICRP dose limit of 20 mSv/a is used for comparison, which is the annual dose limit for the exposure of an occupational worker at a nuclear or radiation facility, and hence is not justifiable. If a facility, say a fertilizer plant, has a potential of EDs exceeding 1 mSv/a to the workers, possibly some dose control measures may be justified. In the case of natural radioactivity in fertilizers (Bq/kg), the computation of absorbed dose using UNSCEAR dose coefficients for soil is not appropriate and rather the radiation survey would give proper dose rate results. It should be noted that the dose conversion factors provided by UNSCEAR are specific to soil radioactivity at 1 m above the ground. They can not be applied to handling of fertilizers and other building materials such as rocks, sand, cement, and concrete.
In summary, the annual EDs derived from soil natural radioactivity should be compared to 0.07 mSv/a (outdoor) and 0.41 mSv/a (indoor) and total of 0.48 mSv/a, depending on the results of the study or research application. The references cited in this editorial are only indicative of the information, and there could be many such articles on this topic, published elsewhere.
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