Ingestion Of Natural Radionuclides Research Articles

Environmental assessment of toxic heavy metals and natural radionuclides in irrigation water from Rustenburg, South Africa

Water is a transparent, tasteless and odourless inorganic compound, essential for life and sustainable development. It is important for food and energy production, socio-economic growth, healthy ecosystems and human existence. Global attention is given to water quality monitoring due to the role it plays in human exposure to different kinds of contaminants, including radioactive and toxic contaminants from industrial, agrochemicals, mining and other anthropogenic activities. This work presents the results of measured natural radionuclides and toxic heavy metals in Rustenburg, and their associated non-carcinogenic and carcinogenic risk. The average concentrations of Cd, Cu, Co, Fe, Ni, Mn, Pb and Zn are 0.00007 mg/l, 0.0087 mg/l, 0.0033 mg/l, 0.0636 mg/l, 0.0052 mg/l, 0.0217 mg/l, 0.0003 mg/l and 0.0047 mg/l respectively and are below the safe limit of toxic heavy metals in water. The activity concentration of 40K and 238U ranges from 7.07 Bq/l to 13.2 Bq/l and 1.24 × 10−04 Bq/l to 1.09 × 10−02 Bq/l, with a mean activity concentration of 11.6 Bq/l and 2.78 × 10−03 Bq/l respectively. 232Th was not found in all measured water samples. The estimated average committed effective dose from ingestion of natural radionuclides was observed to be below 170 μSv/yr for 40K, 120 μSv/yr for 232Th and 238U, and a total of 290 μSv/yr reported by the United Nations Scientific Committee on the Effects of Atomic Radiation. The assessment of human health risks resulting from exposure to toxic heavy metals shows negligible carcinogenic and non-carcinogenic health risks to the exposed population, making the water sources from which the sample was collected, safe for agricultural and domestic use. The obtained results will also serve as reference data for future environmental studies.

Ingestion exposure of public to natural radionuclides and committed effective dose and cancer risk through tuber crops cultivated in Ebonyi State, Nigeria.

Internal exposure of public to radiation arising from ingestion of natural radionuclides in tuber crops of Ebonyi State origin was investigated in this study. Committed effective doses and lifetime cancer risk of 40K, 226Ra, and 232Th in cassava, cocoyam, water yam, and white yam were calculated. The average activity concentrations of 40K, 226Ra, and 232Th determined by gamma spectrometric Na (TI) detector were, respectively, 199.15 ± 23.51, 77.57 ± 7.98, and 118.20 ± 10.72 Bq/kg in cassava; 146.62 ± 40.69, 43.42 ± 7.63, and 75.61 ± 2.89 Bq/kg in cocoyam; 162.81 ± 20.43, 63.17 ± 11.36, and 81.50 ± 10.27 Bq/kg in water yam; and 184.50 ± 20.22, 80.23 ± 10.93, and 116.29 ± 5.93 Bq/kg in white yam. The total committed effective dose via ingestion aligned in this orderof cassava (7.05 mSv/year) > white yam (4.38 mSv/year) > water yam (0.42 mSv/year) > cocoyam (0.21 mSv/year) with overall average dose of 3.12 mSv/year. The values of dose were higher than world average of 0.29 mSv/year given by United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). The lifetime cancer risk values were above tolerance level of >10-4 prescribed by United States Environmental Protection Agency (USEPA), suggesting probable evolvement of radiogenic cancer morbidity. The data presented in this study contributes to baseline information on radiological characteristics of tuber crops in Ebonyi, which would be valuable to WHO/FAO food safety policy in Nigeria and rest of the world.

Estimation of radiation dose and cancer risk from ingestion of natural radionuclides in groundwater of Ekiti State, Southwest Nigeria

Groundwater contains natural radionuclides in varying concentrations and its consumption may present a risk to human health. To evaluate the radiation dose due to the ingestion of the radionuclides in groundwater, 100 groundwater samples from wells used for drinking purposes were obtained spatially across Ekiti State. The concentrations of 226Ra, 232Th, and 40K were measured using a calibrated thallium-doped sodium iodide scintillation detector. The average values of the radionuclide concentrations in the drinking water samples are 4.1 ± 3.4 BqL−1, 3.4 ± 1.6 BqL−1, and 62.6 ± 44.0 BqL−1 for 226Ra, 232Th, and 40K respectively. Most of the groundwater samples have radionuclide concentrations above the WHO guidance level for the three radionuclides. The estimated average radiation dose due to ingestion of radionuclides in the groundwater to adults, children, and infants are 971.48 μSvy−1, 680.03 μSvy−1, and 291.44 μSvy−1 respectively. The lifetime cancer risk values of 1.05×10−3 and 1.10×10−3 due to ingestion of radionuclides in groundwater were evaluated for male and female respectively. These risk values are considered to be significant. Remedial actions may be necessary to ensure the safe consumption of groundwater in the area.

Total annual effective dose and health risk due to intake of natural radionuclides of some vegetables cultivated in suburban Ho Chi Minh City, Vietnam

Total annual effective dose and health risk due to intake of natural radionuclides of some vegetables cultivated in suburban Ho Chi Minh City, Vietnam

Natural radionuclide and toxic metal contents of rock salts from mines in Central Anatolia of Turkey

ABSTRACT In this study, the natural radionuclide andpotentially toxic heavy metal contents of rock salt samples collected from three different salt mines located in Çankırı city in the Central Anatolia of Turkey were determined using gamma-ray and X-ray fluorescence (XRF) spectrometry.Also, concentrations of radionuclides and heavy metalsin different table salts (sea, lake, and Himalayan) consumed in Turkey were determined to compare with rock salt. The average activity concentration of 226Ra, 232Th and 40K measured in rock salt samples were found as 1.4, 5.6 and 34.0 Bqkg−1, respectively.The average concentration of Ti, Mn, Fe, Ni, Cu, Zn, Zr and Pb in rock salt samples was analysed as 64.8, 12.9, 504.6, 3.6, 1.8, 2.0, 1.6 and 1.4 µgg−1, respectively. The concentration of Cr, Co, As, Cd, and Hg was below the detection limits.Also, the total annual effective dose (AED) and daily intake of potentially toxic heavy metals due to an intake of the radionuclides and heavy metals from rock salt samples wereestimated. The average value of AED was estimated as 8.4 μSv, which is significantly lower than the average annual effective dose of 290 μSv received by the ingestion of natural radionuclides.

Uranium Concentration in Some Medical Herbs

Uranium concentration and the annual committed effective dose in some selected medicinal plants commonly used in Iraq have been determined using fission tracks technique etch in twelve medical plants samples using CR-39 track detector. The results show that the uranium concentration ranged from 0.044±0.021 ppm in Thyme sample to 0.2±0.03 ppm in Black Pepper and Cardamom samples with an average value of 0.14 ±0.0 4ppm. The average annual effective dose due to ingestion of uranium radionuclide was 13.77x10 -5 mSv/y, which is below the world average annual committed effective dose of 0.3 mSv/y for ingestion of natural radionuclides.

Radiological assessment of internal exposure resulting from ingestion of natural radionuclides in Arachis hypogaea L. grown in Turkey

Groundnut (Arachis hypogaea L.) is one of the most important of all legumes and contains appreciable amounts of dietary oil and protein. Groundnut is added to many foods to enhance their levels of high-quality protein in diets lacking in nutrition. In this study, 51 groundnut samples were collected from the Mediterranean region of Turkey and analysed for naturally occurring radioactive isotopes of radium (226Ra), thorium (232Th) and potassium (40K). The activity concentrations of 226Ra, 232Th and 40K in groundnut samples varied from 2.9 ± 0.8 to 7.6 ± 1.0 Bq kg−1 (dw), with an average of 5.4 Bq kg−1 (dw); 4.4 ± 0.9 to 10.7 ± 1.2 Bq kg−1 (dw), with an average of 6.9 Bq kg−1 (dw) and 246.3 ± 18.2 to 541.8 ± 40.1 Bq kg−1 (dw), with an average of 427.1 Bq kg−1 (dw), respectively. The annual effective radiation dose was estimated to assess the health hazards caused by the ingestion of groundnut samples based on the measured activity concentrations of the radionuclides contained in them. The annual effective radiation dose varied from 6.5 to 10.1 µSv y−1, with an average of 8.3 ± 0.1 µSv y−1. The results revealed that consumption of Turkish groundnuts does not pose any radiological health hazards.

Public ingestion exposure to 226Ra in Ramsar, Iran

Ramsar, in the north of Iran by the Caspian Sea, has been known for the highest natural radiation background on Earth due to the local geology and hydrogeology. The residents and visitors use the hot springs that distribute the natural radionuclides especially 226Ra and its decay products in the areas. Many studies have been undertaken to measure the absorbed dose rate in Ramsar's air, however, no survey has been done to assess public internal exposure from ingestion of natural radionuclides, such as, a broad survey for 226Ra was conducted in foodstuffs and drinking water. This study presents the results of public annual activity intake (Bq) and effective dose (μSv) from ingestion of 226Ra in foodstuffs and drinking water in Ramsar city. The total mean annual intake for adults was found to be 24.8 Bq. The annual average effective dose due to ingestion exposure to 226Ra was found to be 6.9 μSv for adults that were slightly more than the estimated global averages reported by UNSCEAR, 2000 (6.3 μSv). The contribution of drinking water and foodstuffs represent respectively about 30% and 70% of the total mean annual effective dose due to 226Ra. The highest effective dose from ingestion of 226Ra for adults was estimated to be 80.6 μSv y−1. Based on the results of this study, even with the largest 226Ra value in our survey, maximum annual effective dose due to consumption of foodstuffs and drinking water for children was 164.2 μ b Sv y−1.

Assessment of natural radioactivity in rice and their associated population dose estimation

ABSTRACTThe concentration of natural radioactivity in rice is an important parameter for the determination of population exposure by the ingestion of natural radionuclides during habitual consumption of food. All types of food including rice contain a detectable amount of natural radioactivity which successively relocate into the human body via the ingestion pathway. Rice is the main cultivated crop in Bangladesh and most of the Bangladeshi people consume rice as their staple food. Hence, studies on the evaluation of natural radioactivity in rice have been performed by gamma-ray spectrometry using High Purity Germanium (HPGe) detector in order to estimate various radiation hazards due to rice consumption. The average activity levels of natural radionuclides 226Ra, 228Ra and 40K in the rice samples were 1.09 ± 0.31, 0.17 ± 0.21 and 4.70 ± 1.59 Bq kg−1, respectively. The estimated effective doses for the respective radionuclides caused by the rice consumption were 43.69, 16.39 and 4.15 µSv y−1, respectively which was below the UNSCEAR compiled value. The calculated excess lifetime cancer risk (ELCR) values via rice consumption were found below the acceptable limit of 0.29 × 10−3 for radiological risk.

Assessment of Natural Radioactivity Levels and Radiological Significance of Bottled Drinking Water in Bangladesh

This study deals with the assessment of natural radioactivity due to 226Ra, 232Th and 40K in bottled drinking water samples, marketed in Bangladesh. High purity germanium (HPGe) detector based spectrometry system coupled with Multi Channel Analyzer (MCA) was used to obtain gamma-ray spectra. The mean concentrations of 226Ra, 232Th and 40K were found to be 3.28±2.10, 6.40±2.92 and 18.26±17.49 Bq.L-1 respectively. The committed effective doses due to ingestion of natural radionuclides from the consumption of bottled water for four different age groups were estimated. The annual cumulative effective doses due to all three natural radionuclides for different age groups of children (2–7 y), 7–12 y, 12–17 y and adults (>17 y) were estimated to be 1.54, 2.07, 3.41 and 1.83 mSv y-1 respectively. Among the three natural radionuclides, annual effective doses due to intake of 226Ra for age groups 7-12 y and 12-17y are significant. For children (2-7 y) and adults (>17 y), annual effective doses due to intake of 232Th are significant. The obtained results are compared with the reported and recommended values from other countries and international organizations respectively. However, annual estimated effective doses for all four age groups from the intake of natural radionuclides in bottled drinking water were higher than the World Health Organisation (WHO) recommended limit of 0.10 mSvy-1 as well as the average radiation dose of 0.29 mSvy-1 received per head worldwide due to ingestion of natural radionuclides assessed by UNSCEAR -2000.

Radon (222Rn) in underground drinking water supplies of the Southern Greater Poland Region.

Activity concentration of the 222Rn radionuclide was determined in drinking water samples from the Sothern Greater Poland region by liquid scintillation technique. The measured values ranged from 0.42 to 10.52 Bq/dm3 with the geometric mean value of 1.92 Bq/dm3. The calculated average annual effective doses from ingestion with water and inhalation of this radionuclide escaping from water were 1.15 and 11.8 μSv, respectively. Therefore, it should be underlined that, generally, it’s not the ingestion of natural radionuclides with water but inhalation of the radon escaping from water which is a substantial part of the radiological hazard due to the presence of the natural radionuclides from the uranium and thorium series in the drinking water.

Measurement Of Activity Concentrations Of 226Ra, 232Th, 40K And 137Cs In Some Common Spices Consumed By Inhabitants In Accra Metropolis, Ghana

A spice is a vegetable substance of indigenous or exotic origin which is or has a hot, pigment taste, used to enhance taste of foods or to add to them the stimulant ingredients contained in them. Knowledge of radioactivity levels in human diet is of particular concern for the estimation of possible radiological hazards to human health. In this study, the radioactivity concentrations of 226 Ra, 232 Th, 40 K and 137 Cs in some selected natural and processed spices of different brands in the Madina District of Accra Metropolis were determined and the ingested doses via food consumption were also assessed using gamma spectrometry. The average activity concentration of 226 Ra, 232 Th, 40 K and 137 Cs in the natural spices were 0.81 ± 0.2 BqKg -1 , 0.98 ± 0.4 BqKg -1 , 2.19 ± 0.9 BqKg -1 and 0.06 ± 0.01 BqKg -1 respectively. For the processed spices, the average activity concentration of 226 Ra, 232 Th, 40 K and 137 Cs were 0.18 ± 0.1 BqKg -1 , 0.3 ± 0.1 BqKg -1 , 1.3 ± 0.8 BqKg -1 and 0.02 ± 0.01 BqKg -1 respectively. The levels of 137 Cs observed in the samples are within the range of 'background' concentrations. The estimated total annual effective dose received from 226 Ra, 232 Th and 137 Cs, due to consumption of natural (fresh) and processed spices by the inhabitants of Madina in the Accra metropolis was 8.47 µSvy -1 and this is far below the average radiation dose of 0.29 mSvy -1 received per caput worldwide due to ingestion of natural radionuclides provided in UNSCEAR (2000) report. The results indicate insignificant radiologi - cal health hazard to the public due to the consumption of spices via foods. 226 Ra; 232 Th; 40 K; 137 Cs; Spices; Hpge; Effective Dose.

Estimation of annual effective dose due to ingestion of natural radionuclides in foodstuffs and water at a proposed uranium mining site in India

Purpose: To study the distribution of 210Po, 226Ra, 230Th and U(nat) (naturally occurring radioisotopes of uranium [234U, 235U and 238U]) in food and water around the Bagjata uranium mining area in India.Materials and methods: Radionuclides were analyzed in food samples of plant and animal origin after acid digestion. Intake and ingestion dose of the radionuclides were estimated.Results: 210Po, 226Ra, 230Th and U(nat) in all the dietary components ranged widely from < 0.2–36, < 0.02–1.58, < 0.01–2.8 and < 0.017–0.39 Bqkg-1, respectively. The range of 226Ra and U(nat) in water was < 3.5–206 and < 12.6–693 mBql−1, respectively. The intake of radionuclides considering food and water was calculated to be 760 BqY−1 while the ingestion dose was 601 μSvY−1. The estimated doses reflect the natural background dose via route of ingestion, which is below the 1 mSvY−1 limit set by the International Commission on Radiological Protection (ICRP). However, the doses are more than the dose constraint of 300 μSvY−1 as suggested by the ICRP for members of the public for planned disposal of long-lived radioactive waste.Conclusion: The study confirms that current levels of radionuclides do not pose significant radiological risk to the local inhabitants, but they need close investigation in the near future.

A Study on Natural Radioactivity in Khewra Salt Mines, Pakistan

The Khewra Salt Mines, the second largest salt mines in the world, are located 160 km south of Islamabad, the capital of Pakistan. Around 1000 workers are involved in the removal of salt from these mines. More than 40,000 visitors come annually to see the mines. The visitors and workers are directly exposed to the internal and external radiological hazards of radon and gamma rays in these mines. The general public is affected by the intake of the salt containing the naturally occurring radionuclides. Therefore the concentration of radon (²²²Rn) in the Khewra Salt Mines and activity concentrations of the naturally occurring radionuclides in the salt samples from these mines were measured. Both active and passive techniques were employed for the measurement of radon with Radon Alpha Detector (RAD-7) and SSNTD respectively. The concentration of ²²²Rn was 26 ± 4 Bq m⁻³ measured by the active method while 43 ± 8 Bq m⁻³ was measured by the passive method. The activity concentration of the radionuclides was measured using gamma ray spectrometry with HPGe detector. The mean activity of ⁴⁰K in salt samples was found to be 36 ± 20 Bq kg⁻¹ and the concentration of ²²⁶Ra and ²³²Th in the salt samples was below the detection limits. Gamma radiation hazard was assessed in terms of the external gamma dose from salt slabs and the rooms made of salt and the annual effective dose due to gamma radiation. The exposure to radon daughters, annual effective dose and excessive lifetime cancer risk due to radon in the mines were estimated. The mean annual effective dose due to an intake of ⁴⁰K from the salt was calculated as 20.0 ± 11.1 µSv, which is lower than the average annual effective dose rate of 0.29 mSv, received by the ingestion of natural radionuclides. Due to the low concentration values of primordial radionuclides in the salt and radon ²²²Rn) in the mines, a 'low level activity measurement laboratory' is suggested to be established in these mines.

Assessment of health safety from ingestion of natural radionuclides in seafoods from a tropical coast, India

The activities of 210Po and 210Pb were determined in commonly consumed seafoods to evaluate the internal exposure and risk to humans residing Kudankulam coast where a mega nuclear power plant is under construction. The concentration of 210Po in seafoods ranged from 1.2±0.7 to 248±8.1Bqkg−1. Meanwhile, 210Pb ranged between 1.1±0.05 and 14.8±1.6Bqkg−1. The committed effective dose (CED) due to 210Po and 210Pb varied from 11.04 to 515.6 and 3.93 to 23.5μSvyr−1, respectively. The lifetime cancer risk for the public due to 210Po was in the range of 3.47×10−5–1.62×10−3 and it was 4.03×10−5–1.96×10−4 due to 210Pb. The activity intake, effective dose and cancer risk was found lesser than international guidelines and the seafood intake was considered to be safe for human consumption.

RADIOMETRIC ANALYSIS OF SAMPLES OF DOMESTIC FISH SPECIES AND RADIOLOGICAL IMPLICATIONS

Radiometric analysis of samples of commonly sold fish species in Pakistan were carried out for the measurement of concentrations of naturally occurring and artificial radionuclides. For this purpose, a high resolution Ge detector was employed. Mean concentrations of (226)Ra, (228)Ra, and (40)K in fish samples were 1.3 +/- 0.3, 1.0 +/- 0.2, and 90 +/- 15 Bq kg, respectively, whereas concentration of (137)Cs was not detected. The annual effective dose due to ingestion of these radionuclides through fish diet was evaluated to be 2.3 microSv y(-1). This value of effective dose is found much below the average radiation dose of 0.29 mSv y(-1) received per capita worldwide through ingestion of natural radionuclides during the consumption of food assessed by the United Nations Scientific Committee on the Effects of Atomic Radiation. It is concluded that fish supplies in the markets from the domestic fish farms are free from radiological risks. These results may contribute to the national and regional data regarding radioactivity levels in domestic fish species.

Determination of natural radioactivity in rock salt and radiation doses due to its ingestion

The Khewera Mines located in Pakistan contain the world’s second largest reserves ofrock salt. Rock salt is used in Pakistan in food recipes. It was decided toinvestigate the concentrations of naturally occurring radionuclides in rock saltfrom the Khewera Mines. Samples of rock salt were collected from 10 differentlocations and analysed by gamma spectrometry. The mean activity concentrations of226Ra,232Th and 40K were 790 ± 262, 640 ± 162 and 23 000 ± 6000 mBq kg−1, respectively. The mean annual effective dose due to the intake ofnatural radionuclides from rock salt for adults was estimated to be0.0638 ± 0.015 mSv, which is lower than the average annual effective dose of 0.29 mSv received percaput worldwide due to the ingestion of natural radionuclides, as reported bythe United Nations Scientific Committee on the Effects of Atomic Radiation in2000.

Measurements of 134Cs and 137Cs in urine and estimation of the internal dose of an adult exposed to the Chernobyl Accident

To estimate the internal dose of a Chinese visiting scholar internally contaminated because of the Chernobyl Accident, the contents of 134Cs and 137Cs in urine were measured using a Ge(Li) γ-spectrometer. The internal doses were calculated based on data from the ICRP Publications. The effective doses from 134Cs and 137Cs were estimated to be 61 μSv and 98 μ Sv, respectively. The sum of 159 μSv was lower than the total effective dose (310 μSv), from the inhalation and ingestion of natural radionuclides. The dose of 131I was also reviewed referring to the UNSCEAR 2000 Report. The equivalent effective dose of 131I was estimated to be 2.9 mSv, 18 times more than the amount of 134Cs and 137Cs. Therefore, it is considered that the earlier estimation of internal doses of 131I is important in evaluating radiation injuries from a nuclear reactor accident.

Concentration of natural radionuclides (&lt;SUP align=right&gt;40&lt;/SUP&gt;K &lt;SUP align=right&gt;228&lt;/SUP&gt;Ra and &lt;SUP align=right&gt;226&lt;/SUP&gt;Ra) in seafood and their dose to coastal adult inhabitants around Kudankulam, Gulf o

The concentrations of naturally occurring radionuclides, such as 40K 228Ra and 226Ra, were determined in the edible fin fish and shellfish, consumed by most coastal inhabitants around Kudankulam. Fin fish and shellfish were collected from eight different sampling points within 30 km radius from the Kudankulam Nuclear Power Project site. The samples were processed and counted using HPGe γ-ray spectrometry. In fin fish, 40K activity ranged from 34.16 to 360.7 Bq kg−1 fresh, 228Ra ranged from Below Detectable Limit (BDL) ≤ 0.12 to 1.9 Bq kg−1 fresh and 226Ra ranged from BDL ≤ 0.06 to 0.18 Bq kg−1 fresh. In shellfish, 40K, 228Ra and 226Ra activities ranged from 15.6 to 96.6 Bq kg−1 fresh; from 0.61 to 6.61 Bq kg−1 fresh and from BDL ≤ 0.06 to 1.15 Bq kg−1 fresh, respectively. The total daily intake resulting from consumption of fin fish and shellfish was 63.15 Bq kg−1 d−1 for 40K, 0.54 Bq kg−1 d−1 for 228Ra and 0.096 Bq kg−1 d−1 for 226Ra. The annual dosage resulting from ingestion of natural radionuclides was 1.76 ? 10−4 Sv yr−1 for fishermen, 2.51 ? 10−5 Sv yr−1 for farmers and 2.01 ? 10−5 Sv yr−1 for labourers. In general, 40K 228Ra and 226Ra contribute a total dose of 1.43 ? 10−4 Sv yr−1, 5.61 ? 10−5 Sv yr−1 and 2.35 ? 10−5 Sv yr−1, respectively.

Measurement of natural radioactivity in bottled drinking water in Pakistan and consequent dose estimates

Natural radioactivity was determined in 11 different brands of commonly sold bottled drinking water in the federal capital Islamabad and Rawalpindi city of Pakistan using gamma spectrometry technique. Mean concentrations of 226Ra, 232Th and (40)K were found to be 11.3 +/- 2.3, 5.2 +/- 0.4 and 140.9 +/- 30.6 mBq l(-1), respectively. The annual cumulative effective doses due to all three natural radionuclides for different age groups of 1-5 y, 5-10 y, 10-15 y and adults (>or=18 y) were estimated to be 4.0, 3.4, 3.1 and 4.1 microSv y(-1), respectively. Among the three natural radionuclides, annual effective doses for all age groups from 226Ra were significant. Children in the age group of 1-5 y appeared to be at risk with respect to the annual effective doses from 226Ra as compared to the other age groups. Results obtained in this study are compared with the reported values from other countries of the world and it was observed that measured activity concentrations of three natural radionuclides in the bottled drinking water were lower than these values. Annual estimated effective doses for all four age groups from the intake of natural radionuclides in bottled drinking water were also found below the World Health Organisation (WHO) recommended limit of 0.1 mSv y(-1) as well as the average radiation dose of 0.29 mSv y(-1) received per head worldwide due to ingestion of natural radionuclides assessed by UNSCEAR (2000).