Contents:
Eight new products were sampled. These included replacements for the products no-longer sold, and examples of a new category of STP US snus that was not available during the exercise. In total 78 different STPs were sampled. Both samplings included the major products in each category of STP; where there were multiple flavored variants, the base product was sampled and analyzed. Barium internal tracer was supplied by Amersham International. Polonium, thorium and plutonium internal tracers were supplied by the National Physical Laboratory UK. To convert measurements made on a wet-weight basis wwb to a dry-weight basis dwb , the water content of all STPs was measured by near-infrared NIR spectroscopy using a standard technique wherein water was extracted from the STPs using dry methanol.
Also summarized in the table are their sources, main radioactive decay modes, measurement methods in this study, half-lives and specific activities. All radionuclide analyses were conducted by Environmental Scientifics Group Didcot, UK , from whom further method details can be obtained. This was repeated, then nitric acid was added and the sample taken to dryness to remove any traces of hydrofluoric acid.
The residue was dissolved in hydrochloric acid, and polonium was isolated by auto deposition onto a silver disc Fourjay Limited, UK under reducing conditions. The radioactivity on the silver disc was measured by alpha spectrometry to determine the ratio of Po to Po.
Levels of Ra were determined by adding a known activity of Ba tracer to a dried and ground aliquot of the sample, which was then ashed in a furnace overnight. The sample was then digested in aqua regia 3: The radium radionuclides were initially co-precipitated with lead and barium sulfates from a faintly acidic water sample. The precipitate was isolated by centrifuging, then redissolved in an alkaline solution of ethylenediaminetetraacetic acid EDTA and triethanolamine TEA.
The radium radionuclides were then co-precipitated with barium sulphate from acetic acid medium free of lead contamination. The ashed residue was dissolved in hydrofluoric acid. Thorium was concentrated by co-precipitation with ferric hydroxide. Following dissolution of the precipitate using nitric acid, the thorium was purified using ion-exchange chromatography disposable plastic columns with Analytical Grade ion exchange resin, Eichrom Technologies, Inc. Uranium internal yield tracer was added to a dried and ground aliquot of the sample and ashed in a furnace overnight.
The ashed residue was dissolved in hydrochloric acid following pre-treatment with hydrofluoric and nitric acids. After co-precipitation of the uranium with ferric hydroxide, ion-exchange chromatography disposable plastic columns with Analytical Grade ion exchange resin, Eichrom Technologies, Inc.
Measurement of the uranium isotopes was carried out by alpha-spectrometry. Plutonium yield tracer was added to a dried and ground aliquot of the sample and ashed in a furnace overnight. The sample was then digested in aqua regia. After co-precipitation of the nuclides of interest with ferric hydroxide, ion-exchange chromatography disposable plastic columns with Analytical Grade ion exchange resin, Eichrom Technologies, Inc was used to further purify and separate the plutonium from americium.
The plutonium was then electrodeposited onto stainless-steel discs. Measurement of the plutonium isotopes was carried out by alpha-spectrometry. A sub-sample of known weight was taken from each sample and then burnt in an oxygen rich atmosphere in the presence of a copper oxide catalyst. Under these conditions, the hydrogen species were converted to water vapor, which was then selectively trapped in a series of gas-bubblers containing 0.
The tritium activity was corrected for the proportion of the bubbler trapping solution taken and for the weight of sample combusted to yield the specific activity in the sample. Under these conditions, the carbon species were converted to carbon dioxide. This was then selectively trapped in a series of gas-bubblers containing a trapping medium. The carbon activity was corrected for the proportion of the bubbler trapping solution taken and for the weight of sample combusted.
Application of decay corrections for the naturally occurring daughter radionuclides of uranium and thorium assumes that the series daughter radionuclides are all in secular equilibrium and therefore decay with the half-life of the first radionuclide in the series. All instruments are calibrated using certified standards traceable to national standards. The radioactive controls and internal tracers are also made from certified standards and are supplied by various manufacturers: The generic formulae for the detection limit can be simplified by setting a value for the coverage factor chosen to be 1.
Where the symbols are defined as follows: When calculating the limits of detection in gamma ray spectrometry, it is important to take into account the increased uncertainty from estimating the continuum from a smaller number of channels when peaks are located close together. This is therefore incorporated into the recommended formula above for the peak integration case as follows and in a re-arranged format:.
Where n 2 m is usually about 1. However if gamma ray peaks are close together and the number of channels available for continuum estimation is reduced then n 2 m could increase to possibly 4 or more. A single measurement on each sample was made and a full uncertainty budget calculated as described in the Measurement Good Practice Guide No.
Different LoDs were calculated for different samples of the same analyte; these arise from the factors used in the calculation for the limit of detection in the formula shown above. The values of some factors, such as b, differed from measurement to measurement, resulting in different LoDs for many samples. The background for most techniques is fairly constant, but this is not the case for analysis by gamma ray spectrometry.
Here the individual sample background is the Compton continuum produced by the gamma rays in the spectrum. If, for example, the K level is low in one sample, the Compton continuum will be low and therefore the background will be low. Conversely if the K activity is high, the Compton continuum will be higher and therefore the background will be higher. Measured values for radionuclides in STPs were obtained as measurements of the radioactivity of the sample as received or wet weight basis, wwb. Mass concentrations allow direct comparison of the data reported here with levels of other chemical toxicants in tobacco.
The data are also given on a dry weight basis dwb , i. The wwb values reflect the radionuclide content of the STP as experienced by the user and measured in this study , whereas the dwb values refer to the radionuclide content of the solid matter of the STP predominately tobacco and is reported here to facilitate a comparison both across different types of STP and with published values, which are predominantly reported historically as dwb.
Where reported activity levels were below limits of quantification LOQ , randomly imputed values between the LOQ and zero generated using Microsoft Excel were used for the purpose of these comparisons. Although only Pb, Po and uranium have been previously reported in STPs, many other radionuclides have been reported to be present in the tobacco plant and tobacco products [ 8 ]. Tables S2—S4 and the corresponding mass of these radionuclides presented in Additional file 1: Summary of current findings in contemporary STPs and historic values for uranium and radionuclides of the uranium decay series.
Summary of current findings in contemporary STPs and historic values for radionuclides of the thorium decay series and for potassium and cosmic ray generated radionuclides. Summary of current findings in contemporary STPs and historic values for anthropogenic radionuclides. The activity values of uranium and radionuclides of the uranium decay series are presented in Additional file 1: Table S2, and the corresponding mass concentrations in Additional file 1: Uranium U, Uranium U, 0. In the current work these three radionuclides are discussed together because of the way in which uranium levels have been historically reported, sometimes as total uranium and sometimes as the individual radionuclides.
In the samples where both U and U were present, the two radionuclides had very similar activities; however, owing to the greater specific activity of U, a substantially greater mass concentration of U 6. For the individual isotopes U, U our results are of the same order of magnitude but slightly higher than those reported for Brazilian and Egyptian tobaccos [ 20 , 21 ]. Given that the majority of the samples measured in the current study did not have measurable levels of uranium radionuclides, it is of value to estimate the upper limits for their presence in these STPs based on the current analytical capabilities.
Thorium Th has not previously been reported in tobacco. Radium Ra was identified in all but three of the samples at an activity of 0. On a dry-weight basis, there were generally similar Ra contents among the STPs analyzed, except that loose and pouched snus had higher levels than CT. Among the naturally-occurring radionuclides that become incorporated into tobacco plants, polonium Po has received the greatest attention of any radionuclide due to its transfer to smoke in cigarettes [ 22 ], and potential for causing lung cancer [ 23 ].
The measured activities ranged from 1. Two snus portion products and 2 CT products evaluated in our study were below the detection levels. The Po content of both loose and pouched snus was lower than the other product categories except for CT. Our results for Po activities in DS A number of authors have reported secular equilibrium between Pb and Po due to the length of time between harvesting of tobacco leaves and tobacco product production [ 24 — 27 ].
Consequently, Pb is likely to be present in the current sample set, at activity levels comparable to the Po measurements. The activity values for thorium decay series radionuclides are presented in Additional file 1: Table S3, and the corresponding mass concentrations in Additional file 1: The corresponding dwb values 1. The current method is insensitive to the levels of Ac reported historically of 0. In the current work, 47 of the STPs had detectable levels of Th with activity ranging from 1. Table S6 , with many of the measured activities similar in magnitude to the limit of quantification of the analysis.
When expressed as a dry weight basis, there were no significant differences among the product categories. Although not detected here, trace levels of Pb and Tl have been reported in Swiss cigarettes [ 28 ], and Pb and Bi levels have been quantified [ 29 ] in Iraqi cigarettes at 6—9 and 9. For these naturally occuring radionuclides the activity values in the analysed STPs are presented in Additional file 1: Potassium 40 K , present at 0.
Furthermore, 40 K was the radionuclide present at the highest mass concentrations, 1. Comparing product categories on a wwb showed higher activity levels for DS products, with all other products having similar or lower levels of activity. On a dwb the differences between STP categories diminished, although DS products were still at the higher end of 40 K content. Compared to the other snus products the material in the Oomph pouch was lighter in colour and contained a substantial content of a white material Fig. This was probably due to the cellulose powder and vegetable fibre ingredients reported on the package.
Hence, the lack of detectable 40 K may well reflect the diluted tobacco content of this STP. Visual comparison of typical Swedish portion snus left and Oomph portion snus right. Shown are cross-sections of the cut products. Carbon 14 C is largely a product of cosmic ray irradiation of the atmosphere. The 14 C content of tobaccos has not been reported previously, although an assimilation study [ 30 ] has shown that 14 C is readily taken-up and distributed within the tobacco plant.
In the current work, 14 C was detected in all but one of the STPs, making it one of the more pervasive radionuclides examined in this study. The product without measurable 14 C was Romeo y Julieta Habanos Nordics , a portion snus; for this product, the detection limit was higher than for many of the other STPs; thus, 14 C may have been present at a level slightly below the limit of detection.
Comparison of 14 C activity levels across different product categories showed no significant differences on either a dwb or wwb. Although tritium 3 H which is also produced by cosmic ray interaction with the atmosphere, has not been reported in tobacco, it was considered a potential contaminant via generation in the atmosphere, and incorporation into the growing tobacco plant as 3 H-incorporated water. Phosphorus 32 P is another radionuclide generated by cosmic ray interaction with the atmosphere, and has been categorized by IARC as a Group 1 carcinogen. However, its short half-life The activity values for man-made radionuclides in the analysed STPs are presented in Additional file 1: Table S4, and the corresponding mass concentrations in Additional file 1: The synthetic radionuclide americium Am is generated within nuclear waste.
The STPs were analyzed for three plutonium radionuclides, Pu, Pu and Pu, which are products of the nuclear reactions of uranium. These values correspond to 7. Among the STPs found to contain plutonium in the present study, the activity levels of , Pu were considerably higher than those reported [ 32 ] for Finnish cigarette tobaccos in the s 0. Both caesium Cs and caesium Cs are products of nuclear fission reactions, and are contaminants produced in nuclear incidents. It has been suggested that geographic source is a determinant of its presence or absence in tobacco [ 28 ].
Iodine I co-evolves with caesium radionuclides after nuclear reactor incidents. Cobalt 60 Co , which is also a product of nuclear fission, was not detected in any of the STPs. It has not been reported previously as a natural contaminant in tobacco but has been detected in neutron activated tobacco in laboratory studies [ 33 ]. The present study represents the most comprehensive assessment of the radionuclide content of STPs published to date. Seventy-eight contemporary STPs from the USA and Sweden, covering the main product categories and manufacturers, were assessed for the presence of 28 radionuclides, encompassing all of the major sources of environmental radioactivity.
Three of the species for which we found quantifiable amounts 14 C, 3 H, and Th have not previously been reported in tobacco. In contrast to the conclusions of recent literature reviews of radionuclides in STPs [ 1 , 6 ] focusing on Po, U and U, this study has revealed a plurality of radionuclides in contemporary STPs. However, none of the radionuclides investigated were detected in all STPs. Other than 40 K, the mass of radionuclides measured in these STPs were very low in comparison with other toxicants identified in STPs [ 1 , 6 ], often by many orders of magnitude.
For the Th series, only Th and Th were detected, with Th showing greater activity.
Radionuclides resulting from cosmic ray irradiation of the atmosphere were also found in the STPs. In these two samples, although 3 H was present at much lower mass concentrations than 14 C, its radioactivity levels were similar to 14 C. The substantially lower mass concentrations of 3 H than 14 C probably reflect differences in atmospheric production rates and subsequent uptake by the growing tobacco plant. Among the man-made radionuclides examined, some STPs showed measurable quantities of three plutonium radionuclides. Although some members of the U and Th decay series were present, others Th, Pa, Bi, Pb, and Ac, Pb, Bi, Tl respectively , as well as U, I and the two caesium radionuclides, showed no activity in any of the STPs examined.
When a species was not detected it may be due either to the absence of the species in the analyzed matrix or to insufficient sensitivity of the analytical method for the sample being examined. There are some indications to the reasons underlying the absence of measured activity from specific radionuclides in some samples. The presence of members of the U and Th decay series, particularly the originating radionuclides, in an STP means the presence of other members of the decay series cannot be precluded, albeit at levels below the detection limit of the assay.
This is exemplified by the uranium isotopes examined in this study. Natural sources of uranium contain these radionuclides at a ratio of Therefore, U and U will also be present, even if not detectable, in the samples containing U. Moreover, given the very short half-lives of many of the progeny of the U decay series such as Pb and Bi it is reasonable to assume that such species may be present, however fleetingly, at some point between production and consumption of an STP. In contrast, some of the man-made radionuclides with relatively short half-lives e. Chernobyl in and the date of this study — The analytical method is sufficiently sensitive to detect the levels reported in many of the historical observations, and therefore Cs may not be present in these STPs.
Appreciable quantities of plutonium radionuclides were released into the atmosphere during atmospheric nuclear weapons testing in the mid to last half of the 20th century, and their presence has subsequently been detected in several plant species [ 32 ]. In the present work, upper bounds for the possible presence of undetected radionuclides were calculated from the reporting limits of the activity counting method. For some radionuclides with very short half-lives, the upper reporting limit corresponds to a few atoms of the radionuclide within the STP sample.
This may either point to an effective cut-off point, based on radionuclide half-life, for the analytical capability of the current approach, or perhaps reflect the age of tobacco at the time of measurement. Examination of the Hoffmann et al.
Unlike the potential risk from more volatile radionuclides such as Po in cigarette tobacco, transfer to smoke is not a factor in assessing exposure to radionuclides in STPs. Although as depicted in Fig. Activities below LoD are displayed as 0 in the graph. Some variations in radionuclide content were observed among the different STP categories.
HP products also had higher levels of Ra than the other categories on a wwb. The higher levels of these radionuclides likely reflect the presence of non-tobacco such as calcium carbonate [ 38 ] materials within the HP products. Estimation of the inorganic content via ashing of the STPs showed higher inorganic contents in the HP products than in CT, MS, plug, SP, loose snus and all pouched snus other than the low moisture brands.
However, the inorganic contents of the DS, dry pouched snus and HP products were comparable. Hence these measurements suggest that the nature of the non-tobacco materials in the HP products may be more important than the quantity. Uranium is known to interchange with calcium in bone samples [ 39 ], and the presence of calcium salts in the HP products may act as a source of uranium and daughter radionuclides in STPs.
For the most abundant radionuclide present, 40 K, the highest levels were found in DS products, and the lowest in an STP whose tobacco content appeared diluted with other materials. No differences were found among product categories for 14 C or Th when adjusted for the moisture content of the STPs. The STPs in which Pu and , Pu were detected had similar levels of these man-made radionuclides.
A review of the literature generally indicates that the radionuclides we identified in STPs are similar to levels historically reported in tobacco, except, as noted above, where non-tobacco materials appear to be included in the STP. However, we identified several radionuclides in STPs that have not previously been reported in tobacco. Establishing the radionuclide content of STPs is an essential first step in understanding the incremental contribution of radionuclides associated with STP use to the background exposure from radionuclides in our diet, water and air.
A key step is to calculate the radiation dose to tissues of STP users, because it allows estimation of the relative risk profiles of different STP product categories, and in principle it facilitates estimation of the risks associated with radionuclides in STPs. Models exist for calculating the radiation dose exposure energy divided by mass of exposed tissue resulting from exposure to radionuclides present in our diet, water and air, as well as from occupational exposure e.
Perhaps the closest established model is that used to calculate the exposure to, and risk from, ingested radionuclides. However, models of ingestion assume rapid mouth transit of the ingested material, and also incorporate the metabolic processes of the body that lead to dispersal of the radionuclide from the gastrointestinal tract to the physiologically preferred accumulation site e.
STP-use typically involves extended mouth residence e. During residence in the mouth, radionuclides in the STP may also potentially directly irradiate the tissues adjacent to the STP. Some STPs are dispersed in saliva and not designed to be expectorated; these STPs and their radionuclides will be more readily absorbed or ingested. In those STP categories that are designed for expectoration of the used product, some loose tobacco particles may be swallowed during use.
When use of a non-dispersing product is complete, the remaining STP solids which are heavily loaded with saliva are removed by the user and discarded. Therefore only those radionuclides lying very close to the periphery of the STP portion could possibly lead to direct irradiation of oral tissue. In addition, the average thickness of the salivary film, 0. These estimations highlight the need for more sophisticated exposure models to assess radiological dose in STP users.
These models should consider the committed effective dose arising from exposure to alpha and beta generating radionuclides; internal exposure to alpha radiation is considered more damaging than beta radiation due to the way in which energy is imparted to tissue by these two types of radiation. Several further aspects of direct irradiation need to be considered.
The emitted gamma radiation can introduce an additional radiation dose to the STP user, as gamma radiation can penetrate further and potentially interact with critical biological tissue; this both widens the area of potential radiation exposure but also introduces a relatively low potential for tissue damage due to the comparatively weak interaction of gamma radiation with tissue. Some further, potentially important, exposure mechanisms are also important to consider in the development of a model and are described below. Extracted radionuclides may come into closer contact with oral tissues than those remaining within the STP [ 48 ], and therefore may more readily expose STP users to radiation.
However, for most categories of STP other than dispersable products, for which complete ingestion can be assumed , uncertainties exist over the extent of extraction of individual radionuclides into saliva. It is also difficult to estimate the solubility of these species in tobacco because the exact chemical forms are unknown: Environmental studies have shown that radium is only moderately soluble in water, but is most soluble under chloride-rich reducing aqueous systems with a high total content of dissolved solids, a condition that might relate to STPs that have a high salt and water content [ 51 ].
Environmental thorium has very low aqueous solubility [ 46 ]. Aqueous solubilities of uranium, plutonium and neptunium are low but pH dependent [ 52 ]. Similarly, no measurable level of lead extraction was found during use of snus by US snus consumers [ 54 ]. However, 14 C is incorporated chemically into the tobacco plant in several soluble organic species such as sugars, sugar esters and starches [ 30 ], and 3 H can be present as tritiated water or organic species [ 41 ].
Therefore it is likely that these two species would be bioavailable from STPs, although the extent of availability is unclear at present. Overall, these data suggest that most of the radionuclide content of STPs may remain within the STP during use, but some extraction of radionuclides into saliva, particularly 40 K, 3 H and 14 C, will occur.
Once released into saliva, the radiation emitted by saliva-soluble radionuclides will have to overcome the physical shielding effects of saliva, air and non-vital epithelium cells within the oral cavity in order to encounter biologically-important tissue. Radionuclides extracted from STP portions may potentially be absorbed into oral cavity tissues Fig. If tissue clearance mechanisms are relatively slow compared with STP usage duration, this may lead to a localized build-up of radionuclide in the oral tissue during use, particularly as STP users generally position the tobacco portion at a fixed location within the mouth.
Standard radiological models do not account for this potential source of exposure, and this is an area requiring further attention. In contrast, the incremental exposure to radionuclides after swallowing during STP use, is within the scope of the standard radiological dose models for ingested radionuclides from the diet.
Systemic dispersion of radionuclides after ingestion is well understood. Increased exposure to radiation from 40 K may arise in the GI tract of STP users during transit of swallowed materials; however, comparison to the recommended USA adult daily dietary intake of 4. Hence the risk of systemic exposure to 40 K from STPs will be small. In contrast, STP use can add to the body concentrations of 3 H, 14 C, and the progeny of U and Th, at levels corresponding to their extractability.
Depending upon the effectiveness of fractional absorption from the gut there may also be some GI exposure to radionuclides that undergo extended intestinal transit. The extent of these sources of exposure is unclear, as noted above, but is likely to present a minimal increase in exposure and hence risk in comparison to dietary intake. The greatest potential radiological risk from radionuclides in STPs therefore appears to be from 40 K, and to a lesser degree 14 C.
With the uncertainties surrounding STP portion size and geometry and the resulting attenuation of radiation emitted from within STPs , and the differential extent and kinetics of extraction into saliva by users of different STPs, it is challenging to establish an accurate estimate for effective dose to the oral cavity. Clearly, more sophisticated models that account for localized exposure are desirable to quantify radionuclide exposure within the oral cavity, and their development would represent an advance in understanding the potential for oral toxicity of STP use.
Ultimately, epidemiology provides the most informative insights into the risks associated with STP use. Environmental radioactivity is produced by radioactive materials in the human environment. While some radioisotopes , such as strontium 90 Sr and technetium 99 Tc , are only found on Earth as a result of human activity, and some, like potassium 40 K , are only present due to natural processes, a few isotopes, e. The concentration and location of some natural isotopes, particularly uranium U , can be affected by human activity.
Radioactivity is present everywhere , and has been since the formation of the earth. According to the IAEA , soil typically contains the following four natural radioisotopes: Synthetic radioisotopes also can be detected in silt. Busby [ citation needed ] quotes a report on the plutonium activity in Welsh intertidal sediments by Garland et al. The additional radioactivity in the biosphere caused by human activity due to the releases of man-made radioactivity and of Naturally Occurring Radioactive Materials NORM can be divided into several classes.
Just because a radioisotope lands on the surface of the soil, does not mean it will enter the human food chain. After release into the environment, radioactive materials can reach humans in a range of different routes, and the chemistry of the element usually dictates the most likely route. Using milk as an example, if the cow has a daily intake of Bq of the preceding isotopes then the milk will have the following activities.
If the radioactivity is tightly bonded to by the minerals in the soil then less radioactivity can be absorbed by crops and grass growing in the soil. One dramatic source of man-made radioactivity is a nuclear weapons test. The glassy trinitite formed by the first atom bomb contains radioisotopes formed by neutron activation and nuclear fission.
In addition some natural radioisotopes are present. A recent paper [5] reports the levels of long-lived radioisotopes in the trinitite. The trinitite was formed from feldspar and quartz which were melted by the heat. Two samples of trinitite were used, the first left-hand-side bars in the graph was taken from between 40 and 65 meters of ground zero while the other sample was taken from further away from the ground zero point.
The Eu half life Some of the 60 Co half life 5. This 60 Co from the tower would have been scattered over the site reducing the difference in the soil levels. The Ba half life The barium was present in the form of the nitrate in the chemical explosives used while the plutonium was the fissile fuel used. The Cs level is higher in the sample that was further away from the ground zero point — this is thought to be because the precursors to the Cs I and Xe and, to a lesser degree, the caesium itself are volatile. The natural radioisotopes in the glass are about the same in both locations.
The action of neutrons on stable isotopes can form radioisotopes , for instance the neutron bombardment neutron activation of nitrogen forms carbon This radioisotope can be released from the nuclear fuel cycle ; this is the radioisotope responsible for the majority of the dose experienced by the population as a result of the activities of the nuclear power industry. Nuclear bomb tests have increased the specific activity of carbon, whereas the use of fossil fuels has decreased it.
See the article on radiocarbon dating for further details. Discharges from nuclear plants within the nuclear fuel cycle introduce fission products to the environment. The releases from nuclear reprocessing plants tend to be medium to long-lived radioisotopes; this is because the nuclear fuel is allowed to cool for several years before being dissolved in the nitric acid.
The releases from nuclear reactor accidents and bomb detonations will contain a greater amount of the short-lived radioisotopes when the amounts are expressed in activity Bq. An example of a short-lived fission product is iodine , this can also be formed as an activation product by the neutron activation of tellurium. In both bomb fallout and a release from a power reactor accident, the short-lived isotopes cause the dose rate on day one to be much higher than that which will be experienced at the same site many days later.
This holds true even if no attempts at decontamination are made. In the graphs below, the total gamma dose rate and the share of the dose due to each main isotope released by the Chernobyl accident are shown. An example of a medium lived is Cs, which has a half-life of 30 years. Caesium is released in bomb fallout and from the nuclear fuel cycle. A paper has been written on the radioactivity found in oysters found in the Irish Sea , these were found by gamma spectroscopy to contain Ce, Ce, Ru, Ru, Cs, 95 Zr and 95 Nb.
An important part of the Chernobyl release was the caesium, this isotope is responsible for much of the long term at least one year after the fire external exposure which has occurred at the site. The caesium isotopes in the fallout have had an effect on farming. The accident could have been stopped at several stages; first, the last legal owners of the source failed to make arrangements for the source to be stored in a safe and secure place; and second, the scrap metal workers who took it did not recognise the markings which indicated that it was a radioactive object.
This paper also reports details of the effect of potassium , ammonium and calcium ions on the uptake of the radioisotopes. Caesium binds tightly to clay minerals such as illite and montmorillonite ; hence it remains in the upper layers of soil where it can be accessed by plants with shallow roots such as grass. Hence grass and mushrooms can carry a considerable amount of Cs which can be transferred to humans through the food chain. One of the best countermeasures in dairy farming against Cs is to mix up the soil by deeply ploughing the soil.
This has the effect of putting the Cs out of reach of the shallow roots of the grass, hence the level of radioactivity in the grass will be lowered. Also, after a nuclear war or serious accident, the removal of top few cm of soil and its burial in a shallow trench will reduce the long term gamma dose to humans due to Cs as the gamma photons will be attenuated by their passage through the soil.
The more remote the trench is from humans and the deeper the trench is the better the degree of protection which will be afforded to the human population. In livestock farming, an important countermeasure against Cs is to feed to animals a little prussian blue. This iron potassium cyanide compound acts as an ion-exchanger. The cyanide is so tightly bonded to the iron that it is safe for a human to eat several grams of prussian blue per day. The prussian blue reduces the biological half-life not to be confused with the nuclear half-life of the caesium.
The physical or nuclear half-life of Cs is about 30 years, which is a constant and can not be changed; however, the biological half-life will change according to the nature and habits of the organism for which it is expressed. Caesium in humans normally has a biological half-life of between one and four months. An added advantage of the prussian blue is that the caesium which is stripped from the animal in the droppings is in a form which is not available to plants.
Hence, it prevents the caesium from being recycled.