The application of atomic layer deposition in the production of sorbents for 99Mo/99mTc generator.

New production routes for 99Mo are steadily gaining importance. However, the obtained specific activity is much lower than currently produced by the fission of U-235. To be able to supply hospitals with 99Mo/99mTc generators with the desired activity, the adsorption capacity of the column material should be increased. In this paper we have investigated whether the gas phase coating technique Atomic Layer Deposition (ALD), which can deposit ultra-thin layers on high surface area materials, can be used to attain materials with high adsorption capacity for 99Mo. For this purpose, ALD was applied on a silica-core sorbent material to coat it with a thin layer of alumina. This sorbent material shows to have a maximum adsorption capacity of 120 mg/g and has a99mTc elution efficiency of 55 ± 2% based on 3 executive elutions.

64Cu enrichment using the Szilard-Chalmers effect - The influence of γ-dose.

Cu is an important trace metal which plays a role in many biological processes. The radioisotope 64Cu is often used to study such processes. Furthermore, 64Cu finds applications in cancer diagnostics as well as therapy. For all of these applications 64Cu having high specific activity is needed. 64Cu can be produced in cyclotrons or in nuclear reactors. In this paper we study the effect of gamma dose on the production of 64Cu according to the Szilard-Chalmers reaction using Cu(II)-phthalocyanine as a target. For this purpose, irradiations were performed in the nuclear reactor of the Delft University of Technology using a novel irradiation facility helping to limit the dose produced by gammas present in the reactor pool. The obtained 64Cu activity yield was in general above 60% in accordance to the theoretical expected value. An increase in gamma dose has no significant influence on the obtained activity yield but increases the loss of Cu from Cu(II)-phthalocyanine up to 0.9% and hence decreases the specific activity that can be obtained. However, without optimisation, when reducing the gamma dose specific activities in the order of 30 TBq/g can be achieved.

Fluorescent nuclear track detectors for alpha radiation microdosimetry.

BACKGROUND: While alpha microdosimetry dates back a couple of decades, the effects of localized energy deposition of alpha particles are often still unclear since few comparative studies have been performed. Most modern alpha microdosimetry studies rely for large parts on simulations, which negatively impacts both the simplicity of the calculations and the reliability of the results. A novel microdosimetry method based on the Fluorescent Nuclear Track Detector, a versatile tool that can measure individual alpha particles at sub-micron resolution, yielding accurate energy, fluence and dose rate measurements, was introduced to address these issues. METHODS: Both the detectors and U87 glioblastoma cell cultures were irradiated using an external Am241 alpha source. The alpha particle tracks measured with a Fluorescent Nuclear Track Detector were used together with high resolution 3D cell geometries images to calculate the nucleus dose distribution in the U87 glioblastoma cells. The experimentally obtained microdosimetry parameters were thereafter applied to simulations of 3D U87 cells cultures (spheroids) with various spatial distributions of isotopes to evaluate the effect of the nucleus dose distribution on the expected cell survival. RESULTS: The new experimental method showed good agreement with the analytically derived nucleus dose distributions. Small differences (< 5%) in the relative effectiveness were found for isotopes in the cytoplasm and on the cell membrane versus external irradiation, while isotopes located in the nucleus or on the nuclear membrane showed a substantial increase in relative effectiveness (33 - 51%). CONCLUSIONS: The ease-of-use, good accuracy and use of experimentally derived characteristics of the radiation field make this method superior to conventional simulation-based microdosimetry studies. Considering the uncertainties found in alpha radionuclide carriers in-vivo and in-vitro, together with the large contributions from the relative biological effectiveness and the oxygen enhancement ratio, it is expected that only carriers penetrating or surrounding the cell nucleus will substantially benefit from microdosimetry.

Alpha particle spectroscopy using FNTD and SIM super-resolution microscopy.

Structured illumination microscopy (SIM) for the imaging of alpha particle tracks in fluorescent nuclear track detectors (FNTD) was evaluated and compared to confocal laser scanning microscopy (CLSM). FNTDs were irradiated with an external alpha source and imaged using both methodologies. SIM imaging resulted in improved resolution, without increase in scan time. Alpha particle energy estimation based on the track length, direction and intensity produced results in good agreement with the expected alpha particle energy distribution. A pronounced difference was seen in the spatial scattering of alpha particles in the detectors, where SIM showed an almost 50% reduction compared to CLSM. The improved resolution of SIM allows for more detailed studies of the tracks induced by ionising particles. The combination of SIM and FNTDs for alpha radiation paves the way for affordable and fast alpha spectroscopy and dosimetry.

Improved 225Ac daughter retention in InPO4 containing polymersomes.

Alpha-emitting radionuclides like actinium-225 (225Ac) are ideal candidates for the treatment of small metastasised tumours, where the long half-life of 225Ac enables it to also reach less accessible tumours. The main challenge lies in retaining the recoiled alpha-emitting daughter nuclides, which are decoupled from targeting agents upon emission of an alpha particle and can subsequently cause unwanted toxicity to healthy tissue. Polymersomes, vesicles composed of amphiphilic block copolymers, are capable of transporting (radio)pharmaceuticals to tumours, and are ideal candidates for the retention of these daughter nuclides. In this study, the Geant4 Monte Carlo simulation package was used to simulate ideal vesicle designs. Vesicles containing an InPO4 nanoparticle in the core were found to have the highest recoil retention, and were subsequently synthesized in the lab. The recoil retention of two of the daughter nuclides, namely francium-221 (221Fr) and bismuth-213 (213Bi) was determined at different vesicle sizes. Recoil retention was found to have improved significantly, from 37 ± 4% and 22 ± 1% to 57 ± 5% and 40 ± 2% for 221Fr and 213Bi respectively for 100nm polymersomes, as compared to earlier published results by Wang et al. where 225Ac was encapsulated using a hydrophilic chelate (Wang et al. 2014). To better understand the different parameters influencing daughter retention, simulation data was expanded to include vesicle polydispersity and nanoparticle position within the polymersome. The high retention of the recoiling daughters and the 225Ac itself makes this vesicle design very suitable for future in vivo verification.

Separation of nuclear isomers for cancer therapeutic radionuclides based on nuclear decay after-effects.

177Lu has sprung as a promising radionuclide for targeted therapy. The low soft tissue penetration of its ß- emission results in very efficient energy deposition in small-size tumours. Because of this, 177Lu is used in the treatment of neuroendocrine tumours and is also clinically approved for prostate cancer therapy. In this work, we report a separation method that achieves the challenging separation of the physically and chemically identical nuclear isomers, 177mLu and 177Lu. The separation method combines the nuclear after-effects of the nuclear decay, the use of a very stable chemical complex and a chromatographic separation. Based on this separation concept, a new type of radionuclide generator has been devised, in which the parent and the daughter radionuclides are the same elements. The 177mLu/177Lu radionuclide generator provides a new production route for the therapeutic radionuclide 177Lu and can bring significant growth in the research and development of 177Lu based pharmaceuticals.

Pharmacokinetics of Polymersomes Composed of Poly(Butadiene-Ethylene Oxide); Healthy versus Tumor-Bearing Mice.

Vesicles composed of block copolymers (i.e., polymersomes) are one of the most versatile nano-carriers for medical purposes due to their tuneable physicochemical properties and the possibility to encapsulate simultaneously hydrophobic and hydrophilic substances, allowing, for instance, the combination of therapy and imaging. In cancer treatment, these vesicles need to remain long enough in the blood stream to be sufficiently taken up by tumors. Here, we have investigated the biodistribution and the pharmacokinetics of polymersomes, composed of poly(butadiene-b-ethylene oxide) having dimensions around 80 nm. The polymersomes have been radiolabeled with ¹¹¹In via the so-called active loading method achieving a loading efficiency of 92.9 ± 0.9% with radionuclide retention in mouse serum of more than 95% at 24 h. The optimized ¹¹¹In containing polymersomes have been intravenously administered in healthy and tumor bearing mice for pharmacokinetic determination using microSPECT (Single Photon Emission Computed Tomography). In healthy mice these polymersomes have been found to exhibit relatively long blood circulation (> 6 h), low liver uptake (6 ± 1.5%ID/g, 48 h p.i.) and elevated spleen uptake (188 ± 30%ID/g). The blood circulation in tumor bearing mice is dramatically reduced (< 1.5 h) most likely due to elevated splenic filtration, clearly indicating the importance of in vivo studies in diseased mice. Finally, the polymersomes have been injected subcutaneously in tumor bearing mice revealing retention of 77% in the mice, primarily accumulated at the site of injection, up to 48 hours after administration.

Source apportionment of indoor PM10 in Elderly Care Centre.

Source contribution to atmospheric particulate matter (PM) has been exhaustively modelled. However, people spend most of their time indoors where this approach is less explored. This evidence worsens considering elders living in Elderly Care Centres, since they are more susceptible. The present study aims to investigate the PM composition and sources influencing elderly exposure. Two 2-week sampling campaigns were conducted-one during early fall (warm phase) and another throughout the winter (cold phase). PM10 were collected with two TCR-Tecora(®) samplers that were located in an Elderly Care Centre living room and in the correspondent outdoor. Chemical analysis of the particles was performed by neutron activation analysis for element characterization, by ion chromatography for the determination of water soluble ions and by a thermal optical technique for the measurement of organic and elemental carbon. Statistical analysis showed that there were no statistical differences between seasons and environments. The sum of the indoor PM10 components measured in this work explained 57 and 53 % of the total PM10 mass measured by gravimetry in warm and cold campaigns, respectively. Outdoor PM10 concentrations were significantly higher during the day than night (p value < 0.05), as well as Ca(2+), Fe, Sb and Zn. The contribution of indoor and outdoor sources was assessed by principal component analysis and showed the importance of the highways and the airport located less than 500 m from the Elderly Care Centre for both indoor and outdoor air quality.

Estimating the inhaled dose of pollutants during indoor physical activity.

BACKGROUND: It is undeniable that many benefits come from physical activity. People exercise in fitness centers to improve their health and well-being, prevent disease and to increase physical attractiveness. However, these facilities join conditions that cause poor indoor air quality. Moreover, increased inhalation rates during exercise have influence on inhaled doses of air pollution. OBJECTIVES: This study aims to calculate the inhaled dose of air pollutants during exercise, by estimating minute ventilation of participants and measuring air pollutant concentrations in fitness centers. METHODS: Firstly, the 20 participants performed an incremental test on a treadmill, where heart rate and minute ventilation were measured simultaneously to develop individual exponential regression equations. Secondly, heart rate was measured during fitness classes and minute ventilation was estimated based on the calculated regression coefficients. Finally, the inhaled dose of air pollutants was calculated using the estimated minute ventilation and the concentrations of the pollutants measured in a monitoring program performed in 63 fitness classes. RESULTS: Estimated inhaled doses were higher in aerobic classes than in holistic classes. The main difference was registered for PM10 inhaled dose that presented an average ratio between aerobic and holistic classes greater than four. Minute ventilation and PM10 concentrations in aerobic classes were, on average, 2.0 times higher than in holistic classes. Results showed that inhalation of pollutants is increased during heavy exercise, demonstrating the need to maintain high indoor air quality in fitness centers. CONCLUSIONS: This study illustrates the importance of inclusion minute ventilation data when comparing inhaled doses of air pollution between different population groups. This work has estimated for the first time the minute ventilation for different fitness classes. Also constitutes an important contribution for the assessment of inhaled dose in future studies to be performed in fitness centers.

Determination of airborne nanoparticles in elderly care centers.

According to numerous studies, airborne nanoparticles have a potential to produce serious adverse human health effects when deposited into the respiratory tract. The most important parts of the lung are the alveolar regions with their enormous surface areas and potential to transfer nanoparticles into the blood stream. These effects may be potentiated in case of the elderly, since this population is more susceptible to air pollutants in general and more to nanoparticles than larger particles. The main goal of this investigation was to determine the exposure of institutionalized elders to nanoparticles using Nanoparticle Surface Area Monitor (NSAM) equipment to calculate the deposited surface area (DSA) of nanoparticles into elderly lungs. In total, 193 institutionalized individuals over 65 yr of age were examined in four elderly care centers (ECC). The occupancy daily pattern was achieved by applying a questionnaire, and it was concluded that these subjects spent most of their time indoors, including the bedroom and living room, the indoor microenvironments with higher prevalence of elderly occupancy. The deposited surface area ranged from 10 to 46 µm(2)/cm(3). The living rooms presented significantly higher levels compared with bedrooms. Comparing PM10 concentrations with nanoparticles deposited surface area in elderly lungs, it is conceivable that living rooms presented the highest concentration of PM10 and were similar to the highest average DSA. The temporal distribution of DSA was also assessed. While data showed a quantitative fluctuation in values in bedrooms, high peaks were detected in living rooms.

Levels and spatial distribution of airborne chemical elements in a heavy industrial area located in the north of Spain.

The adverse health effects of airborne particles have been subjected to intense investigation in recent years; however, more studies on the chemical characterization of particles from pollution emissions are needed to (1) identify emission sources, (2) better understand the relative toxicity of particles, and (3) pinpoint more targeted emission control strategies and regulations. The main objective of this study was to assess the levels and spatial distribution of airborne chemical elements in a heavy industrial area located in the north of Spain. Instrumental and biomonitoring techniques were integrated and analytical methods for k0 instrumental neutron activation analysis and particle-induced x-ray emission were used to determine element content in aerosol filters and lichens. Results indicated that in general local industry contributed to the emissions of As, Sb, Cu, V, and Ni, which are associated with combustion processes. In addition, the steelwork emitted significant quantities of Fe and Mn and the cement factory was associated with Ca emissions. The spatial distribution of Zn and Al also indicated an important contribution of two industries located outside the studied area.

Indoor and outdoor biomonitoring using lichens at urban and rural primary schools.

Monitoring particulate matter (PM) and its chemical constituents in classrooms is a subject of special concern within the scientific community in order to control and minimize child exposure. Regulatory sampling methods have presented several limitations in their application to larger number of classrooms due to operational and financial constraints. Consequently, passive sampling methodologies using filters were developed for indoor sampling. However, such methodologies could not provide parallel information for outdoors, which is important to identify pollution sources and assess outdoor contribution to the indoors. Therefore, biomonitoring with transplanted lichens, a technique usually applied for outdoor studies, was used both indoor and outdoor of classrooms. Three main objectives were proposed, to (i) characterize simultaneously indoor and outdoor of classrooms regarding inorganic air pollutants, (ii) investigate spatial patterns of lichen conductivity, and (iii) assess pollution sources that contribute to a poor indoor air quality in schools. Lichens Flavoparmelia caperata were transplanted to indoor and outdoor of classrooms for 59 d. After exposure, electric conductivity of lichens leachate was measured to evaluate lichen vitality and cell damage. Outdoors lichen conductivity was higher near the main highways, and indoors there was great variability in levels, which indicates different emissions sources and different ventilation patterns. Chemical content of lichens was assessed by instrumental neutron activation analysis (INAA), and As, Br, Ca, Ce, Co, Cr, Cs, Eu, Fe, Hf, K, La, Na, Rb, Sb, Sc, Sm, Sr, Ta, Th, Yb, and Zn were determined. Element accumulation, crustal enrichment factors, and spatial variability of elements were analyzed and contaminants from anthropogenic sources, such as traffic (As, Sb, and Zn) and indoor chalk (Ca) found. Classrooms with potential indoor air quality problems were identified by presenting higher accumulations of inorganic pollutants in exposed biomonitors.

Fungal contamination assessment in Portuguese elderly care centers.

Individuals spend 80-90% of their day indoors and elderly subjects are likely to spend even a greater amount of time indoors. Thus, indoor air pollutants such as bioaerosols may exert a significant impact on this age group. The aim of this study was to characterize fungal contamination within Portuguese elderly care centers. Fungi were measured using conventional as well as molecular methods in bedrooms, living rooms, canteens, storage areas, and outdoors. Bioaerosols were evaluated before and after the microenvironments' occupancy in order to understand the role played by occupancy in fungal contamination. Fungal load results varied from 32 colony-forming units CFU m(-3) in bedrooms to 228 CFU m(-3) in storage areas. Penicillium sp. was the most frequently isolated (38.1%), followed by Aspergillus sp. (16.3%) and Chrysonilia sp. (4.2%). With respect to Aspergillus genus, three different fungal species in indoor air were detected, with A. candidus (62.5%) the most prevalent. On surfaces, 40 different fungal species were isolated and the most frequent was Penicillium sp. (22.2%), followed by Aspergillus sp. (17.3%). Real-time polymerase chain reaction did not detect the presence of A. fumigatus complex. Species from Penicillium and Aspergillus genera were the most abundant in air and surfaces. The species A. fumigatus was present in 12.5% of all indoor microenvironments assessed. The living room was the indoor microenvironment with lowest fungal concentration and the storage area was highest.

Retention studies of recoiling daughter nuclides of 225Ac in polymer vesicles.

Alpha radionuclide therapy is steadily gaining importance and a large number of pre-clinical and clinical studies have been carried out. However, due to the recoil effects the daughter recoil atoms, most of which are alpha emitters as well, receive energies that are much higher than the energies of chemical bonds resulting in decoupling of the radionuclide from common targeting agents. Here, we demonstrate that polymer vesicles (i.e. polymersomes) can retain recoiling daughter nuclei based on an experimental study examining the retention of (221)Fr and (213)Bi when encapsulating (225)Ac.

Assessment of exposure to metals in lead processing industries.

Inhalation of particulate matter in industrial environments has been associated with respiratory symptoms and lung diseases, which continues to lead to long- and short-term hazardous health effects on exposed subjects. The main objectives of this study were (a) to determine the dust exposure of workers from the lead industry in different operations and (b) to evaluate if the Exhaled Breath Condensate (EBC) can be used as a non-invasive tool to evaluate this exposure. Therefore, this cross-sectional study not only measured the exposure to Airborne Particulate Matter (APM) and to the associated elements but also analysed the EBC elemental composition. APM was collected in Ind1, Ind2, Offices and outdoor with Gent samplers, which delivers two size fractions: fine particulate matter (< 2.5 µm), and coarse particulate matter (between 2.5 and 10 µm). EBC samples were collected from the workers and from a non-exposed group working in Offices. The techniques INAA and PIXE were used for the APM element characterization and ICP-MS for EBC elemental content. The PM2.5 and PM2.5-10 mass concentrations were significantly higher in the industries studied than in Offices and in the environment. At the industrial sites surveyed the coarse fraction dominated and both factories had different fingerprints: APM elements with higher expression were Pb, Sb, Na, Cl and Fe in Ind1 and Pb, Si, Br, Ca, Al, Cl and Na in Ind2. Most of these elements revealed a gradient of concentration where Ind.1 > Ind.2 > Offices and EBC revealed a clear translation of this exposure, suggesting the latter to be a potential good indicator of exposure to metals in occupational settings. Pb in EBC presented the most representative results. Even though EBC was found to reflect predominantly the inhaled coarser fraction it is more related to concentration levels of exposure than to the predominance of APM fraction. The present study demonstrated not only the ability of EBC to reflect environmental exposure to metals but also the importance of measuring and characterizing different fractions of APM for a correct assessment.

Effect of particulate matter, atmospheric gases, temperature, and humidity on respiratory and circulatory diseases' trends in Lisbon, Portugal.

This study addresses the significant effects of both well-known contaminants (particles, gases) and less-studied variables (temperature, humidity) on serious, if relatively common, respiratory and circulatory diseases. The area of study is Lisbon, Portugal, and time series of health outcome (daily admissions in 12 hospitals) and environmental data (daily averages of air temperature, relative humidity, PM(10), SO(2), NO, NO(2), CO, and O(3)) have been gathered for 1999-2004 to ascertain (1) whether concentrations of air pollutants and levels of temperature and humidity do interfere on human health, as gauged by hospital admissions due to respiratory and circulatory ailments; and (2) whether there is an effect of population age in such admissions. In general terms, statistically significant (p < 0.001) correlations were found between hospital admissions and temperature, humidity, PM(10), and all gaseous pollutants except CO and NO. Age appears to influence respiratory conditions in association with temperature, whereas, for circulatory conditions, such an influence likely involves temperature as well as the gaseous pollutants NO(2) and SO(2).

On the beneficial role of silicon to organisms: a case study on the importance of silicon chemistry to metal accumulation in yeast.

Silicon is involved in numerous important structural and functional roles in a wide range of organisms, including diatoms, plants, and humans, but clear mechanisms have been discovered only in diatoms and sponges. Silicate availability influences metal concentrations within various cell- and tissue-types, but a mechanism has not been discovered so far. In an earlier study on Baker's yeast Saccharomyces cerevisiae it was proposed that a chemical mechanism, rather than a biological one, is important. In the present study, the interaction of silicon with Baker's yeast is further investigated by studying the influence of zinc and magnesium on Si accumulation both at a low and a high silicate concentration in the medium. Si accumulation fitted well with Freundlich adsorption and Si release followed depolymerization kinetics, indicating that silicate adsorbs to the surface of the cell rather than being transported over the cell membrane. Subsequently, adsorbed silicate interacts with metal ions and, therefore, alters the cell's affinity for these ions. Since several metals are nutritional, these Si interactions can significantly change the growth and viability of organisms. In conclusion, the results show that chemistry is important in Si and metal accumulation in Baker's yeast, and suggest that similar mechanisms should be studied in detail in other organisms to unravel essential roles of Si.

Influence of silicon on cobalt, zinc, and magnesium in baker's yeast, Saccharomyces cerevisiae.

Silicon (Si, as silicate) is involved in numerous important structure and function roles in a wide range of organisms, including man. Silicate availability influences metal concentrations within various cell and tissue types, but, as yet, clear mechanisms for such an influence have been discovered only within the diatoms and sponges. In this study, the influence of silicate on the intracellular accumulation of metals was investigated in baker's yeast (Saccharomyces cerevisiae). It was found that at concentrations up to 10 mM, silicate did not influence the growth rate of S. cerevisiae within a standard complete medium. However, an 11% growth inhibition was observed when silicate was present at 100 mM. Intracellular metal concentrations were investigated in yeast cultures grown without added silicate (-Si) or with the addition of 10 mM silicate (+Si). Decreased amounts of Co (52%), Mn (35%), and Fe (20%) were found within +Sigrown yeast cultures as compared to -Si-grown ones, whereas increased amounts of Mo (56%) and Mg (38%) were found. The amounts of Zn and K were apparently unaffected by the presence of silicon. +Si enhanced the yeast growth rate for low-Zn2+ medium, but it decreased the growth rate under conditions of a low Mg2+ medium and did not alter the growth rates in high Zn2+ and Co2+ media. +Si doubled the uptake rate of Co2+ but did not influence that of Zn2+. We propose that a possible explanation for these results is that polysilicate formation at the cell wall changes the cell wall binding capacity for metal ions. The toxicity of silicate was compared to germanium (Ge, as GeO2), a member of the same group of elements as Si (group 14). Hence, Si and Ge are chemically similar, but silicate starts to polymerize to oligomers above 5 mM, whereas Ge salts remain as monomers at such concentrations. Ge proved to be far more toxic to yeast than Si and no influence of Si on Ge toxicity was found. We propose that these results relate to differences in cellular uptake.

Predicting metal toxicity revisited: general properties vs. specific effects.

The present paper addresses the prediction of metal toxicity by evaluation of the relationships between general metal properties and toxic effects. For this, metal toxicity data were taken from 30 literature data sets, which varied largely in exposure times, organisms, effects and effect levels. General metal properties were selected on basis of literature reviewing of basic metal property classifications: used were the electrochemical potential deltaE0; the ionization potential IP; the ratio between atomic radius and atomic weight AR/AW; and the electronegativity Xm. The results suggest that toxicity prediction may be performed on basis of these fixed metal properties without any adoption to specific organisms, without any division of metals into classes, or grouping of toxicity tests. The results further indicate that metal properties contribute to the observed effects in relative importances which depend on specific effects, effect levels, exposure times, selected organisms and ambient conditions. The discussion strongly suggests that prediction should be by interpolation rather than by extrapolation of calibrated toxicity data: the concept here is that unknown metal toxicities are predicted on basis of observed metal toxicities in calibration experiments. Considering the used metal properties, the calibration covers the largest number of metals by the simultanuous use of Ge(IV), Cs(I), Li(I), Mn(VII), Sc and Bi in toxicity studies. Based on the data from the 30 studies considered, metal toxicities could be ordered in a relative way. This ordering indicates that the natural abundance of metals or metal ions in the Earth's crust may be regarded as a general comparative measure of the metal toxicities. The problems encountered in toxicity interpretation and ordering of toxicities indicate that control of the solution acidity, the metal's solubility and the metal's oxidation state may be key problems to overcome in future metal ion toxicity studies.

Accumulation of 99Tc in duckweed Lemna minor L. as a function of growth rate and 99Tc concentration.

This study focuses on the question of whether short-term studies can be used to forecast the accumulation of the long-lived fission product 99Tc in duckweed, Lemna minor L., grown in the field; in other words, are the accumulation parameters independent of changing growth rates typical of natural populations of duckweed. Two processes determine the 99Tc accumulation: (i) uptake and release of 99TcO4-, characterised by a concentration factor, Kd, and (ii) first-order reduction and complexation of TcVII, characterised by kred. At various 99Tc concentrations, the growth, total Tc and TcO4- accumulation were monitored over 10 days; parameters were fitted and compared with earlier results. Both Kd and kred turn out to be independent of time, concentration and growth rate up to a concentration of 10(-6)mol l(-1) 99TcO4-. Concentrations above this level result in toxic effects. The Tc accumulation in field populations of duckweed at Tc concentrations which generally occur in the environment can be forecasted by using the results from short-term experiments.