- Open Access
Environmental and health-related research on application and production of rare earth elements under scrutiny
Globalization and Health volume 18, Article number: 86 (2022)
Unlike most other commodities, rare earth elements (REEs) are part of a wide range of applications needed for daily life all over the world. These applications range from cell phones to electric vehicles to wind turbines. They are often declared as part of “green technology” and, therefore, often called “green elements”. However, their production and use are not only useful but also risky to the environment and human health, as many studies have shown. Consequently, the range of global research efforts is broad and highly variable, and therefore difficult to capture and assess. Hence, this study aims to assess the global parameters of global research on REE in the context of environment and health (REEeh). In addition to established bibliometric parameters, advanced analyses using market driver and scientific infrastructure values were carried out to provide deep insight into incentives, necessities, and barriers to international research.
The focus of REE research is in line with national aspirations, especially from the major global players, China and the USA. Whereas globally, regional research interests are related to market interests, as evidenced by the inclusion of drivers such as electric vehicles, wind turbines, and permanent magnets. The topics receiving the most attention are related to gadolinium used for magnetic resonance imaging and the use of ceria nanoparticles. Since both are used for medical purposes, the medical research areas are equally profiled and mainly addressed in high-income countries. Nevertheless, environmental issues are increasingly in focus.
There is still a need for research that is independent and open-ended. For this, market-independent technologies, substitutes and recycling of REEs need to be addressed scientifically. The results of this study are relevant for all stakeholders, from individual scientists to planners to funders, to improve future research strategies in line with these research mandates.
Entire daily lives are impacted by rare earth elements (REEs) as they are found in every car, computer, cell phone, television, and energy-efficient fluorescent light bulb, naming just the most common applications . This exacerbates the often overlooked fact that they are emerging contaminants (ECs), as they are currently unregulated for humans and the environment, not included in environmental and public health programs, are micropollutants with very low detection limits. Their mechanisms of toxicity are poorly understood .
According to the International Union of Pure and Applied Chemistry (IUPAC), they form a homogenous group of 17 chemical elements forms the REEs, which show a coherent behavior [1, 3]. In addition to the 15 lanthanides, scandium and yttrium also belong to this group. Although a distinction is generally made between light and heavy REEs, there is no consistent definition worldwide. The heavy REEs occur less frequently, have higher atomic masses and are characterized by lower solubility and alkalinity . REEs are not as rare as their designation suggests. Instead, they occur in significant abundance in almost all rock formations . Therefore, the designation as rare elements is actually misleading. It is not based on their geological rarity in the earth’s crust but on their occurrence in the mineral mixture in low concentrations as well as the few occurrences that can be mined economically . However, when converted to atomic concentrations, especially the REEs with odd atomic numbers are rarer, including those of great value .
REEs are highly reactive because they are very easily oxidized. Therefore, they are used as reducing agents for difficult to reduce metals . The exceptional applicability for technical and industrial usages is due to their specific mechanical, chemical, optical, and magnetic characteristics . They are widely used in high technology but also in traditional applications. They are key components for many high-tech applications, reaching from everyday applications to power generation, chemistry, mobility, medicine, defense, and agriculture . Practical applications also provide environmental benefits, such as phosphate removal from aquatic systems to prevent eutrophication with lanthanum hydroxide  or lanthanum carbonate groups  or photocatalytic applications such as pollutant degradation, carbon dioxide reduction, and hydrogen evolution . Given all these uses, people are often exposed to them .
The difficult separation of the elements remains a challenge to this day. Additionally, the eminently increasing use in technical devices raises concerns about the sustainability of REEs, which has stimulated numerous studies on the subject . This difficulty is the cause of the high price of REEs, which is disproportionate to their deposits. In addition, high demand and tight supply due to limited mining areas in China contribute to their value. Sufficient global supply of REEs as critical metals is thus on everyone’s lips . Moreover, recycling rates are still exceedingly low, so that a desirable circular economy unfeasible is currently not feasible .
The increasing use of REEs for industrial applications implicates a corresponding increase in health and environmental impacts. As anthropogenic compounds, they enter soils and waters through disposal, discharges from mining operations, wastewater, and atmospheric emissions. Further dispersal in the environment occurs through wind blowing, runoff, leaching, or irrigation, releasing, for example, radioactivity or other chemical byproducts [2, 9]. Its use as a component of fertilizers and animal feeds also contributes to its widespread use. It is based on the dose-response effect. At low doses it has a growth-prompting effect, while at high doses it is toxic [10, 11].
Adverse health effects on aquatic and terrestrial organisms have been already reported [12,13,14]. However, there is a need for more comprehensive toxicological studies, as the transfer of effects to species or to ecosystems is still not well understood . Another application of REEs is in medicine, e.g., gadolinium as a contrast agent for magnetic resonance imaging (MRI) and yttrium for radioimmunotherapy of non-Hodgkin’s lymphoma (NHL). However, exposure to these REEs has also been shown to be risky. For example, the use of gadolinium in contrast agents has been associated with nephrogenic systemic fibrosis , and its accumulation in the brain can cause adverse effects on the central nervous system . Other reported health effects associated with REE exposure include the occurrences of pneumoconiosis, anti-testicular effects, male sterility, dysfunctional neurological disorders, genotoxicity, and fibrotic tissue damage. However, studies on human health effects are limited, particularly concerning the toxicity of the individual REEs and the long-term effects of their exposure . In addition, knowledge of environmental occurrence and behavior is low, which is especially on demand for developing countries with their potential REE sources , as data is mostly generated in developed countries .
Therefore, an analysis of the scientific landscape of REEs related to environmental and human health (REEeh), as intended in the present study, is useful and important. Scientists can benefit from it to identify, plan, and expand current and future projects. Policymakers and funders can benefit by being able to support scientific efforts more specifically. Thus, the goal is to present the scientific basis on REEs in terms of their impact on the environment and human health by assessing general and advanced features of the research output. That will provide information on the drivers and barriers to the research conducted, as well as the incentives and requirements for future research, and identify and assess key player of global research on REEeh.
For the overall publication output on REEeh, 6941 articles (n) could be retrieved from WoS.
The majority of articles were written in English (98.44%, n = 6833). Only a few articles were published in other languages, e.g., Chinese (n = 32), German (n = 17), and Spanish (n = 16), to name those that contributed more than 15 articles.
In addition to the title synonymous term rare earth elements, the densest keyword clusters were led by the terms toxicity, gadolinium, lanthanum, nanoparticles, and adsorption. Lower density clusters were identified for research areas around the keywords therapy and safety and late gadolinium enhancement and fibrosis. These clusters identify the main areas of REEeh research, which include the effect of cerium oxide nanoparticles (CeNPs), the adverse effects of gadolinium as a contrast agent in magnetic resonance imaging, and accumulation processes in soil, plants, and water (Fig. 1).
In accordance with these results, the five most frequently assigned WoS categories relating to the different research areas were Chemistry, Materials Science, Environmental Sciences & Ecology, Radiology Nuclear Medicine & Medical Imaging, and Engineering (Table 1).
Analysis of publication development over time
The first article found about REEeh was published in 1950. In the following years, there were no or only a few publications. It was only since the 1990s that the annual number of articles increased to double digits and since 2005 the number increased to triple digits with a steep increase since then. The previous peak was reached in 2021 with n = 722 articles on REEeh. The number of annual citations (c) increased similarly since the 1990s, reaching two major peaks in 2008 (c = 11,288) and 2015 (c = 12,798). Afterward, a steep decline could be observed. The average annual citation rate (cr) emphasizes the year with high citation numbers and small publication volumes, leading to the peaks of 1978, 1987, and 1990 (Fig. 2A). A comparison of the increase in publication output of REEeh and all SCIE (Science Citation Index Expanded) articles, measured by the ratio Articles / (SCIE articles / 100,000), shows the above-average growth of articles on REEeh (Fig. 2B).
The comparison of the development of the publication figures and the production of rare earth oxides over time since 1990  show a steady increase for both (Fig. 3A). Chinese mine production equals global production and accounts for by far the largest share. Until 2002, when mining ceased for the time being, the USA accounted for a visible share of production. The period of short-term decline in mining starting in 2009, leading to the low point in 2012, was also accompanied by a decline in citation numbers over the same period, reaching the same low point in 2012. Production then picked up again, also in the USA, mining resumed, and publication patterns also grew disproportionately again (Fig. 3B).
Table 2 lists the most frequently cited articles. The article that has received the most citations so far is on the toxicity of cerium oxide nanoparticles, among others, by Xia et al. and was published in 2008 .
Analysis of publication countries
Of all retrieved articles, the vast majority, n = 6918 (99.67%), could be assigned to at least one country of origin. Of these, China and the USA are by far the most publishing countries at REEeh, with more than 1500 articles each (China n = 1730, USA n = 1558). Germany follows in third place by a wide margin with n = 478. Ranks 4 and 5 were occupied by India (n = 444) and Japan (n = 345). Of note is Iran’s ranking of 11th with n = 225 and Russia’s ranking of 15th with n = 153 articles on REEeh (Fig. 4A).
The two major players also lead in the number of citations, but here the USA is in the lead and China follows at a greater distance (USA c = 63,286; China c = 35,379). This was followed by Germany (c = 17,766), the UK (c = 14,532), and France (c = 11,314) (Fig. 4B). The UK ranked only 8th in the number of publications (n = 341) but had the 4th highest citation rate of countries with at least 30 articles on REEeh. Austria ranked first in citation rate (cr = 58.25), followed by Denmark (cr = 46.80), and Switzerland (cr = 42.94). The UK (cr = 42.62) is followed by the USA in 5th place (cr = 40.62) (Fig. 4C).
The national share of publications has changed over time. The analysis of the contribution of most publishing countries shows the decreasing relative share of the USA and the simultaneously increasing relative share of China. In the first evaluation interval (1987-1991), only a few countries participated in REEeh research, with the US being by far the most publishing country. In the last evaluation interval (2017-2021), the US-American share among the top 10 countries decreased to 16.47%, while China’s share increased to 28.27% (Fig. 5). The participation of India and Iran also increased steadily to 7.50 and 4.91%, respectively, in the last interval.
Over time, a broad international network has also developed for REEeh research. The USA, as the core country, has generated the most collaborations. They collaborated primarily with China (n = 133) and the UK (n = 74). However, joint work with Germany (n = 46) and Canada (n = 39) is also notable. In addition, the partnerships between China and Australia (n = 41) and Saudi Arabian and Indian researchers (n = 36) are worth mentioning (Fig. 6).
The most frequently addressed areas in this analysis were represented differently in the individual countries. In China and India, the most frequently recognized category overall was Chemistry, with 26.04 and 27.12%, respectively. In contrast, in the USA (28.12%), the most publishing European countries (Germany: 25.24%, Italy: 18.50%, France: 19.43%, the UK: 17.76%), and Japan (17.21%), the most frequently assigned area was Radiology, Nuclear Medicine & Medical Imaging. In India and China, this field landed in the last places of the top 10 research areas. Environmental sciences & Ecology was most represented in Canada (23.11%), followed by France (18.79%). However, this field also ranked second in Chinese REEeh research (19.59%). In Western countries and Japan, Cardiovascular System & Cardiology was the second most covered research area, while in China, India, and France, this area is rarely covered and falls to the last place of the top 10. The WoS category Materials Science was assigned most frequently in South Korean articles on REEeh and ranked second in China and India (Fig. 7).
Inclusion of research infrastructural features
When national scientific characteristics are included, the order of the leading countries changes. When calculating RGERD (ratio of the number of articles to gross R&D expenditures in billions of $), the ranking was topped by Iran, followed by Portugal, Pakistan, Ukraine, and Romania. When calculating RRES (ratio of the number of articles to the number of researchers in 10,000 FTEs), Switzerland ranked first, followed by Romania, Italy, Portugal, and Iran (Table 3).
Inclusion of characteristics of global REE production
The inclusion of economically important market variables such as the national stock of electric vehicles, the number of wind turbines, and the production of permanent magnets led to another ranking of the publishing countries (Table 4).
To determine the correlation between the number of articles on REEeh and market values, the correlation and the linear regression with the residual (Spearman) (Fig. 8) were calculated. All correlations were highly significant (articles and wind turbines: r = 0.80, p < 0.0001; articles and export trade value of permanent magnets: r = 0.78, p < 0.0001; articles and electric vehicles: r = 0.65, p = 0.0002).
Most publishing institutions
The institutions that mainly work on REEeh are summarized in Table 5.
Aspects of research development and focus
For the publication output on REEeh, a total of 6941 original articles indexed in WoS could be identified. They can be grouped according to the occurrence of keywords in some main topic areas, which are headed by the term “toxicity”. The first article on REEeh published in 1950 by Cochran et al. addressed the acute toxicity of some REEs and found that the general toxicity observed in rats was consistent with the similarity of the elements . This study has been cited repeatedly over time, 82 times to date, which is a comparably high number for postwar publications. In the 1960s, the research-grade REEs became available in sufficient amounts and at moderate prices  leading to a small over-proportional publication output compared to all SCIE-articles. In 1961, a study on the toxicological effects of gadolinium and samarium chlorides, which contained some evidence of adverse health effects, was also highly cited, with 69 citations .
The next study with a substantial number of citations (c = 420) was published in 1990. It examined the influence of thermodynamics on the toxicity of gadolinium complexes to increase their inherent safety . This publication marked the beginning of an initially moderate and, from 2005, steep increase in work on REEeh. In 2008 and 2015, both years with major citation peaks, particularly high-cited articles were published, including the top-cited studies of Xia et al. on cerium oxide toxicity  and McDonald et al. on gadolinium deposition after MRI . This also points to the identified thematic focus around the term “nanoparticle” in relation to oxidative stress and cytotoxicity, particularly of cerium oxide, and the term “gadolinium” in relation to its risks associated with use as a contrast agent in MRI.
In the 1950s, the nuclear arms race gave impetus to REE production and research. The opening of mines in Russia, South Africa, and California (USA) in the early 1950s was caused by the rising value of REEs. The contrast-enhancing effects of gadolinium were discovered in the 1980s , which later spurred research on its adverse health effects and triggered the steep increase in publications since 1990, as evidenced by the appearance of, for example, the article by Cacheris et al. . The disproportionate trend from 1990 compared to SCIE articles establishes this relationship. The adverse health effects of gadolinium, which can cause nephrogenic fibrosis, identified by Grobner  2006, led to research interest in this area and an increased volume of annual publications subsequently. In 2008, a year with a clearly visible citation spike, CeNPs were in the focus of two of the most cited articles in this analysis [19, 26]. Both analyzed the effect of CeNPs on the cell, noting or focusing on a protective effect against oxidative stress. In 2006 and 2011, the articles by Schubert et al.  and Celardo et al. , which are likewise among the most-cited articles, also addressed the neuroprotective and pharmacological potential of CeNPs.
After this year of high interest in the scientific community in 2008, the number of annual citations declined from 2009 to 2012, representing an apparent interim low point analogous to the declining depletion of REEs. At the same time, the number of publications continued to increase at a high level. In 2008, the share of Chinese articles was rather low compared to the years before or the steady increase after. This rise led China to overtake the USA in terms of the number of publications in 2013. Thus, it is to assume that the trend during this period is not due to an actual drop in citations, but more to the extremely high response in 2008, which was mainly due to the high proportion of US-American and German articles. Accordingly, China’s share of articles assigned to the subject area “Radiology, Nuclear Medicine & Medical Imaging” is small, while US-American and German research is primarily focused on this area. The trend, except for 2008, seems to be coherent. Since 2012, the growth of publications on REEeh has been extremely high, leading to a citation peak in 2015 with two high-profile articles, Kanda et al.  and McDonald et al. . Both articles deal with the deposition or accumulation of gadolinium in the brain.
The subsequent decline in citation numbers is due to the methodological phenomenon of the so-called cited half-life (CHL). This concept describes the estimated time it takes for a publication to reach 50% of its citations. For biomedical research, this period is assumed to be 7-8 years corresponding to the CHL . The citation rate still decreases as the number of Chinese publications increases. This is evidenced by the earlier onset of the falling values compared to the CHL-induced drop. This will certainly change in the future, as Chinese researchers are now aware of the importance of impact factors and other bibliometric key figures. They are obliged by governmental policies to produce not only quantity, but also high-quality studies that are listed in WoS [31, 32].
Global research aspects
The high share held by Chinese and US-American REEeh research is not surprising, as both countries have been competing for key positions in research performance in recent years . The main citation figures are currently still concentrated in the USA. Although, this may change in the future. Until the mid-1980s, the main source of REEs was in California, USA. But this mine was closed in 2002 due to stiff competition from emerging Chinese mining and the environmental problems associated with REE mining . Russian production was also scaled back during this period. The USA and Russia later started the REE mining again . Besides these two countries, some European countries (Germany, France, Italy, UK), India, and Japan dominate the global research landscape. It can be seen that the high-income countries focus mainly on radiological topics. “Nephrology” is also more prevalent in the REEeh research in these countries. They focus on the health effects of gadolinium as a contrast agent.
In comparison, China and India focus more on chemical and material topics. In China and Canada, environmental issues also receive high attention. For this, China, with the most production facilities, is clearly in the best position to study the hazardous effects of unregulated production. The results of these studies led to a rethink by Chinese authorities and the introduction of regulations and restrictions. So, China has shut down some small-scale mines and thinks about gentle mining procedures and the prevention of illegal mining through the Chinese consolidation strategy  to reduce adverse impacts on human health, ecotoxicity, and eutrophication and acidification of soils . To what extent this will lead to an improvement in the environmental impact of REE mining remains to be seen.
Regarding the ranking in terms of the citation rate, Austria is leading. With 1261 citations alone, the article by Thomas Grobner on the health risk of gadolinium  was responsible for this, especially since Austria having 44 articles on REEeh was only just above the analysis threshold of 30 articles. Grobner is a physician and specialist in internal medicine but not a high-profile scientist, having achieved only an h-index of 6 so far. Nevertheless, he set a milestone with his scientific observations in REEeh research.
Denmark (n = 44), which ranked second in terms of citation rate, also achieved this rank with research on the health effects of gadolinium, e.g., during pregnancy and lactation . Some of these studies were carried out in collaboration with Switzerland , which followed in third place in terms of citation rate. With partial participation of Danish scientists, Swiss research has successfully analyzed the use of yttrium additives for radioimmunotherapy of follicular lymphoma. This is the most common form of NHL, which has been shown to cause hematologic toxicities . Switzerland also topped the rankings when the number of researchers per country was included.
Regarding international collaborations, it is noteworthy that Saudi Arabia has been India’s primary partner in REEeh research. One focus of their joint work is the function of some REEs, e.g., for photo-degeneration or the -degradation of environmental pollutants [39, 40]. Apart from these collaborations, most of the cooperation took place between the two major players, China and the USA. Despite their different research foci, they share common interests in the field of environmental science, primarily because of the opportunity to study impacts directly at mining sites, the most located in China (Table 6).
Among REE mining countries, also India is in the top 5 of the publication ranking. It is also leading when the national economic interest concerning the stock of electric vehicles is included in the comparison. India has a long tradition of REE mining, as the government established India Limited (IREL), formerly Indian Rare Earth Limited , back in 1950.
Looking at other countries that mine REE and are ranked in the top 15 of the publication ranking, Brazil, has a long tradition of mining, as does Russia. The former USSR had long mined REEs with its mines in Kyrgyzstan . Scientific collaboration between Kyrgyz, Ukrainian, and Russian scientists is due to their shared history and the close ties that still exist. After closing its mines in 2003, Russia started exploiting REEs again in 2008 , which was accompanied by increasing publication numbers. The strong scientific partnership between Russia and Ukraine will certainly be affected during and after Russia’s war of aggression against Ukraine.
Other mining countries, Myanmar, Madagascar, Vietnam, and Burundi play rather minor roles in REEeh research. In Madagascar and Burundi, mining only started in 2017 and 2018, respectively , so their low contribution to REEeh research can be explained.
In addition to the influence of the countries’ mining status, the strong relationship between publication output and proxy values for market drivers (electric vehicle inventory, trade value of permanent magnets, and number of wind turbines) was demonstrated by significantly high correlation values.
In addition to these ratings, Iran tops the ranking when national research expenditures (GERD) are included in the evaluation of countries’ research performance in REEeh. It also ranks at the top when including interest in wind energy as measured by the number of national wind turbines. The Iranian government announced in early 2020 that it intends to industrialize the production of REEs  – an effort initiated in 2017 at the 9th Symposium of the Iranian Society of Economic Geology . Notably, studies on the suitability of potential mining sites increased accordingly in Iran , especially after the extraction of REEs as a byproduct of uranium mining was considered . This decision was accompanied by increase in research on REEeh in Iran.
Thus, the major publishing countries in absolute terms, the USA and China, fell behind when economic characteristics or the number of researchers per country were taken into account. As nations with the highest science funding and the highest number of researchers worldwide, this drop is not surprising when the ratio to publication output is calculated.
Instead, Portugal ranked second in the GERD rankings after Iran. Environmental research is at the heart of Portuguese studies on REEeh. For example, studies have been conducted in the Tagus Estuary Nature Reserve, one of the most important wetlands in Europe, or in other water bodies to determine REE contamination as an indicator of anthropogenic activities. Another Portuguese research focus was the accumulation of REEs in the environment. It was investigated where the REEs come from, whether from near inactive chemical complexes, effluents from wastewater treatment, or lithogenic sources, as in a coastal lagoon near Aveiro. The University of Aveiro was the institution where most Portuguese research on REEeh has been conducted [46,47,48].
Another country worth mentioning in terms of REEeh publication is Pakistan. It ranks third in regard to the inclusion of GERD and first in the inclusion of the trade value of permanent magnets. Even after REE deposits were discovered in Pakistan, the government did not initially promote mining. The reason given was the lack of financial resources and technical expertise for exploitation, although the reserves were highly estimated. A shift in thinking in this regard began in 2010 and coincided with the start of REE research in Pakistan. But it was not until 2021 that Pakistani publications reached double digits for the first time, at a time when the government finally decided to exploit its REE reserves .
Taking all these historical and regional aspects into account, it is clear that the research efforts on REEeh depend on economic factors on the one hand and interests and accessibility to data from mining sites on the other hand. The exploitation of REEs is still in the hands of a few countries, first and foremost China. Therefore, obtaining data is difficult for independent and open-ended research is difficult, which is necessary for research on the environmental and health risks of REE use and production. In addition, an independent and secure supply of REE must be maintained. Substitutes or reduction possibilities must be sought for the REEs applied, like those currently available are generally even less effective. New separation technologies for energy-saving processes must be sought. There is also a need for research in the area of recycling REE-containing equipment, as only a very small proportion is reused to date. All of these mandatory tasks represent an enormous challenge for future research that can only be met with networked expertise and resources on a global scale.
Methodological platform, data source, and data base generation
This analysis is based on the methodological and technical principles developed by the New Quality and Quantity Indices in Science (NewQIS) platform, which is an established methodology for assessing scientific performance under a variety of basic  and advanced parameters . It combines a representation of the global scientific landscape with sophisticated visualization techniques such as density equalizing map projections (DEMP) by following a method by Gastner and Newman .
The metadata underlying the bibliometric analyses was taken from the Web of Science (WoS) Core Collection, one of the most representative and qualitative sources of bibliometric research.
The data collection took place on 04-08-2022 and included all years from 1900 onwards.
The search strategy included, first, a title search with the various denominations for REE and all elements declared as REE. Only the radioactive promethium was excluded from the search because it does not form stable isotopes and cannot be extracted from geological material because it occurs only in “infinitesimal” concentrations [1, 53].
Title: “rare earth element*” OR “rare earth metal*” OR “tech metal*” OR “high-tech metal*” OR “cerium” OR “dysprosium” OR “erbium” OR “europium” OR “gadolinium” OR “holmium” OR “lanthanum” OR “lutetium” OR “neodymium” OR “praseodymium” OR “samarium” OR “terbium” OR “thulium” OR “ytterbium” OR “yttrium” OR “scandium” OR “lanthanid*” OR “lanthanoid*”.
Second, a combination of relevant terms describing human and environmental health effects of REEs was searched as a topic search in the title, abstract, and keywords. These terms were combined with the title search.
Topic: “Risk*” OR “contamination*” OR “hazard*” OR “*toxicolog*” OR “*toxicit*” OR “adverse effect*” OR “negative effect*” OR “negative health effect*” OR “*toxic effect*” OR “ecological” OR “pollution*” OR “contaminant*” OR “pollutant*” OR “bioaccumulat*” OR “bio-accumulat*” OR “bioavailab*” OR “bio-availab*” OR “threat*” OR “ecology” OR “environmental impact*” OR “oxidative stress” OR “fibrosis” OR “fibrotic” OR “male sterility” OR “pneumoconiosis” OR “neurological disorder*” OR “anti-testicular effect*”
Entries were filtered for original articles and then manually checked for representativeness. Subsequently, the term “price” had to be excluded from the search because it returned not thematically related entries. All metadata were transferred in a structured manner to an MS-Access database, which served as the basis for all analyses performed. Afterward, some data had to be manually standardized, such as the variations in the designation of affiliation, to unify them and thus make them suitable for further analysis.
Performed analyses and visualization techniques
The analyses included absolute parameters such as annual publication parameters, the number of publications, the number of citations, the citation rate per country, and research institutions to identify the major players in REEeh research worldwide. Relative parameters such as the characteristics of the scientific infrastructure and the main drivers of rare earth exploration were additionally used for an enhanced assessment. For this purpose, the number of researchers and the gross expenditure on research and development (GERD)  were used on the one hand. On the other hand, the number of wind turbines in 2018 , the national stock of electric vehicles , and the trade value of permanent magnets  were included as approximate values for relevant market drivers.
The main research areas were identified and analyzed for clustering using the VOSviewer application  and visualized to create density maps of the occurring keywords. Additionally, the assigned WoS categories directing to the research areas. They are assigned to each listed journal by WoS. Every article adopts automatically the category assigned to the journal in which it is published. As there are multidisciplinary journals there are more subject categories than articles in number.
To visualize the geographic results, DEMPs were applied to the national publication patterns. Using this algorithm, the world map is distorted according to the particular evaluation parameter by increasing the size of countries with high values and decreasing the size of countries with respective low values.
Statistically, the relationship between national characteristics and the number of publications was calculated using linear regression and correlation analysis (Spearman).
Methodological limitations and strength
The results presented have to be considered in context to the limitations and strength of the methods applied. WoS has been recommended for bibliometric analyses in numerous studies and has proven its worth by not only providing citation information but also ensuring database quality through strict listing requirements [59, 60]. Hence, it has advantages over the other databases that also provide citation counts, namely Scopus and Google Scholar. Scopus lists only articles since the 1990s, making it useless for the purpose of our study. Google Scholar lists all publications without quality requirements, while WoS requires an impact factor of the journal and proper peer review for the articles. Without these quality criteria, a valid scientific analysis is not possible because there is too much untested data. Another point to consider is the English bias of WoS, which is certainly responsible for the large proportion of English articles [61, 62].
The search strategy also has some limitations. It must be taken into account that, in principle, not all existing publications can be found and integrated into the analyses. However, the strategy can be optimized to find the majority of related entries and the minimum of unrelated entries. To ensure the representativeness of the database generated in this way, the metadata found were additionally checked manually and on a random basis.
Availability of data and materials
The bibliometric data is owned by and has been obtained from the Web of Science database. Therefore, authors are not allowed to share the data publicly or privately. Any researcher with access to the Web of Science database can obtain the data using the methods described in the paper.
Cerium Oxide Nanoparticle
Density Equalizing Map Projections
Full Time Equivalent
Gross Expenditures for Research and Development
India (Rare Earth) Limited
International Union of Pure and Applied Chemistry
Magnetic Resonance Imaging
New Quality and Quantity Indices in Science
Rare Earth Elements
- REEeh :
Rare Earth Element Research in context of environment and health
Science Citation Index Expanded
Web of Science
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Klingelhöfer, D., Braun, M., Dröge, J. et al. Environmental and health-related research on application and production of rare earth elements under scrutiny. Global Health 18, 86 (2022). https://doi.org/10.1186/s12992-022-00879-5
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