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 [20]. 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 [1] 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 [21].

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 [22]. 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 [19] and McDonald et al. on gadolinium deposition after MRI [23]. 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 [24], 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. [22]. 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 [25] 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. [27] and Celardo et al. [28], 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. [29] and McDonald et al. [23]. 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 [30]. 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].

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 [31]. 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 [1]. Russian production was also scaled back during this period. The USA and Russia later started the REE mining again [33]. 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 [34] to reduce adverse impacts on human health, ecotoxicity, and eutrophication and acidification of soils [35]. 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 [25] 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 [36]. Some of these studies were carried out in collaboration with Switzerland [37], 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 [38]. 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 [41], 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 [33]. 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 [33], 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 [18], 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 [42] – an effort initiated in 2017 at the 9th Symposium of the Iranian Society of Economic Geology [43]. Notably, studies on the suitability of potential mining sites increased accordingly in Iran [44], especially after the extraction of REEs as a byproduct of uranium mining was considered [45]. 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 [49].