# Rare Earth Elements Uptake by Synthetic Polymeric and Cellulose-Based Materials: A Review
## 5 Environmental Impact and Health Effects of REEs  
Although the increase in RE demand has also increased REE production and related waste, knowledge about the environmental impact and health effects from exposure to REEs has not been extensively studied, which differs from other heavy metals [3; 4; 5; 19; 20]. Some concerns about REE-containing ores refer to the presence of radioactive elements, such as Th or U, which are also present in REE mineral deposits and storage areas, where high levels of nuclear radiation have been measured [3; 4; 5]. Concerns exist about Gd presence in water sources across Europe and the US due to its usage as a contrasting agent in medicine, which is excreted in urine, and it has been found in water treatment plants [4]. The effects of REE exposure in biological organisms have been studied, which have shown necrosis, hepatic and pulmonary damage, behavioral changes in rats and mice [19], development impairment in fish and toxicity in marine organisms [19; 20; 21; 22], and plant growth inhibition due to disruption in nutrient uptake and plant metabolism [3; 4; 19]. Effects in plants are topics of concern considering the usage of REE oxides and nitrates in Chinese agriculture and should be discussed, especially considering that REE usage in agriculture adds a route of REE exposure to human beings and animals [22]. In humans, REE effects have been reported in some enzymes found in the gastrointestinal tract, which could cause malabsorption, indigestion, and protein deficit [3; 19; 20]. REEs also accumulate in the liver and bones [3; 19; 20]. REE accumulation has been reported in brain tissue, which could cause neurological and nephrological damage [19; 20]. Pneumoconiosis and lung diseases have been reported in REE mining workers [3; 4; 5; 20]. REE exposure causes cognitive impairment, bone alteration, tissue toxicity, and male sterility [3; 4; 17; 20; 22]. Increments in oxidative stress markers after REE exposure have also been reported, which could be dangerous for REE long-term exposure, although these effects have been used as tools for novel cancer treatments [20]. Critical REE toxicity values in human beings have been estimated for Sm (III) between 500 and 2000 mg/L [22].  
The US EPA has a strict regulation for REE mining and processing, which has halted REE mining operations due to environmental concerns [3; 4]. China has the largest REE mining production, and evidence points to the presence of REEs in water and soils near REE mining facilities and noted health effects on human populations near those facilities [3; 4; 5; 22]. However, there is no clarity about maximum exposure limits for REEs in waters for human consumption from environmental regulators and agencies, although limits to REE exposure have been proposed based on background concentrations of REEs in pristine environments [23]. Moreover, if industrial and mining waste derived from exploitation and product manufacture related to REE is considered, the lack of regulations about REE exposure and growing evidence suggesting that REE exposure could be hazardous [3; 4; 19; 20; 21; 22], it is possible to expect an environmental and/or human disaster in the midterm. Considering the facts exposed in this section, recovering REEs from secondary sources, such as mining wastewaters and AMD, could reduce the environmental and human population damage caused by REE exposure, which could be seen as a desirable advantage in the search for more sustainable economic extraction processes.