Rare earth elements (REEs) are a category of metals that group fifteen lanthanides (from lanthanum (La) to lutetium (Lu)), to which scandium (Sc) and yttrium (Y) are often added because of similar geochemical behaviour. Depending on their molecular weight, REEs can be divided into three categories: light REEs (LREEs: La, cerium (Ce), praseodymium (Pr), neodymium (Nd) and promethium (Pm)), medium REEs (MREEs: samarium (Sm), europium (Eu) and gadolinium (Gd)) and heavy REEs (HREEs: terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), tulium (Tm), itterbium (Yb) and Lu).

REEs are mostly used in the industrial sector in permanent magnets, metallurgical alloys, catalysts, lasers, phosphors, batteries or glass polishing, but are also increasingly used in medicine, as contrast agents for MRI (magnetic resonance imaging) or tomography, as well as radionuclide therapy. Although rare earths are naturally present in the earth's crust, the increase in global extraction combined with anthropogenic uses (industrial, agricultural or medical) is inducing more emissions into the environment, causing a gradual "enrichment" of aquatic ecosystems including freshwater and seawater.

More than a third of the literature on the effects of REE on human health relates to cytotoxicity. Hair, blood and blood serum are the most biomonitored matrices. In the near future, research should focus on other tissues and organs potentially targeted by REE. Cerium is the most studied of the REEs.

In a study, the distribution of REEs along the food web of the Loire estuary (France) was assessed. Several species representing different trophic levels were sampled: 8 vertebrates, 3 crustaceans, 2 molluscs, 3 annelids and 4 algae. The study showed the phenomenon of trophic dilution of REEs in the Loire estuary food web, with the lowest concentration recorded in vertebrate strains and the highest in primary producers.

The REE review has shown that there is an impact on human health, although REE levels in the environment and in food are currently present at low levels and do not pose a major risk to human health, they should be considered as a potential future risk as REE concentrations are expected to increase over time. Although the highest levels of REE were found in the study environment at low trophic levels, biomonitoring of these elements has been recommended due to the potential cumulative effects on human health, as they can be transferred to the human body via the trophic chain.