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Mercury vapors released from elemental mercury occur naturally in the environment as a consequence of the removal of gases from the earth’s crust, volcanic eruptions, and evaporation from oceans and soils. Anthropogenic sources such as metal mining, smelting (with mercury, gold, copper, and zinc), coal combustion, municipal incinerators, and the chloralkaline industry contribute significantly to atmospheric mercury, and mercury vapors are stable within the atmosphere for about one year (Tokar et al. 2013). Once released into the environment, the various forms of mercury undergo complex oxidation-reduction and methylation-demethylation reactions known as the mercury cycle, which further gives rise to inorganic or organic species that become globally distributed.

Exposure to inorganic mercury can occur through inhalation and through oral or dermal routes, but inhalation is the most biologically relevant owing to its high absorbance potential in the lungs (up to 80%; see Risher et al. 2003). Inorganic mercury poisoning often occurs as a result of the inhalation of mercury vapor from industrial sources, dental amalgams, or the continued use of mercury compounds in consumer products (Clarkson and Magos 2006). The inhalation of mercury vapor from the use of liquid mercury can follow from occupational exposure in the chloroalkali industry, in the manufacture of scientific instruments, in fluorescent lightbulb manufacturing, in small-scale gold mining, and in dentistry where mercury amalgams are used as fillings for tooth decay (Clarkson and Magos 2006; Risher et al. 2003). Mercury poisoning can also result from inorganic mercury, which is most commonly found as calomel or mercurous chloride (Hg₂Cl₂) and was used widely in diuretics, antiseptics, skin products, laxatives, and teething powder in the twentieth century (Clarkson and Magos 2006; Klaassen 2013). Additionally, the use of phenylmercuric compounds as antifungal agents in paints, although banned in the United States, still continues in other countries (Risher et al. 2003).

      Ding et al. (2016) investigated the plasma miRNA expression profile for female workers in eastern China who were occupationally exposed to inorganic mercury and found that four miRNAs (miR-16-5p, miR-30c-3p, miR-181a-5p, and let-7e-5p) were downregulated and four miRNAs (miR-92a-3p, miR-122-5p, miR-451a, and miR-486-5p) were upregulated, as measured by microarray. Validation of these miRNAs by TaqMan-based RT-qPCR revealed that two miRNAs, namely miR-92a-3p and miR-486-5p, were consistently upregulated when measured by both methods.

      Both metallic and organic mercury compounds are oxidized to inorganic mercury in the blood and in the liver, which plays a key role in animal toxicity (Tchounwou et al. 2003). However, organomercurial compounds are generally more toxic to animals than inorganic mercury on account of their high bioaccumulation potential, and methyl mercury (MeHg) is toxicologically the most important organic form. In humans, consumption of methyl mercury-contaminated fish is the predominant route of exposure to organomercury, but other consumables such as drinking water, cereals, vegetables, and meat can also be sources of exposure (Holmes et al. 2009; Karagas et al. 2012; Yang et al. 2020). Early stages of life are generally more sensitive to mercury, such that exposure to high-level elemental, inorganic, and organic mercury results in severe developmental and neurological defects, depending on the length and dose of this exposure (Yang et al. 2020). However, to date, no literature exists that has examined circulating miRNAs that are due to the consumption of methylmercury-containing food sources.

      Circulating miRNAs Associated with Cadmium Exposure

Cadmium (Cd) is a naturally occurring toxic metal of environmental and occupational concern. The main source of cadmium contamination is related to its use in industries, for example in the production of nickel–cadmium batteries, pigments, chemical stabilizers, and metal coatings (Genchi et al. 2020). Fossil fuel combustion and the use of phosphate fertilizers also contribute to cadmium accumulation in the environment. Human exposure to cadmium occurs mainly through the ingestion of contaminated food and water and through inhalation; cigarette smoking is the most significant source of human exposure to cadmium (Bernhoft 2013; Friberg 1983). Once absorbed, cadmium is bound by metallothionein (MT) within liver cells and is released into circulation in the form of MT-cadmium complexes. These complexes are filtered through the renal glomeruli and reabsorbed by the proximal tubular cells, processes that make the kidney a primary target organ of toxicity (Yuan et al. 2020). Since cadmium remains tightly bound to MT and is almost completely reabsorbed in the renal tubules, excretion in urine is very low, resulting in a twenty-five–thirty-year half-life for cadmium in the body (Genchi et al. 2020). Additionally, women with lower iron status are at risk for greater absorption of cadmium after oral exposure (Olsson et al. 2002). Cadmium can be measured in blood, urine, feces, hair, nails, saliva, and milk. Cadmium in blood indicates recent exposure and its acceptable level ranges between 0.03 and 0.12 μg/dl in blood (Alli 2015). In smokers, cadmium concentrations are typically above 3 μg/L, whereas in the general population the concentration varies between 0.1 and 1.0 μg/L (Fowler et al. 2015). Acute exposure to high levels of cadmium can result from inhalation or ingestion and initial symptoms of inhaled cadmium exposure (8.63 mg/m³ for five hours) include chills, fever, and myalgias. Later, individuals can develop chest pain, cough, and dyspnea (Newman-Taylor 1998). Gastrointestinal symptoms such as nausea, vomiting, diarrhea, abdominal cramps and pain have also been described after the ingestion of cadmium-contaminated food or drinks (Fowler et al. 2015). Long-term or chronic exposure to low levels of cadmium is also of concern. The most common health effects of low-dose chronic exposure include cancers, cardiovascular disease, diabetes, osteoporosis-related fractures, and kidney dysfunction, the kidney being the critical target for cadmium toxicity (Jarup and Akesson 2009). Recent studies have shown adverse kidney effects even at doses lower than the provisional tolerable weekly intake of 1 μg/kg body weight/day (Kobayashi et al. 2006; Noonan et al. 2002; WHO 1993). Hence it is crucial to identify potential biomarkers for the early detection of cadmium exposure.

      Few studies have addressed cadmium-induced alterations in miRNA expression in humans. miR-122-5p and miR-326-3p were found to be upregulated in the serum of a cadmium-exposed population from China (Yuan et al. 2020). The commonly used biomarkers for cadmium exposure, urinary β2-microglobulin and retinol-binding protein, remained in the normal range, which suggests that these two miRNAs exhibited greater sensitivity to cadmium exposure than the urinary markers; and this makes them suitable candidates for the early detection of exposure (Yuan et al. 2020). In another study, levels of miRNAs were measured from the serum and urine of patients in China diagnosed with occupational chronic cadmium poisoning (Chen et al. 2021). In the urine, 16 miRNAs were found to be upregulated and 36 downregulated, while in the serum 46 miRNAs were upregulated and 131 were downregulated. An overlap of 59 abnormally expressed miRNAs was found between serum and urine samples of occupational chronic cadmium poisoning patients; these included miR-122-5p, miR-363-3p, miR-129-5p, miR-204-3p and miR-361-3p (Chen et al. 2021). In individuals exposed to low-levels of cadmium in China, the levels of miR-21 and miR-29b in plasma were upregulated. Serum levels of miR-30 were decreased in Chinese chronic obstructive pulmonary disease (COPD) patients with elevated levels of cadmium in serum by comparison to the levels in healthy control subjects. Additionally, an epithelial-mesenchymal transition was observed in the lung tissue of these patients (Zheng et al. 2021). In another study, individuals in India who were occupationally exposed to cadmium presented elevated levels of serum miR-221 and IL-17, a pro-inflammatory cytokine (Goyal et al. 2021; Zenobia and Hajishengallis 2015). Overall, several studies have identified differentially expressed miRNAs in the urine, serum, and plasma of individuals exposed to cadmium only in the Chinese population.

      Circulating miRNAs Associated with Chromium Exposure

Chromium (Cr) is the seventh most abundant element on the earth. As a transition metal, chromium can occur in any of the oxidation states from -II to +VI, but it is mostly found in oxidation states 0, +III, and +VI (Sperling 2005). In the environment, chromium can occur in hexavalent (Cr(VI)) or trivalent (Cr(III)) species (Nath et al. 2009). Cr(VI) has been classified as Group I human carcinogen (Proctor et al. 2014; Straif et al. 2009). Hexavalent chromium is used extensively in industry, for instance in leather tanning, in textile and pigment manufacturing, in stainless steel production, and in welding, polishing, and grinding (Wang et al. 2017). Occupational exposure is thus a major concern, primarily because of inhalation. Once absorbed, chromium circulates in the blood, getting bound to the β-globulin plasma fraction, and is transported to tissues

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