Mercury is a naturally occurring element. For hundreds of years, it has been directly dispersed by humans into aquatic and terrestrial ecosystems through activities such as biomass burning, mining of metal sulphide ores, use in precious metal extraction, and many other materials where it is present in significant amounts (e.g., paint, electronic devices and chlor-alkali plants). Its release and exposure to humans represent one of the most serious environmental contamination problems known. There are also natural sources of Hg release, including weathering of rocks, volcanic activity and forest fires. Mercury can evaporate from the ocean into the atmosphere, and be deposited back into the oceans or onto land.

Worldwide health concerns on Hg, centre on human consumption of seafood, mainly fish, contaminated with methyl Hg (MeHg), with neurotoxicity being the most important of these concerns. Methyl Hg easily reaches the bloodstream and is distributed to all tissues and can cross the normally protective blood-brain barrier to enter the brain. Mercury that enters the body is of especial concern to pregnant women and women of childbearing age, because it can readily move through the placenta to foetuses and their developing brains. Other effects of Hg exposure in humans include kidney damage, renal impairment, gastrointestinal, genetic, cardiovascular and developmental disorders, and even death in extreme cases.

Despite the prolificity of research worldwide on Hg exposure, there still exist several gaps in our understanding of the element’s emissions-to-impact path, as well as the dynamic interactions during metabolic processes that lead to maladies. For instance, there are significant gaps in the knowledge of chemical processes affecting Hg atmospheric transport and deposition, characteristics of the air-surface exchange and processes responsible for re-emission of Hg to the atmosphere (Travnikov et al., 2013; Leiva González et al., 2022); and properly designed simulation modelling experiments with high quality input data would go a long way in helping to bridge these knowledge gaps. The assemblage of a high quality geomedical database for Hg would also enhance our predictive capacity for future modifications in Hg pertubations in the environment under climate change, such as the still unknown direction of Hg ocean - air fluxes (See, e.g., Custódio et al., 2022) and the consequent implications for the elements’ toxicokinetics.

Mercury is emitted by natural sources, such as volcanoes, geothermal springs, geologic deposits, and the oceans; and by human activities. Human-related sources primarily include coal combustion, waste incineration, industrial uses, and mining.

In the environment, Hg occurs mainly in the form of inorganic Hg. Inorganic Hg species enter the air from the mining of deposits of ores that contain Hg, from the emissions of coal-fired power plants, from burning municipal and medical waste, from the production of cement, and from uncontrolled releases in factories that use Hg (ATSDR, reviewed 2015). Most models evaluating sources of Hg in the oceans have been built using input data on Hg deposited directly from the atmosphere. However, a new study now shows that rivers are actually the main source of this toxic element along the world’s coasts (Liu et al., 2021).

Relatively recently, Ehrlich and Newman (2008) gave an average crustal abundance of Hg by mass of only 0.08 ppm. There is a dearth of recent estimates of Hg content in the various components of the Earth’s crust (igneous, metamorphic and sedimentary rocks), as the following sections (I to III) clearly show.

Since elemental Hg is a liquid at room temperature, its gas phase is very important geochemically, for some of its compounds have relatively high vapour pressures.

Mercury is a chalcophile element (having affinity for a sulphide phase) and extremely active biologically. A complex relationship exists between Hg and Se (also primarily chalcophilic in character) with reference to toxicity effects (Zampetti and Brandt, 2023), with co-exposure leading to reduced toxicity (antagonism; See, e,g., Oliveira et al., 2017; Tinggi and Perkins, 2022), enhanced toxicity (synergism), or no effect at all (See: Wyatt et al., 2016). Different interactions may be related to chemical speciation, concentration, and method of administration (Dang and Wang, 2011); but these have not been thoroughly studied.

According to Timmerman and Omaye (2021), it is likely that the toxicity of Hg can result in Se deficiency, in that the affinity of Hg for Se is strong enough to cause deactivation of Se-dependent enzymes and proteins. Conversely, Se compounds can have prophylactic or antidotal effects, whereby the adverse toxicity action of Hg exposure can be prevented or reversed (Timmerman and Omaye, 2021). According to Gochfeld and Burger (2021) the high mutual binding affinity of Hg and Se is the reason that each can reduce the toxicity of the other. This is an area that needs further extensive research, as results obtained so far have been inconclusive.

Other metals/metalloids shown to have synergistic and/or antagonistic relationship with Hg include - Cr: e.g., synergistic action with Hg on fish (See: Selvan et al., 2012; Dwivedi et al., 2012); As: e.g., synergistic action in attenuating canonical and non-canonical NLRP3 inflammasome activation (See: Ahn et al., 2018); Pb: combined exposure (synergism) to Pb, inorganic Hg and MeHg showing deviation from additivity for cardiovascular toxicity in rats (See: Dwivedi et al., 2012) and Ni: synergistic effect of Ni and Hg on fatty acid composition in the muscle of fish Lates calcarifer (See: Senthamilselvan et al., 2016).

Before working out the degree of mobility, availability, and toxicity of Hg it is of critical importance to determine its speciation (See: Amde et al., 2016; Natasha et al., 2020). In nature, Hg exists in three oxidation states, viz., metallic or elemental Hg (Hg°), mercurous ion (Hg₂ ²⁺), and mercuric ion (Hg²⁺⁾. These oxidation states determine the properties and behaviour of Hg in the environment as well as in metabolic processes in humans, other animals and plants. Inorganic Hg¹⁺ species can also occur but are generally believed to play an inconsiderable role in natural systems; it is found mainly as shorter-lived intermediates in redox cycling between Hg° and Hg²⁺ species.

All three forms of Hg convey some degree of toxicity to life forms - including humans - but it is the Hg compounds containing the mercuric ion that are the most toxic, especially the organic-mercury compound, methyl mercury (CH₃Hg⁺) or simply, MeHg.

Mercury has seven stable isotopes, making it unique among the heavy elements - these isotopes are: Hg¹⁹⁶, Hg¹⁹⁸, Hg¹⁹⁹, Hg²⁰⁰, Hg²⁰¹, Hg²⁰² and Hg²⁰⁴, and they span a relative mass difference of 4%. On this score, Bergquist and Blum (2009) recognised the great promise held by Hg isotope systematics - both mass-dependent (MDF) and mass-independent fractionations (MIF) - for tracking this toxin’s geochemical transformations, and by extension, its adverse metabolic interactions (See Section 8, on: “Metabolic function (clinical toxicity)”, this article. As of today, however, we have only just begun exploring the potential of using Hg stable isotope signatures to provide new insights into Hg biogeochemistry (AuYang et al., 2022; McLagan et al., 2022; Ecley et al., 2023).

A lot has been written about the biogeochemical cycling of Hg in terms of our understanding of several aspects of the cycle, such as oxidation processes in the atmosphere, land-atmosphere and ocean-atmosphere cycling, and methylation processes in the ocean. Some of the more recent accounts include those of Gustin et al. (2020), Chételat et al. (2022) and Wang et al. (2022).

Most of the Hg that enters the atmosphere (from mining deposits of ores that contain Hg, from the emissions of coal-fired power plants, from burning municipal and medical waste, from cement production, and from uncontrolled releases in factories that use Hg), is in the form of metallic (Hg°) and inorganic compounds (ATSDR, reviewed 2015). The fate of Hg after it is emitted depends on several factors such as: the form of Hg emitted, location of the emission source, the height above the landscape from which the Hg is emitted (e.g., the height of a power-plant stack), the surrounding terrain, and the weather. Redeposition of Hg⁰ mainly as Hg⁺⁺ (because Hg⁰ is oxidised in the atmosphere) takes place on terrestrial surfaces by dry deposition, including foliar uptake, sorption, and gas dissolution (Enrico et al., 2016). The ultimate deposition of Hg, probably as HgS ore, is believed to be in ocean sediments (Sanei et al., 2021; Zaferani and Biester, 2021), where part of the inorganic Hg emitted becomes oxidised to Hg⁺⁺ and then methylated or in other ways transformed into organomercurials (WHO, 2000; Ignatavičius et al., 2022).

Some of the effects of climate change on the environmental perturbations of Hg include: its impact on Hg deposition, viz., a synchronous change in global Hg deposition as reported by Li et al. (2020); changes in cycling of Hg, engendering an increase in MeHg and hence increased contamination (See: Fahnestock et al., 2019; Zhou et al., 2021; Chai et al., 2022; Chételat et al., 2022; Wang et al., 2022).); and increase in Hg uptake in fish and marine mammals in regions experiencing glacial retreat (See, e.g., Wang et al., 2020; McKinney et al., 2022).

The solubility, mobility as well as availability of Hg in soils are quite low, and dependent primarily on factors such as soil texture, the amount of organic matter and soil pH (See, e.g., Różański et al., 2016; Gworek et al., 2020). A wide array of physical, chemical, biological and isotopic exchange methods has been developed to estimate the (bio)-available pools of Hg in soil in an attempt to offer a plausible assessment of its risks. Unfortunately, many of these methods do not mirror the (bio)-available pools of soil Hg and suffer from technical drawbacks (Huang et al., 2020).

The main reservoir of Hg in aquatic environments appear to be sediments, which may also serve as a source of Hg to aquatic food webs (See: Jonsson et al., 2022). In aqueous systems and anoxic sediments, the geochemical forms of Hg (and subsequent bioavailability) are controlled primarily by reactions between Hg (II), inorganic sulphides, and natural organic matter (Hsu-Kim et al., 2013). Inorganic Hg compounds are, in general, water soluble with a bioavailability of 7%–15% after ingestion (Park and Zheng, 2012).

Plants differ (from species to species) in their ability to take up Hg and can develop a tolerance to high concentrations on contaminated sites, with corresponding elevated concentrations in edible parts compared with natural soils (Kabata-Pendias and Mukherjee, 2007).

Studying the process of methylation of inorganic forms of Hg constitutes the first step in understanding the process of bioaccumulation of MeHg in aquatic food webs (Figure 2). Because the process is primarily mediated by anaerobic bacteria (Hsu-Kim et al., 2013) knowledge about the bioavailability of inorganic Hg to these microbes is important in understanding of MeHg bioaccumulation in fish and other high trophic-level species (See, e.g., Li et al., 2021).

As already stated, the dominant route of exposure of humans and other higher order wildlife to MeHg is through the ingestion of fish and seafood products, with most fish - both freshwater and saltwater - containing varying levels of MeHg, which can enter and accumulate in the food chain (Ullrich et al., 2001; Al-Sulaiti et al., 2022). Although Hg is not known to be an essential part of the food chain, it is assimilated by organisms living in environments which contain it.

The absorption, transport and distribution of Hg are dependent on the Hg species. The main entry route of elemental Hg (Hg⁰) into the body is inhalation, with up to 80% of the inhaled vapours directly entering the bloodstream (Martinez-Finley and Aschner, 2014). Absorption of elemental Hg by the gastrointestinal tract, however, is weak (less than 0.01% in rats) (WHO, 2000; Fowler and Zalups, 2022; US EPA, 2022). Upon absorption, elemental Hg enters all tissues and accumulates in the central nervous system and kidneys. Inorganic mercury (HgS) has a lower liposolubility, engendering lower organism absorption and limited passage through the blood-brain barrier (Lohren et al., 2016).

Oral exposure is the primary route for inorganic Hg salts (Park and Zheng, 2012). Mercury absorption from oral ingestion also depends on the form of Hg. Absorption and accumulation of Hg in its metallic form have already been described at the beginning of Section 7 (this Section).

Dermal penetration is usually not a significant route of exposure to inorganic Hg (Bjørklund et al., 2019; Tincu et al., 2022).

The main route of absorption of organic Hg (MeHg) is the GI, but some absorption of MeHg through the skin and the lungs also occurs (Posin et al., 2022).

Once absorbed, the majority of Hg²⁺ accumulates in the kidney and liver (Martinez-Finley and Aschner, 2014; Zhao et al., 2022). Subsequent to absorption into the circulation, MeHg enters erythrocytes where a substantial amount binds to haemoglobin and a lesser amount to plasma proteins. A small amount of Hg (some 10% of the burden of MeHg) gets to the brain where demethylation to an inorganic Hg form proceeds slowly. Methylmercury readily crosses the placenta to the foetus, where deposition within the developing foetal brain can take place (See: ATSDR, reviewed 2015; Bjørklund et al., 2019) with serious neurological consequences for the newborn, though other recent studies (e.g., Dack et al., 2022) have ruled out the likelihood of dietary Hg exposure during pregnancy being a risk factor for low neurodevelopmental functioning in early childhood.

The toxicity of Hg is strongly influenced by microbially mediated biotransformation between its organic (MeHg) and inorganic [Hg (II) and Hg⁰] forms.

Mercury is not classified as an essential element and no known benefits have been reported for life processes in animals and plants.However, a number of studies have revealed the presence of Hg in significant quantities in herbal dietary supplements and in several therapeutic preparations (See, e.g., Puścion-Jakubik et al., 2021; Brodzial-Dopierala et al., 2018), such as to warrant further research that would possibly lead to implementation of a stricter quality control of dietary supplements. The use of Hg in traditional medicine is also becoming widespread (See, e.g., Street et al., 2015).

Reported therapeutic uses appearing in the literature include various medications, ointments, dental fillings, contact lenses and cosmetics. A number of instruments for medical applications such as thermometers and sphygmomanometers also make use of Hg (See: Iqbal and Asmat, 2012), though a number of healthcare institutions and clinicians worldwide have been calling for a replacement of these instruments even before the year 2010 (See, e.g., Buchanan et al., 2011; WHO, 2015; Asayama et al., 2016).

Excretion of Hg takes place primarily through the urine and faeces, with smaller amounts leaving the body through exhaled breath, sweat, and saliva.Following short-term exposure to Hg vapour [elemental Hg (Hg°)], about a third of the absorbed Hg will be eliminated in unchanged form through exhalation, while the rest will be eliminated mainly through the faeces (Ishihara, 2000; Abass et al., 2018; Paduraru et al., 2022).

Another form of excretion, albeit an undesirable one, is through lactation (ATSDR, reviewed 2015). Wu et al. (2022) have found that AnGongNiuHuang Pill (AGNHP) (a Hg-containing traditional Chinese drug for the treatment of “febrile disease”) increased blood Hg absorption and reduced urinary Hg excretion. Other studies, e.g., the one by Li et al. (2012) have noted that organic Se supplementation could increase Hg excretion and decrease urinary malondialdehyde and 8-hydroxy-2-deoxyguanosine levels in local residents. For synergistic/antagonistic relationships between Se and Hg, see Section 3: “Associations (synergism/antagonism)”, this article.

Mercury concentrations in humans and other animals have been determined in blood, urine, body tissues, hair, breast milk, and umbilical cord blood. Most of the earlier methods used atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), or neutron activation analysis (NAA), although mass spectrometry (MS), spectrophotometry, and anodic stripping voltammetry (ASV) have also been employed. The most commonly used method, pre-2000s was cold vapour (CV) AAS introduced in 1968 by Hatch and Ott (1968). As this technique (CV AAS) went through further development, it became possible to reliably determine Hg concentrations below the ppb level (>76% recovery) through either direct reduction of the sample or reduction subsequent to pre-digestion (See: Risher, 2003).

Today, the concentration of Hg can be accurately determined in air, water, soil, and biological samples (blood, urine, tissue, hair, breast milk, and breath) by a variety of analytical methods. Most of these methods are for total Hg (inorganic as well as organic Hg compounds) and are based on wet oxidation followed by a reduction step. However, relatively modern methods also exist for the separate determination of inorganic Hg compounds and organic Hg compounds (See, e.g., Risher, 2003).

There continues to be a huge and continuous interest in improvements on the determination of Hg and its speciation, mainly due to the element’s high toxicity and biomagnification (Liem-Nguyen, 2016). Developments in analytical procedures for Hg, its speciation analysis and its applications in environmental and biological studies since the year 2008 were extensively reviewed by Gao et al. (2012). Despite these relatively recent advances in analytical techniques, interpretation of Hg geochemistry in environmental systems still remains a challenge (See, e.g., McLagan et al., 2022). This is largely due to our inability to identify specific Hg transformation processes and species using established analytical methods in Hg geochemistry (total Hg and Hg speciation). Recently, Yu et al. (2019) and Favilli et al. (2022), respectively reviewed and critically examined different approaches for the quantification of Hg species in different samples using HPLC-ICP-MS.

Further improvements in analytical techniques for determining Hg in biological and environmental samples are now particularly focussed on the attainment of lower detection limits and ease of operation of analytical instrumentation, as exemplified in the following overview which encapsulates selected works that represent these developments:

• In 2022, Santos et al. (2022) described a simple and cost-effective wet digestion method for the determination of total Hg in samples of small plankton material using a cold vapor atomic fluorescence spectroscopic (CV AFS) technique.

In this section, a brief mention is made of selected recent geomedical research on Hg toxicity in Africa, focussing on work done in the last two decades (post-2000), or so. Most of the research on Hg exposure in Africa has been done in South Africa, despite the existence of multiple sources of Hg pollution in several other African countries. Much of this research centres around the impact of Hg exposure during alluvial Au mining, which is also a common activity in a number of countries across the Continent. A few selected examples follow:

Bonzongo et al. (2004) present a case study on health impacts due to Hg exposure in artisanal Au mining in Ghana, West Africa. Oosthuizen et al. (2010) studied Hg exposure in a low-income community in South Africa associated with Au mining activities to determine whether significant exposure to Hg occurs. Spiegel (2009) looked at socio-economic dimensions of Hg pollution abatement in artisanal mining communities in Sub-Saharan Africa; and in 2013, Lusilao-Makiese et al. (2013) assessed the impact of historical use of Hg for the extraction of Au in watersheds from an abandoned mine in Randfontein, a town located some 45 km west of Johannesburg (South Africa).

In 2007, Learner (2007) reported on the development of the South African Mercury Assessment (SAMA) Programme during the Stockholm Water Week that year. The research areas addressed in the SAMA Programme included: “(a) regulatory framework; (b) analytical methods; (c) source, speciation, fate, and transport; and d) impacts (ecological and human health)”. In 2010, Papu-Zamxaka et al. (2010) looked at the consequences of Hg contamination in the vicinity of a Hg processing plant in the KwaZulu-Natal Province of South Africa.

In 2011, Walters et al. (2011) reviewed the status of Hg as a pollutant in the aquatic ecosystems of South Africa.

In 2011, AJUA Environmental Consultants prepared on behalf of the Department of Environmental Affairs of South Africa, a report entitled: “Situation analysis of mercury in South Africa: Development of an inventory of mercury uses and sources”. Mercury release calculations used UNEP (2011) Toolkit level 1 (revised January 2011) were based on the mass balance principle, where the quality of input data impinged strongly on the system outputs. A further review of this ‘Toolkit’ appeared in 2019 (UNEP, 2019b). In 2014, Hanna et al. (2015) compiled published data on Hg contamination in African freshwater fishes, invertebrates, and plankton, and examined factors that may be responsible for concentrating Hg in these organisms. Despite revealing strong trends in Hg concentration, the work revealed a marked paucity of data on a number of other aspects of Hg dynamics in Africa.

The work of Street et al., in 2015 looked at the perceptions and practices in the use of metallic Hg by South African traditional health practitioners, stressing the need for putting in place, public education messages and regulatory measures.

In 2015, Bosch et al. (2016) outlined gaps in the then current knowledge and research on the accumulation of metal contaminants including Hg in commercially consumed marine fish worldwide and especially in South Africa, that affect both the fishing industry and fish consumers.

In 2016, Garnham and Langerman (2016) calculated the amount of Hg emitted from each of the South Africa Electricity Supply Commission (Eskom) coal-fired power stations, based on the amount of coal burnt and the Hg content in the coal. These authors calculated Emission Reduction Factors (ERF’s) from two sources, in working out the co-benefit derived from the emission control technologies at the stations.

In 2019, Belelie et al. (2019) performed a pilot study to characterise ambient total gaseous Hg concentrations over the South African Highveld (a portion of the South African inland plateau). In the same year (2019), Panichev et al. carried out large-scale assessment of atmospheric air pollution by Hg using the lichen Parmelia caperata as biological indicator. In the study by Panichev et al. (2019), five provinces of South Africa were sampled between 2013 and 2017; and the lichens analysed to provide time-integrated data, which correspond to the mean Hg concentration in air at a specific location over a long time period. Nevondo et al. (2019) suggested that Hg coming from contaminated groundwater from landfill leachate seepage could carry significant amounts of Hg, especially in landfills that are not lined with a geomembrane.

Bieser et al. (2020) investigated sources and sinks of Hg at Cape Point, South Africa by combining atmospheric Hg data with a trajectory model; and found that continent is the major sink, and the ocean, especially its warm regions (i.e., the Agulhas Current), is the major source for Hg.

Astolfi et al. (2020) used Hg in hair as biomarkers for understanding of Hg exposure in a population living in Asmara, capital of Eritrea; and to evaluate the influence of certain demographical factors on the Hg levels in hair. These authors (Astolfi et al. (2020)) found hair Hg concentration to be significantly higher among women compared to men (p < 0.001), underlining once again the need for routine biomonitoring surveys and the promotion of health education by municipal authorities, decision makers and workers.

Bredenkamp’s 2021 ‘Thesis’ (North-West University, South Africa) pointed out the paucity of long-term measurements of atmospheric Hg at inland background sites in South Africa; and held that such measurements, in conjunction with data from the coastal CP GAW (Cape Point Global Atmosphere Watch) station will be essential in providing credible estimates on background atmospheric Hg concentrations in South Africa; as well as contribute to formulation of cogent policies and standards on Hg emission limits.

In 2022, Elumalai et al. (2022) performed a chemometric analysis of exposure and human health on tourist beaches in Durban, South Africa, coming up with observations that lend credence to the criticality of erecting long-term monitoring schemes for Hg in order to provide requisite data for formulating improved public health management strategies. The emergence of relatively new techniques such as Hg stable isotopes now enables us perform more accurate biomonitoring exercises (See: Li et al., 2022).

Ansah et al. (2022) constructed a model for Hg in soil and Hg leaching to waters for use in assessing present and future pollution levels for the environment in Ghana, a country where artisanal small scale gold mining (ASGM) activities are intense, and the use of Hg in the extraction process, widespread. The model, which according to Ansah et al. (2022), can be run with a 5 × 5 km² scale resolution and a yearly or monthly time-step; and is considered to be an important tool for policy support in future regulation of the mining sector.

A noteworthy case description can be given of Cato Ridge, a small industrial village in the KwaZulu Natal Province of South Africa. Mercury waste shipments from other countries have been received at a Cato-Ridge plant owned by Thor Chemicals in South Africa, a subsidiary of Thor Chemicals, Inc. of Great Britain (See, e.g., Phalane and Steady, 2009). Investigation into the international shipment of hazardous wastes has focused attention on the consignment and its delivery to South Africa. This Firm has been accused of poisoning workers and putting surrounding communities at risk from Hg exposure. As of December, 2,000, the Hg wastes were still stockpiled at Thor Chemicals, Inc., in Cato-Ridge. Research on the fate of residual Hg at Cato-Ridge is perhaps a worthwhile proposition.

Mercury pollution is a global problem, not least in the Continent of Africa. Despite the prolificity of research in the past two decades or so, there are still several uncertainties and variabilities in our knowledge of both the element’s exposure dynamics and its health effects.

Understanding the intricacies of the element’s emissions-to-impact path is rendered intractable by its varied environmental fate and the overarching influences of environmental, geological/geochemical, biological and socioeconomic drivers (See: Budnik and Casteleyn, 2019). This knowledge is vital as it carries the potential of swaying the course of diagnosis and treatment proffered by medics for Hg-induced maladies. In this context, the use of simulation modelling techniques is thought to carry a high potential for eliminating or reducing some of these knowledge gaps.

Here, a list comprised of key uncertainties, future research needs, and recommendations is presented to assist researchers in readily identifying important gaps in knowledge in the various intricacies of Hg exposure and its outcomes. A comprehensive list of the more recent references is given, to aid the search process of those researchers who seek additional information on areas not covered in this article.

• The evaluation of the content of Hg in the Earth’s crust as well as in the different rock types is critical in any simulation modelling of the elements’ source-emission-impact path. Many Hg simulation models still use Hg concentration values in source rocks that were determined prior to the 2000s (e.g., Turekian and Wedepohl, 1961; Jonasson and Boyle, 1972), when instrument detection limits and operating parameters were not yet at their optimum; nor were techniques for data interpretation and compilation at their most refined.