Mercury Exposure and Health Impacts among Individuals in the Artisanal and Small-Scale Gold Mining Community: A Comprehensive Review

1In February 2009, the Governing Council of the United Nations Environment Programme (UNEP) began development of a legally binding global instrument on mercury (Hg). In January 2013, governments agreed to text for this instrument, thus giving birth to the Minamata Convention on Mercury. In October 2013, the convention was signed in Minamata, Japan. Article 7 and Annex C of the convention address artisanal and small-scale gold mining (ASGM) and the development of national plans for ASGM. Included in the outline for national plans is development of a public health strategy on the exposure of artisanal and small-scale gold miners and their communities to Hg. Such a strategy should include the gathering of health data, training for health-care workers, and raising awareness through health facilities.

2Hg is used in gold mining to extract gold from ore by forming “amalgam”—a mixture composed of approximately equal parts Hg and gold. The amalgam is heated, evaporating the Hg from the mixture, leaving the gold. This method of gold extraction is used in the ASGM community because it is cheaper than most alternative methods, can be used by one person independently, and is quick and easy. The dramatic rise in the cost of gold over the last decade has fueled a gold rush by poverty-driven miners in many countries. ASGM occurs primarily in South America, Africa, and Asia, but it can also be found in North America and Australia. Approximately 15 million people, including approximately 3 million women and children, participate in the ASGM industry in 70 countries. ASGM is the largest source (37%) of global Hg emissions. Between 2005 and 2010, Hg emissions from ASGM doubled. Although most uses of Hg are declining throughout the world, the ASGM demand for Hg is expected to increase. ASGM accounts for the largest percentage of global Hg demand.

3Hg vapors in the air around amalgam burning sites can be alarmingly high and almost always exceed the World Health Organization (WHO) limit for public exposure of 1.0 μg/m3. These exposures affect not only the workers but also those in the communities surrounding the processing centers. reported that the range of the 8-hr time-weighted average airborne Hg exposure at gold mining operations in Venezuela was 0.1–6,315 μg/m3 with a mean of 183 μg/m3. The reported that tremor has been observed in workers exposed to 30 μg/m3 Hg and that renal tubular effects and changes in plasma enzymes are estimated to occur at 15 μg/m3. The vaporized Hg eventually settles in soil and the sediment of lakes, rivers, bays, and oceans and is transformed by anaerobic organisms into methylmercury (MeHg). In bodies of water, the MeHg is absorbed by phytoplankton, which is ingested by zooplankton and fish, thereby contaminating the food chain. It especially accumulates in long-lived predatory species such as shark and swordfish.

4Elemental Hg and MeHg are toxic to the central and peripheral nervous system. The inhalation of Hg vapor can produce harmful effects on the nervous, digestive, and immune systems and the lungs and kidneys and may be fatal. Children are especially vulnerable and may be exposed directly by eating MeHg-contaminated fish. MeHg bioaccumulates in fish and when consumed by pregnant women may lead to neurodevelopmental problems in the developing fetus. Transplacental exposure is the most dangerous because the fetal brain is very sensitive. Neurological symptoms include mental retardation, seizures, vision and hearing loss, delayed development, language disorders, and memory loss. In children, acrodynia, a syndrome characterized by red and painful extremities, has been reported to result from chronic Hg exposure.

5Elemental Hg, the form of Hg used in gold amalgamation, is a liquid that volatilizes rapidly. In humans, elemental Hg is typically measured in blood or urine. MeHg, the form of Hg that contaminates fish, is typically measured in blood, cord blood, or hair. Sample collection of hair is the preferred method of biomonitoring for MeHg because it is less invasive than blood sampling. Blood Hg concentrations characterize recent or current exposure. In populations where exposure to Hg occurs primarily through consumption of contaminated fish, most of the total Hg in the blood is organic and can be used as a measure of MeHg exposure.

6Given the prominence of ASGM as a significant source of global Hg and that national plans, as specified by the Minamata Convention, call for the collection of health data, this review was undertaken to identify studies that describe health effects and exposure in ASGM communities. We hoped that such information can serve as a resource to health authorities in those countries where ASGM is currently practiced.

7To identify relevant studies, we searched PubMed (http://www.ncbi.nlm.nih.gov/pubmed/) as well as ScienceDirect (http://www.sciencedirect.com/) and Google Scholar (http://scholar.google.com/). The following search terms were included: Hg, MeHg, fish consumption, hair, blood, urine, neurological effects, kidney effects, health effects, and gold mining. The names of numerous countries where ASGM is known to occur were also included in searches. A total of 1,317 potentially relevant studies were identified. Studies were considered relevant if they identified health effects or measured hair or urinary Hg concentrations in individuals residing in an area affected by ASGM. We identified 72 studies that fit our criteria for relevance. We divided the studies into two categories: a) studies that identified health effects in ASGM communities (Table 1), and b) studies that reported hair or urine Hg in ASGM communities or communities affected by ASGM (see Supplemental Material, Tables S1, S2, and S4). Several studies included measurements of blood Hg as well as hair Hg (see Supplemental Material, Tables S2 and S3). We included studies published through December 2012.

8We compared hair concentrations with 2.5 μg/g, the hair concentration associated with the provisional tolerable weekly intake (PTWI). The PTWI was established to protect the developing fetus from neurotoxic effects. In 2006, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) concluded that life stages other than the embryo and fetus may be less sensitive to the effects of MeHg. Available data did not allow the JECFA to make firm conclusions with respect to children ≤ 17 years of age and therefore stated that the PTWI also applied to children.

9In the present study, we compared urine concentrations with 50 μg Hg/g-creatinine, a concentration at which renal tubular effects and changes in plasma enzymes are expected to occur. Urine concentrations were also compared with 100 μg Hg/g-creatinine, a urinary concentration at which the probability of developing the classical neurological signs of Hg intoxication is high. For purposes of presentation, only studies that reported ranges and mean urinary or hair Hg were included in figures to demonstrate the extremes of Hg exposure. All study results, however, are presented in Supplemental Material, Tables S1–S4.

10We identified 17 studies in the literature that described health effects in ASGM communities (Table 1). The health effects studies were conducted in 10 different countries on three continents (South America, Africa, and Asia). All of the studies were cross-sectional.

11Numerous biomarker (hair, blood, urine) studies have been conducted in ASGM populations. The biomarker studies were conducted in 19 different countries on three continents. Studies that reported hair Hg concentrations among residents of ASGM communities, miners, and environmentally exposed populations are reported in Supplemental Material, Table S1. These studies were conducted in 14 different countries. Supplemental Material, Table S2, includes studies that examined hair and blood Hg in the same population; 8 different countries are represented by these studies. Supplemental Material, Tables S3 and S4 contain studies that reported Hg concentrations in blood and urine, respectively. The studies on blood and urinary Hg were conducted in 5 and 13 countries, respectively.

12Figure 1 shows urinary Hg concentrations from studies where both a mean (horizontal blue line) and a range (vertical line) were reported. “Neurological symptoms” designates the 100 μg/g-creatinine value for neurological effects identified by. “Kidney effects” designates the 50 μg/g-creatinine concentration at which renal tubular effects are expected to occur.

13Figure 2 shows hair Hg concentrations in female residents of ASGM communities from studies where both a mean and a range (where shown) were reported. Most of the means and all of the maximum concentrations reported were above the PTWI. Figure 3 shows means and ranges of hair Hg of children and infants in ASGM communities. Women and children were selected for Figures 2 and 3, respectively, because the developing fetus and children are considered more vulnerable to the effects of MeHg.

14The most common health effect reported among workers engaged in ASGM are neurological effects (Table 1). These include tremor, ataxia, memory problems, and vision disorder and were found to occur not just among those engaged in mining activities but also among fish consumers living downstream of mining activities.

15Increased urinary excretion of the enzyme N-acetyl-β-d-glucoaminidase (NAG), a biomarker of damage to the proximal tubules of the kidney, was found among occupationally exposed individuals as well as among those living in a community where gold mining had been practiced. Kidney dysfunction was clinically diagnosed in 9 of 103 individuals in a gold mining population in Peru. Those reporting kidney dysfunction had higher urine total Hg concentrations [geometric mean (GM) = 12.0 μg/g-creatinine] than those not reporting kidney dysfunction (GM = 5.1 μg/g-creatinine; p < 0.05). See Table 1 for further information on kidney effects.

16Four studies conducted in Brazil suggest that Hg exposure among gold miners and in gold mining communities is associated with an increase in the prevalence of markers of autoimmune dysfunction. See Table 1 for the details of each study.

17Ten of the study populations in Table 1 were in South America, six were in Brazil alone. Only two of the study populations are in Africa; four are in Asia. The number of studies in a geographic region should not be construed to represent the magnitude of the problem in that region, however. The number of artisanal and small-scale gold miners and the amount of Hg released from ASGM is as great, if not greater, a problem in Asia and Africa as it is in South America.

18,,,,, ),, and all reported urinary Hg concentrations well above 100 μg Hg/g-creatinine. Data on urinary concentrations of Hg are provided in Figure 1 and Supplemental Material, Table S4. The considers 100 μg Hg/g-creatinine to be the level above which the probability of developing classical neurological signs of Hg intoxication is high. High urinary Hg concentrations were particularly evident among those who amalgamate Hg or heat Hg to remove it from the amalgam. As an example of the elevated urinary concentrations found among small-scale gold mining operations, reported that the mean urinary Hg among gold dealers in Burkina Faso was 299.1 μg Hg/g-creatinine. Gold dealers were believed to have the most frequent exposure to Hg vapor. reported that among self-employed gold miners in Venezuela, the mean urinary Hg concentration was 148 μg Hg/g-creatinine; the high end of the range was 912 μg Hg/g-creatinine. reported that the mean urinary Hg concentration among a sample of 80 children working with Hg was 36.50 μg Hg/g-creatinine; the high end of the range was 666.87 μg Hg/g-creatinine. The children who worked in small-scale gold mining operations in Indonesia and Zimbabwe ranged in age from 9 to 17 years. found that Thai schoolchildren living near, but not working in, small-scale gold mining operations had increased urinary Hg concentrations.

19Hair Hg reflects the ingestion of Hg from fish (MeHg). The mean hair Hg concentration in virtually all of the 55 studies conducted in ASGM areas or areas affected by ASGM are above the concentration (2.5 μg/g) associated with the WHO’s PTWI (Figures 2 and 3; see also Supplemental Material, Tables S1 and S2). Many of the studies reported hair concentrations > 14 μg/g, which the considers a no observed effects level (NOEL). The NOEL is based on neurotoxic effects in the fetus; the PTWI was established to protect the fetus from neurotoxic effects. In 2006, the JECFA concluded that life stages other than the embryo and fetus may be less sensitive to the effects of MeHg. Because of a lack of data on children < 17 years of age, the JECFA concluded that the PTWI should also apply to children. Hair Hg concentrations above those that reflect the PTWI and NOEL should, therefore, not be indicative of health effects in adults. The concluded that a hair Hg concentration of 50 μg Hg/g would indicate a low (5%) risk of neurological damage to adults. Some of the studies listed in Table 1 [e.g., and ], however, suggest that neurological effects may be evident in adults at hair Hg concentrations of < 50 μg Hg/g. Blood concentrations of Hg may reflect exposure to either MeHg or inorganic Hg. The background level of Hg in blood for those who do not eat fish is 2 μg Hg/L.

20Individuals involved in the gold mining operations, their families, and those in the gold mining communities are exposed to dangerous levels of elemental Hg vapor, as evidenced by urinary Hg concentrations. This evidence includes extremely elevated urinary Hg concentrations in children who work in the mines and children who live in the areas where small-scale gold mining occurs. Residents in the gold mining communities and downstream of the gold mining communities consume fish that may be heavily contaminated with MeHg, as demonstrated by hair Hg measurements. Current studies indicate that those in the ASGM communities experience neurological effects, kidney effects, and possibly immunotoxic/autoimmune effects from Hg exposure. Not only is the danger widespread globally, but the problem is expected to grow. National public health strategies on ASGM, as required by the Minamata Convention, should be implemented immediately.