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National Institute of Environmental Health Sciences

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New Insights on Mercury Exposure from Artisanal Gold Mining

How mercury can enter our environment

According to the World Bank, more than 100 million people, most of them in developing countries, rely on artisanal and small-scale gold mining (ASGM) for income, and roughly 20 to 30 million people have been estimated to participate directly in the activity. The scale of ASGM has grown considerably over past decades. While roughly 25% of the world's gold production comes from ASGM, it is also the largest source of mercury emissions to the environment. Roughly 35% of the world’s total mercury emissions come from ASGM.

During the gold production process, they make an amalgam of gold-containing silt and metallic mercury to extract the gold, and then heat the amalgam to recover the gold. This process releases toxic mercury vapor into the air. This mercury vapor is not only directly inhaled by miners and others nearby, but it can also travel for miles before being deposited on plants, soil, and water. Converted into methylmercury by bacteria, it then enters the food chain, especially through aquatic environments. This methylmercury bioaccumulates as it moves higher up the food chain, making predatory fish especially toxic if eaten by humans.

Three recently published papers look at how mercury exposure from ASGM has impacted populations in South America, including indigenous villages in Brazil, communities in Peru, and villages in Suriname. These studies shed new light on variations and modifying factors that can either enhance exposures or reduce them.

Indigenous Villages in the Brazilian Amazon Face Unique Challenges

Yanomani Reserve
The indigenous Yanomani Reserve near the Brazil-Venezuela border.
Map Courtesy of Wikimedia

The first study examined rural villages in the region inhabited by the indigenous Yanomani people near the Brazil-Venezuela border. Three villages were included, one with active AGSM, one with historic AGSM from the 1980’s, and a third where inhabitants denied any past or current AGSM activities. The village with active AGSM had considerably higher levels of mercury, with 92.3% surpassing a level of 6 mcg/g in hair. In contrast, the one village with only historic AGSM had only 6.7% exceeding 6 mcg/g. Children and women had elevated levels as well as men.

In one village, the researchers measured mercury in sequential hair segments to assess whether exposures changed over the course of the year. They found indications of lower exposure during the rainy season and hypothesized that consumption of carnivorous fish might be lower then.

In their discussion, the authors point to the challenges in assessing the health risks from these exposures, and how the lack of access to health care and high prevalence of infectious diseases and malnutrition may interact with the toxicity of mercury. They conclude with a call to address the complex web of problems associated with AGSM and mercury through an increased focus on achieving the United Nations (UN) Sustainable Development Goals.

In Peruvian Communities, Dietary Factors Associated with Mercury Exposure

Madre de Dios area of Peru
The Duke University research team focused on the Madre de Dios area of Peru.
Map Courtesy of Wikimedia

A second recent study, led by an NIEHS grantee at the Duke University Global Health Institute, looked at 12 villages in the Madre de Dios area of Peru. In relation to the location of mining sites near the Madre de Dios river, three villages were located upstream, three mid-river near the sites, and six downstream. The study measured levels of mercury in adults and children with a focus on women of child-bearing age and children. The authors also conducted expanded dietary surveys, measured mercury levels in fish from local rivers as well as markets, and assessed seasonality by segmental hair analysis.

The team found that mercury levels in fish were highest around the mining activities, with declining levels further downriver. Notably, they found the highest and the lowest levels of hair mercury in two of the upstream communities and in the village with the highest levels of hair mercury, the fish mercury was low. The authors hypothesized that human migration, genetic variation that impeded mercury excretion, or a natural geologic source contributed to the observed high mercury levels upstream. Finally, the Peru study observed lower mercury levels in people who consumed grains, like quinoa and kiwicha, and some fruits, like tomatoes and bananas. The authors hypothesized these foods facilitated the excretion of ingested mercury.

In Suriname, Mercury Levels Characterized in Women and Children

Suriname
Researchers focused on how ASGM has impacted five interior villages of Suriname.
Map Courtesy of Wikimedia

The third study funded through the GEOHealth program examined five interior villages of Suriname on the eastern coast of South America. The study sampled hair mercury from women under 50 and children of both sexes. Fish samples were also analyzed for mercury content.

Several researchers, who were part of the Suriname team, published a 2012 study assessing the region for mercury in sediments and predatory fish. That study found high levels of mercury in “pristine” areas, where ASGM was not occurring. They hypothesized that prevailing winds dispersed mercury vapors from AGSM in these regions, with subsequent deposition and bioaccumulation.

The Minamata Convention on Mercury

The Minamata Convention on Mercury requires countries to reduce emissions, including those from AGSM, and to monitor the exposures of vulnerable populations. Over 52 countries have ratified the Convention, including Peru, Brazil, and the United States, but not Suriname.

By documenting widespread and serious exposures from AGSM and linking them to health impacts, NIEHS grantees and researchers are giving local environment and public health officials the necessary data to design targeted efforts to protect those most affected.

The human results in the recent study were similar to the Peru environmental sampling study in some respects. Mercury levels in hair were very high in a remote village upstream from AGSM activity. Even though the mercury levels in the fish were not highest there, the consumption of local predatory fish was higher than other regions, leading the authors to hypothesize that differences in fish consumption explained the difference in biomonitoring results.

The Suriname study was conducted as part of the Caribbean Consortium for Research in Environmental and Occupational Health (CCREOH), a prospective environmental epidemiological cohort study of 1000 pregnant women. The research team aims to gain a better understanding of the link between prenatal and early life mercury exposures to neurocognitive and other health outcomes.

“Mercury contamination associated with artisanal goldmining not only results in irreversible ecosystem destruction, but also exposes vulnerable pregnant women and their children to mercury levels that can impact birth outcomes and lead to adverse neurodevelopmental effects in infants and young children, “said Maureen Lichtveld, M.D., principal investigator of the CCREOH study.

“However, this risk of lifetime adversity is underrecognized. Our study is the first of its kind to characterize the sources of contamination, conduct comprehensive exposure assessments, and examine the relationship between exposure to such high levels of mercury and birth outcomes, as well as neurocognitive trajectories of the children from this cohort.”