Mercury and artisanal and small-scale gold mining: Review of global use estimates and considerations for promoting mercury-free alternatives

Artisanal and small-scale gold mining (ASGM) is present in over 80 countries, employing about 15 million miners and serving as source of livelihood for millions more. The sector is estimated to be the largest emitter of mercury globally. The Minamata Convention on Mercury seeks to reduce and, where feasible, eliminate mercury use in the ASGM. However, the total quantity of mercury used in ASGM globally is still highly uncertain, and the adoption of mercury-free technologies has been limited. This paper presents an overview of new data, derived from Minamata ASGM National Action Plan submissions, that can contribute to refining estimates of mercury use in ASGM, and then assesses technologies that can support the phase out mercury use in ASGM while increasing gold recovery. The paper concludes with a discussion of social and economic barriers to adoption of these technologies, illustrated by a case study from Uganda.

Signs and symptoms of mercury-exposed gold miners

OBJECTIVES

Gold miners use mercury to extract gold from ore adding liquid mercury to the milled gold-containing ore. This results in a mercury-gold compound, called amalgam. Miners smelt this amalgam to obtain gold, vaporizing it and finally inhaling the toxic mercury fumes. The objective was to merge and analyze data from different projects, to identify typical signs and symptoms of chronic inorganic mercury exposure.

MATERIAL AND METHODS

Miners and community members from various artisanal small-scale gold mining areas had been examined (Philippines, Mongolia, Tanzania, Zimbabwe, Indonesia). Data of several health assessments were pooled. Urine, blood and hair samples were analyzed for mercury (N = 1252). Questionnaires, standardized medical examinations and neuropsychological tests were used. Participants were grouped into: Controls (N = 209), living in an exposed area (N = 408), working with mercury as panners (N = 181), working with mercury as amalgam burners (N = 454). Chi² test, linear trend test, Mann-Whitney test, Kruskal-Wallis test, correlation coefficient, Spearman's rho, and analysis of variance tests were used. An algorithm was used to define participants with chronic mercury intoxication.

RESULTS

Mean mercury concentrations in all exposed subgroups were elevated and above threshold limits, with amalgam burners showing highest levels. Typical symptoms of chronic metallic mercury intoxication were tremor, ataxia, coordination problems, excessive salivation and metallic taste. Participants from the exposed groups showed poorer results in different neuropsychological tests in comparison to the control group. Fifty-four percent of the high-exposed group (amalgam burners) were diagnosed as being mercury-intoxicated, compared to 0% within the control group (Chi² p < 0.001).

CONCLUSIONS

Chronic mercury intoxication, with tremor, ataxia and other neurological symptoms together with a raised body burden of mercury was clinically diagnosed in exposed people in artisanal small-scale mining areas. The mercury exposure needs to be urgently reduced. Health care systems need to be prepared for this emerging problem of chronic mercury intoxication among exposed people. Int J Occup Med Environ Health 2017;30(2):249-269.

The burden of chronic mercury intoxication in artisanal small-scale gold mining in Zimbabwe: data availability and preliminary estimates

BACKGROUND

Artisanal small-scale gold mining (ASGM) is a poverty-driven activity practiced in over 70 countries worldwide. Zimbabwe is amongst the top ten countries using large quantities of mercury to extract gold from ore. This analysis was performed to check data availability and derive a preliminary estimate of disability-adjusted life years (DALYs) due to mercury use in ASGM in Zimbabwe.

METHODS

Cases of chronic mercury intoxication were identified following an algorithm using mercury-related health effects and mercury in human specimens. The sample prevalence amongst miners and controls (surveyed by the United Nations Industrial Development Organization in 2004 and the University of Munich in 2006) was determined and extrapolated to the entire population of Zimbabwe. Further epidemiological and demographic data were taken from the literature and missing data modeled with DisMod II to quantify DALYs using the methods from the Global Burden of Disease (GBD) 2004 update published by the World Health Organization (WHO). While there was no disability weight (DW) available indicating the relative disease severity of chronic mercury intoxication, the DW of a comparable disease was assigned by following the criteria 1) chronic condition, 2) triggered by a substance, and 3) causing similar health symptoms.

RESULTS

Miners showed a sample prevalence of 72% while controls showed no cases of chronic mercury intoxication. Data availability is very limited why it was necessary to model data and make assumptions about the number of exposed population, the definition of chronic mercury intoxication, DW, and epidemiology. If these assumptions hold, the extrapolation would result in around 95,400 DALYs in Zimbabwe's total population in 2004.

CONCLUSIONS

This analysis provides a preliminary quantification of the mercury-related health burden from ASGM based on the limited data available. If the determined assumptions hold, chronic mercury intoxication is likely to have been one of the top 20 hazards for population health in Zimbabwe in 2004 when comparing with more than 130 categories of diseases and injuries quantified in the WHO's GBD 2004 update. Improving data quality would allow more accurate estimates. However, the results highlight the need to reduce a burden which could be entirely avoided.

Polymorphisms in genes encoding potential mercury transporters and urine mercury concentrations in populations exposed to mercury vapor from gold mining

BACKGROUND

Elemental mercury (Hg0) is widely used in small-scale gold mining. Persons working or living in mining areas have high urinary concentrations of Hg (U-Hg). Differences in genes encoding potential Hg-transporters may affect uptake and elimination of Hg.

OBJECTIVE

We aimed to identify single nucleotide polymorphisms (SNPs) in Hg-transporter genes that modify U-Hg.

METHODS

Men and women (1,017) from Indonesia, the Philippines, Tanzania, and Zimbabwe were classified either as controls (no Hg exposure from gold mining) or as having low (living in a gold-mining area) or high exposure (working as gold miners). U-Hg was analyzed by cold-vapor atomic absorption spectrometry. Eighteen SNPs in eight Hg-transporter genes were analyzed.

RESULTS

U-Hg concentrations were higher among ABCC2/MRP2 rs1885301 A-allele carriers than among GG homozygotes in all populations, though differences were not statistically significant in most cases. MRP2 SNPs showed particularly strong associations with U-Hg in the subgroup with highest exposure (miners in Zimbabwe), whereas rs1885301 A-allele carriers had higher U-Hg than GG homozygotes [geometric mean (GM): 36.4 µg/g creatinine vs. 21.9; p = 0.027], rs2273697 GG homozygotes had higher U-Hg than A-allele carriers (GM: 37.4 vs. 16.7; p = 0.001), and rs717620 A-allele carriers had higher U-Hg than GG homozygotes (GM: 83 vs. 28; p = 0.084). The SLC7A5/LAT1 rs33916661 GG genotype was associated with higher U-Hg in all populations (statistically significant for all Tanzanians combined). SNPs in SLC22A6/OAT1 (rs4149170) and SLC22A8/OAT3 (rs4149182) were associated with U-Hg mainly in the Tanzanian study groups.

CONCLUSIONS

SNPs in putative Hg-transporter genes may influence U-Hg concentrations.