Small-scale gold mining in the Peruvian Amazon poses a health hazard not only to miners but also to nearby communities. Contrary to common assumption that communities closest to mining bear the brunt of exposure, new evidence shows that the highest non-occupational mercury exposures occur in native communities hundreds of kilometers away from mining.

In communities where fish is an important part of the diet, children under 12 with the highest levels of mercury in their hair (exceeding the World Health Organization guideline) have been found to have intellectual deficits amounting to a loss of 4.68 IQ points. Even children with exposure below this guideline show effects amounting to a 0.8 IQ point drop for every 1 part per million increase in hair mercury. This effect is roughly four times larger than detected in an earlier study of prenatal mercury exposure in the Republic of Seychelles.

Both findings come from a series of studies conducted by scientists in and around the Amarakaeri Communal Reserve in the Madre De Dios region of Peru. They appear in a pair of papers published 20 May issue of the AGU journal GeoHealth and 28 May in the Journal of Exposure Science and Environmental Epidemiology.

Duke researcher Helena Frischtak, (right front) administers psycholocal assements with a pair of Peruvian children during a study of mercury contamination near small-scale gold mining. (Bill Pan, Duke)

The studies show that common assumptions about mercury exposure should be reexamined, and that native people in the region are more vulnerable to harm, probably because of their greater reliance on river fish, but also perhaps because their healthcare and standard of living is not as high.

“We can’t just rely on assumptions or ‘common sense’ in science,” said lead author Caren Weinhouse, an assistant professor at Oregon Health & Science University. Many studies have looked at communities closest to the mines on the assumption that they would have the greatest mercury exposures. “We assumed that people closest to mining would have the highest exposure, but we were wrong,” she said. “The lesson is that if we only focus on the people we assume are at risk, we might end up missing the big picture.”

Artisanal and small-scale gold miners in the Peruvian Amazon use liquid elemental mercury to extract gold from soils and sediments. The mercury binds to the gold to form an amalgam, which is then extracted by burning, creating gaseous mercury that enters the atmosphere. The rest of the mercury ends up dumped on the landscape, which is ravaged and eroded by the mining. Miners are also known to simply pour excess mercury directly into surface waters.

As the mercury travels and mixes with the environment, it becomes methylmercury, which is more readily taken up by animals and tends to “bio-accumulate” or add up in tissues, and then “biomagnify” as bigger fish eat contaminated little fish. Although all native communities showed high exposure, their river locations shed light on the likely exposure source. In two sampled native villages that were on tributaries of the Madre de Dios River, mercury exposure was lower than in native people living on the main stem of the river where mining runoff is concentrated, leading the scientists to conclude that fish is the likely exposure source and mining is the likely culprit.

Mercury is a neurotoxic metal that can lead to muscle weakness and problems with coordination in high doses, and neurodevelopmental delay, hyperactivity and IQ deficits in lower doses.

To gather data on hair and blood concentrations of mercury, both near mining operations and farther away, the researchers visited 1,221 Peruvian households in 23 communities in 2015 and returned to resample 900 of those households the following year.

Some of the children in a subsample of the population had higher mercury levels and were on average lower in cognitive ability. A third of the children were found to have mercury levels higher than the World Health Organization’s exposure guidelines.

“We knew going in that mercury caused IQ deficits. What we didn’t know was whether the risk was the same in this setting as it is in prior studies, which were done in “healthy, wealthy” populations,” said Duke University graduate student Aaron Reuben, who led the smaller pilot study on children. Reuben explained that children in Madre de Dios are already at high risk for IQ deficits, because they have poor nutrition and socioeconomic status.

An earlier, benchmark study of prenatal mercury exposure conducted in the Republic of Seychelles reported a loss of 0.18 IQ points for every 1 part per million increase in maternal hair mercury, an effect about four times lower than those in the Peru study. Hair mercury levels in both the Seychelles and Peru studies likely reflect fetal exposure, which allows comparisons of their results.

 “This study suggests that mercury may affect brain development more at the same doses in higher risk populations,” said Reuben. He noted that the group didn’t account for the effects of possible prenatal exposure in the tested children.

The researchers also found initial evidence that the higher the mercury levels in a child’s blood, the lower their hemoglobin levels, corroborating an earlier study from this research team.

“Although mercury is not generally considered a risk factor for anemia, it might be in a population with other, pre-existing risk factors for the disease,” said William Pan, associate professor of environmental sciences and policy at Duke University. “Given that anemia affects over 2 billion people and mercury is a global pollutant, this is a priority research area.” Another earlier study by the Duke University team found that children with higher mercury exposures were less responsive to vaccines, especially if they were also malnourished.

“Taken together, the message is that we can’t assume that we know who is exposed unless we look, and we can’t assume that health risks will translate from developed countries,” Weinhouse said. “These studies show that highest exposures happen in vulnerable, native communities and that they might be at risk for even greater harm than healthy people with the same exposures.”

This post was originally published on the Duke University website.