Some of the world’s most vulnerable communities are currently experiencing high rates of mercury poisoning as a result of the activities of illegal miners. A recent study (in Portuguese) involving 19 communities of indigenous Yanomami and Yekuana people in the Brazilian Amazon revealed that 90 percent have been severely affected by mercury poisoning.

The participants in the study live in protected territories deep in the Amazon rainforest, and typically have very limited contact with the outside world. Legal recognition of these tribes’ rights to protection from intruders was gained in the early 1990s, when the Yanomami indigenous territory was established following a long campaign to ban illegal mining. Over the previous decade, it is thought that the indigenous population in the region fell by around 20 percent as irresponsible miners brought disease and violence to the area.

However, in spite of the legal status of this land, an alarming recent increase in unauthorized gold mining has once again brought the vulnerability of local inhabitants into focus. Last year, researchers monitoring the health of Yanomami communities found that some villages had been abandoned following an intensification of contact between indigenous tribes and outsiders.

Subsequently, researchers collected hair samples from those living in 19 different subpopulations within this territory, finding alarmingly high levels of mercury poisoning in the vast majority of these. It is thought that most of this is caused by the mercury used by miners during the gold extraction process, which then finds its way into local rivers, polluting aquatic ecosystems.

While ingesting large quantities of inorganic mercury – such as that used by the miners – can be harmful, the majority of this is not actually absorbed from the gut, and simply passes straight through the body. However, if this mercury pollutes rivers and lakes, anaerobic microorganisms convert it into organic compounds such as methylmercury.

This then moves up the food chain, accumulating in fish, and is passed on to people who feed on them. Alarmingly, 90 percent of ingested methylmercury is absorbed into the bloodstream, causing a range of harmful health effects. Since fish make up a significant part of Yanomami and Yekuana diets, the dangers posed to these people by illegal mining is severe.

Mercury poisoning can affect the brain and central nervous system, as well as the heart, kidneys and lungs. Symptoms of acute poisoning include muscle weakness, speech impairment, reduced coordination and a loss of feeling in some parts of the body.

Earlier this year, an indigenous child died in the Peruvian Amazon after exhibiting symptoms associated with mercury poisoning. There, as in Brazil, the activities of illegal miners are thought to be having a major detrimental impact on local ecosystems and indigenous populations. Highlighting the widespread extent of the problem across the region, a recent study revealed that around 80 percent of the Nahua tribe in Peru is currently suffering from mercury poisoning.