Mercury poisoning can cause memory loss, muscle weakness, tremors and twitching, headaches and reduced mental abilities. It affects the development of a central nervous system in fetuses, damages the brain and kidneys and the immune system. In large enough doses, it can kill.
There is a known relationship between exposure to mercury and endemic infectious diseases in developing countries, Silbergeld said.
No one has done epidemiological studies of the area, in part because of the cost and relative inaccessibility of Madre de Dios. Fraser, who has been there often, describes it as mostly tropical forest with some Amazon lowlands. There is one paved road into the area, but most transportation is by air.
That mercury is present in dangerous amounts is not debatable. One study by the Peruvian Ministry of Production found mercury in the rivers three to 25 times the legal amount. Tests have found many fish contaminated.
In 2009, Luis Fernandez of the Carnegie Institution for Science in Stanford, Calif. found levels of mercury in the air outside of gold shops in Puerto Maldonado, the largest city in the area, greatly exceeded safety standards.
In one shop the mercury was literally off the scale. His equipment couldn’t register amounts that high. Even shoppers passing by were breathing it.
What is happening in Peru also is a problem in the forests of Brazil, where Silbergeld does her research. People down river from the mines are affected.
There are ways of mitigating the damage, Fraser says. Using filters on the ovens would cut down on mercury, for instance. Regulation would be difficult. There are as many as 100,000 miners and they refuse regulation, sometimes violently.
There are examples of local government corruption as well.
The safer ways to extract the gold costs money, Silbergeld said.
"There's just enough gold to make people what to get it, but not enough to do it seriously," she said.