BRASILIA, Brazil — Many Yanomami, the Amazon’s largest Indigenous tribe in relative isolation, have been contaminated with mercury coming from widespread illegal gold mining, according to a report released last week by Brazil’s top public health institute.

The research was conducted in nine villages along the Mucajai River, a remote region where illegal mining is widespread. Mercury, a poison, is commonly used in illegal mining to process gold.

The researchers collected hair samples from nearly 300 Yanomami of all ages. They were then examined by doctors, neurologists, psychologists and nurses.

The vast majority, 84% of Yanomami tested, had contamination equal to or above 2 micrograms per gram, a level of exposure that can lead to several health problems, according to standards by the U.S. Environmental Protection Agency and World Health Organization.

A smaller part of the group, 10%, surpassed the 6 micrograms-per-gram threshold, a contamination level associated with more severe medical conditions.

Research teams also tested fish in the area and found high levels of mercury. Eating fish with high mercury levels is the most common path of exposure.

Exposure studies usually test for methylmercury, a neurotoxin formed when bacteria, in this case in rivers, metabolize inorganic mercury. Ingestion of large amounts over weeks or months damages the nervous system. The substance also can pass through a placenta of a pregnant woman, exposing a fetus to developmental abnormalities, according to the U.S. Centers for Disease Control and Prevention.

Health effects can include decreased sensitivity in the legs, feet and hands; overall weakness; dizziness; and ringing in the ears. In some cases, a compromise of the central nervous system can lead to mobility issues.

“Chronic exposure to mercury settles in slowly and progressively,” Paulo Basta, an epidemiologist with the Oswaldo Cruz Foundation, which led the testing, told The Associated Press. “There’s a wide spectrum of clinical actions that range from mild to severe symptoms.”

Concerted global efforts to address mercury pollution led to the 2013 Minamata Convention, a U.N.-backed agreement signed by 148 parties to curb emissions. The treaty is named for the Japanese city of Minamata, whose population was contaminated by decadeslong emissions of mercury dumped along with wastewater.

The Brazilian government report has not been peer-reviewed but synthesizes three papers published recently in the journal Toxics, all based on the same fieldwork. One of the studies noted that determining what long-term mercury exposure levels constitute a significant risk for health remains a challenge.

The study’s findings align with prior research in other areas of the Amazon, said Maria Elena Crespo López, a biochemist at the Federal University of Pará who has studied the subject for 20 years.

“The mercury problem is widespread throughout the Amazon,” she said. “Since the 1970s, when the first major gold rush happened here, mercury has been released for decades and ends up being transported over long distances, entering the food chain.”