Decades of mercury exposure has been linked to the high youth suicide rates in an Indigenous community in Canada, in the latest finding to underscore the catastrophic legacy of environmental contamination.

Researchers who studied three generations of mothers and their children from the community of Grassy Narrows, Ontario, have concluded that sustained exposure to the toxic metal helped cause a suicide rate three times higher than any other First Nations community – which are already far higher than among the country’s general population.

Grassy Narrows was the site of mercury dumping for nearly a decade after 1963, when a paper company released more than 20,000lb of mercury into the Wabigoon and English river systems. Fish, including walleye, were soon too poisonous to consume. The dumping is believed to have contaminated more than 150 miles of watershed. A single gram of mercury is sufficient to make all fish in a 20-hectare radius unsafe for consumption – but the Grassy Narrows dumping was 9m times larger.

Before the mercury dumping, the community had not reported any youth suicide attempts, but in the generations after the contamination, the rate of youth suicide attempts in Grassy Narrows far surpassed that of other First Nations in the country. Mothers from Grassy Narrows reported that more than 40% of young adolescents from the community had attempted suicide.

The researchers, who published their findings in the peer-reviewed Environmental Health Perspective journal, examined 80 mothers and 162 children between the ages of five and 17. They initially suspected that the consumption of mercury-tainted fish led to nervous system disorders and psychological distress.

“You can see this cascade of effects,” Donna Mergler, the study’s lead author, said at a press conference on Wednesday. “We found that the mother’s childhood mercury exposure is associated with today’s nervous system disorders, as well as a psychological distress.”

Mergler and other researchers studied the mothers’ umbilical cords, which had traces of their grandmothers’ mercury exposure.

“When you eat fish with mercury and you’re pregnant, the mercury is actively transported across the placenta … that affects the [foetus] development,” she said.

For residents, the crisis is clear. Days before the study was published, the community lost a young person to suicide.

“Even on social media you see people saying that they feel like they don’t want to live or they don’t know how to deal with what they’re going through,” Chrissy Issacs said at the press conference. “It’s not their fault. It’s a part of the sickness from the dumping of mercury and I feel like we need to make people aware of that.”

For years, Grassy Narrows has battled the provincial and federal governments to have their water system cleaned. In 2021, the federal government agreed to fund a $C90m “mercury care home” to help those living with the effects of poisoning, a project that faced repeated delays.

On Wednesday, the Grass Narrows First Nation chief, Rudy Turtle, demanded “fair” compensation for the effects of the poisoning, including counselling and mental health workers.

Turtle also said he wants the provincial government to withdraw from its territory after fixing the problems it created.

“You’ve done enough damage already, why do more?” he said.