Nalcor Energy, the provincial utility behind the dam, doesn’t deny that methylmercury would spike. But levels would rise only modestly before falling back down to pre-dam levels, they contend, and that the neurotoxin wouldn’t make it far into Lake Melville or people’s diets. “We’ve been studying various aspects of the river system since the 1990s, including more than 2000 samples of fish collected to date, and over 100 samples of seal tissues, to understand the current levels of methylmercury in the local species,” Karen O’Neill, a spokesperson for Nalcor, told me in an emailed statement.

On Wednesday, Environmental Science & Technology published a paper by Sunderland’s lab at Harvard that argues differently. The new study predicts that flooding for the dam will expose Inuit and Innu communities along Lake Melville to double the amount of methylmercury they currently ingest, placing the average person in the area near what the U.S. Environmental Protection Agency suggests as a safe reference dose for the substance. (The EPA says that ingesting up to one ten-millionth of a gram of methylmercury per kilogram of body weight every day may be safe; Canada’s own recommendation is twice this.)

According to the study, the town likely to be the most severely affected would be Rigolet, a remote Inuit community of about 300 people. There is a grocery store there, but residents still go out on the water or the ice, depending on the season, to harvest salmon, seals, trout, char, cod, smelt, sculpin, dolphin, whale, and seabirds and their eggs. “Eating whatever you get from the water, the seal or fish, is just part of the way of life,” says Darryl Shiwak, a Rigolet native and environmental minister for the indigenous Nunatsiavut Government, which represents Labrador’s Inuit and autonomously governs five coastal communities.

In Rigolet, the study finds, the average methylmercury exposure for women of childbearing age and children younger than 12 could rise to double the EPA reference dose, pushing Canada’s limit. Residents with more traditional diets would be the worst off. “For some people who eat local seafood every day, it might push them into the zone of acute poisoning, where you start to see tremors and possible hair loss,” says Ryan Calder, a Ph.D. student in Harvard’s School of Public Health, and the new paper’s first author.

Calder’s work was partly funded by the Nunatsiavut Government. Methylmercury spikes are a well-established consequence of hydroelectric-dam construction; the disruption caused by flooding converts elemental mercury in soil into the neurotoxin, which washes downstream. Unlike elemental mercury, methylmercury can waltz through living tissues, including the blood-brain barrier, and its concentration builds up in animals high on the food chain. Since the Muskrat Falls project gained momentum in the late 2000s, Nalcor Energy has conducted its own studies that have minimized the proposed dam’s methylmercury impacts on fishing communities in Lake Melville. So, in 2012, the Nunatsiavut Government reached out to Sunderland’s lab for a second opinion.