Several years after scientists thought they had put the problem to rest, they have once again discovered increasing concentrations of mercury, this time in rainwater. “It’s a surprising result,” says David Gay from the University of Illinois at Urbana–Champaign, who is a co-author on the new study. “Everybody expected [mercury levels] to continue going down. But our analysis shows that may not necessarily be the case.”

The results, recently published in Science of the Total Environment, is surprising because long-term trends had shown a decrease in mercury emissions whereas data collected between 2007 and 2013 indicate an unsettling upturn from the Rocky Mountains to the Midwest. The trend, however, is not due to regional activity. The authors speculate that because the U.S. has controlled its emissions since the 1970s, the toxic element was initially released via coal-burning power plants in Asia, drifted through the upper atmosphere for months, hit turbulence over the Rocky Mountains and was then pulled from the air in the form of rain.

Hannah Horowitz from Harvard University, who was not involved in the study, was surprised to see a similar—albeit less detailed—trend in her own work. After seeing the regional effects in this new study, she agrees that the toxic element likely comes from outside the U.S. “We see for other types of pollutants that [the Rocky Mountain region] tends to be more influenced by nonlocal sources because of its higher elevation—it has access to the free tropospheric air,” she says.

Given that it is such a recent phenomenon, scientists cannot be sure what an increase will mean for the environment and public health. “As a general rule, we are very concerned about mercury because it can be present at very dilute levels in the environment, parts per trillion, but in the food chain—in a food that we eat and that other animals eat—it can reach levels that are toxic,” says Peter Weiss-Penzias from the University of California, Santa Cruz, who led the study. Mercury magnifies as it travels up the food chain from minnows to fish to large mammals, including human beings. Pregnant women, for example, are often discouraged from eating fish because the element poses extreme risks to the developing fetus, particularly the central nervous system and brain.

It is possible that the calculated rise of 2 percent per year will result in a large accumulation in the ecosystem over the coming years, Weiss-Penzias says. “And once an ecosystem is contaminated with mercury, it can take decades for it to become uncontaminated.”