Thanks, China. Any Chance You Could Stop Messing Us Up?

If you're looking for Chinese exports, you won't find all of it lining Wal-Mart shelves. It's even present in our rainwater in the form of potentially dangerous levels of mercury.

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.

“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.”