A new study published in the journal Science of the Total Environment finds that mercury levels in rainfall are rising in parts of North America.

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A new report in the journalScience of the Total Environmentfoundmercury concentrations are "rising in rainfall and other forms of precipitation in North America," primarily being detected inthe central and western United States.Peter Weiss-Penzias, lead researcher and environmental toxicologist at UC Santa Cruz, worked with a team of scientists to analyze wet deposition samples from multiple locations throughout the United States andwas astonished to find that in spite of strengthened federal regulation of mercury in recent years, "increases at many sites."

"We didn't know what we were going to see," Weiss-Penzias told weather.com. "We did basic trend tests and looked at the data, and we were surprised not to see more decreases."

Weiss-Penzias couldn't sayfor certain whether the mercury pollution was coming from Asia, but noted that increases were likely not due to U.S. activities.

"It could be changes in atmospheric chemistry due to climate change," he said. "Long-range transport of pollutants from Asia would affect regions in the Rocky Mountains and central U.S. since the mountains intercept upper atmosphere air, which is where the pollutants tend to travel."

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Charles Driscoll, an environmental engineering professor and mercury expert at Syracuse University who didn't participate in the study, was also curious about the origin of increased mercury levels.

"It is difficult to know what is driving this," Driscoll told weather.com."Studies in the east are showing decreases in mercury deposition. Possible causes could be the drought and increased fire in the west."

Philip Landrigan, epidemiologist and pediatrician at Mount Sinai in New York, who also didn't participate in the study, felt the report was "well conducted by reputable and highly credible scientists," and anticipated that over the next several years, scientists would start finding levels of highly neurotoxic methylmercury increasing in fish.

"What happens here is that microscopic droplets of metallic mercury that are released to the atmosphere from coal-burning power plants and artisanal gold-mining operations travel long distance in the upper atmosphere, and eventually precipitate into rivers and lakes with rainfall," Landrigan explained to weather.com in an email.

He observed that in rivers and lakes, marine microorganisms convert mercury into "neurotoxic methylmercury."

"Methylmercury is environmentally very persistent, and it is also bioaccumulative," Landrigansaid. "It accumulates in fish and reaches its highest concentrations in predatory fish at the top of the food chain."

There, when exposed to pregnant women, it can cause brain injury in developing fetuses, he added.

Weiss-Penzias said that one of the biggest takeaways from his study was that mercury pollution knows no borders.

"Mercury is a global problem," he said. "The U.S. emits 7 percent of the world's total. If we see increases in areas upwind, we can fairly guess that it's coming from upwind sources like Asia and the Middle East, places where there's emerging economies. The U.S. has been striving to reduce mercury, the rest of the world not so much. Because mercury is a global pollutant, we need cooperation with other countries."

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