For years, scientists assumed that mercury crept into crops through the soil, but new research suggests that the true culprit has been the air all along.
New research conducted by the Fire, Earth, Water, Air Contaminant Biogeochemistry Lab (FEWALab) led by Dr. David McLagan, assistant professor in the Department of Geological Sciences and Geological Engineering at Queen’s School of Environmental Studies, and published in the European Geosciences Union journal Biogeosciences, confirms that mercury pollution from artisanal and small-scale gold mining (ASGM) isn’t contaminating crops through soil as previously believed, but rather through the air.
“A lot of scientists assume that it [the mercury] is being taken up by the roots, but no one has tested it. There’s essentially no monitoring of atmospheric mercury around these sites, which is a real shortcoming,” McLagan said in an interview with The Journal.
In collaboration with the University of Lagos and the Nigerian Department of Mines and Steel, FEWALab’s researchers deployed passive air samplers at a legal ASGM operation in Nasarawa State, Nigeria. To track the uptake pathway of mercury, they used concentration data from the air, soil, foliage, stems, and roots and combined it with isotope analysis, which uses isotopic signatures to identify the uptake pathway.
The presence of lighter isotopes in the foliage suggested that the dominant pathway was atmospheric uptake. “That really big shift in the isotopes was how we confirmed this was the dominant pathway,” McLagan added.
While mercury levels in the crops studied were below global thresholds, McLagan stressed the risks shouldn’t be dismissed.
“This was quite a small ASGM activity,” McLagan noted. “There are much bigger areas that produce much more gold and use much more mercury. In those areas, exposures could begin to cross thresholds and present health risks.”
Even before toxicity becomes a concern, which can cause detrimental health effects such as high blood pressure, bleeding gums and even loss of vision, McLagan warned of a more immediate crisis: competition for land.
“These communities rely on agriculture,” he said. “ASGM activities come in and take land resources—the same land people depend on for food; it’s another critical aspect.”
Artisanal small-scale gold mining is the world’s largest source of mercury emissions. With gold prices climbing from $400 USD per ounce in 2005 to more than $4,000 USD per ounce in 2025, economic pressure is accelerating its expansion, according to McLagan.
“Gold is going up, and that’s a big driver why ASGM is going up,” McLagan noted. “These are run, in many cases, by organized crime syndicates. While it provides income, it’s not really a secure income.”
McLagan argued that global monitoring systems—including those informing United Nations treaties—are missing the problem entirely.
“Globally, mercury in the atmosphere is going down, but those monitoring sites are biased to the northern hemisphere,” McLagan said. “Small-scale gold mining is biased to the southern hemisphere, so there’s a discrepancy there.”
Treaties such as the Minamata Convention, established by the United Nations in 2013 to control global mercury emissions, have been touted as a success, but McLagan warned, “This problem isn’t solved, and acting otherwise is a concern because these activities are growing exponentially.”
According to McLagan, the team plans to expand atmospheric monitoring using passive samplers and assess whether crops can be used as bio-indicators of exposure, which could help shape how governments track mercury pollution.
“This is a major environmental and social issue. Ignoring it won’t make it disappear,” McLagan said.
Tags
environmental science, global south, mercury pollution
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