Emissions from smokestacks, wildfires and other sources are raising the levels of mercury in marine fog, according to new research from a group of scientists.
The elevated levels of the metal pose no direct threat to humans, but can accumulate in the bodies of animals, including those that people eat.
While some levels of mercury are naturally present in the atmosphere, human activity has increased the number of mercury atoms in the atmosphere by a factor of five over the last century, according to research presented this week at the fall meeting of the American Geophysical Union.
Many human activities have raised concentrations of the pollutant, including coal-fired power plants, mining operations and agriculture, among others.
The concentrations of mercury in fog are 20 times higher than they are in rain, and the researchers estimate that plants and animals from foggy regions may have 10 times more mercury than those from other areas.
The concentrations of mercury in fog — on the order of 10 parts per trillion — are not an immediate health risk to organisms — including humans, said Peter Weiss-Penzias, a researcher of microbiology and environmental toxicology at the University of California, Santa Cruz, and one of the scientists involved with the research.
But mercury can accumulate in tissue over time, and marine fog can provide a constant source of the pollutant. That means there could be long-term ecological effects, or effects on food supplies. Large fish, such as tuna and tilefish, have long been known to have perilously high levels of mercury, for example.
"The top-level predators we have, whether bears, pumas or birds — they have a lot of stressors, and chemical stressors are one of them," said Weiss-Penzias. A classic example would be the pesticide DDT, which had toxic effects on bird populations before it was banned, he added.
"Mercury is a very unusual element," Weiss-Penzias said. "Most metals, if they do get in the air, are associated with particles that quickly settle out. But mercury is an inert gas in its elemental form, so it can travel great distances. It is really a global atmospheric pollutant."
The element can undergo a process called "re-emission," he said. It can enter the atmosphere, settle on the ground or in water, and then be released again into the atmosphere.
"It is not just today's emissions, which are dominated by countries in East Asia," Weiss-Penzias said. "Historic emissions also play a role; you have to consider the total emissions from the last hundred years."
Once it is out there, mercury can hang around in the atmosphere for a long, long time. That is what Weiss-Penzias and his colleagues observed in offshore marine fog near the West Coast.
The process works like this: Inorganic mercury atoms from emissions — be it from factories, fuel, or other sources — settle in the ocean. By what Weiss-Penzias calls an "accident of nature," certain bacteria deep in the ocean convert the mercury into an organic form called methylmercury when the bacteria are starved of oxygen. Methylmercury is both water soluble and fat soluble, and it is the form that can build up in tissue — such as in the tissue of large, long-living fish like tuna.
A portion of this methylmercury can take a volatile form, and that can escape from the ocean and back into the atmosphere, where it mixes with the water molecules in fog.
"This recycling mechanism is really what is going on in fog," Weiss-Penzias said. "It is a natural process, but the levels of mercury in the air and in the fog have increased over the last hundred years."
However, it is possible to curb mercury emissions.
"The news is not all bad," he said. "In fact there is evidence that because of the regulations of mercury emissions in the United States and Europe, [mercury] levels in fish have been dropping."