Fog rolling in off the Pacific brings iconic beauty to San Francisco, but scientists say it also carries with it something much less pleasant: toxic mercury.

Scientists who studied the fog along the coast of California found that it deposits a neurotoxin called monomethyl mercury — at a concentration about 20 times that of rain — as it sweeps across the city.

The scientists said the finding reveals a new pathway to land of a compound that comes largely from burning coal and other fossil fuels.

“On a relative scale, the levels of mercury are quite low and of no health concern,” said Peter Weiss-Penzias, a professor at UC Santa Cruz. “But it does bioaccumulate,” or build up in organisms.

Weiss-Penzias, an environmental toxicologist, first thought to study fog as he rode his bike to work early one morning. With condensation building up on his glasses, a question came to his mind: “What exactly is in this stuff?”

He and several other researchers, including Kenneth Coale of the Moss Landing Marine Laboratories, formed the FogNet project and sampled fog over the past two summers at collection stations along the coast, including one at San Francisco State University. Among the chemicals that turned up in the stations’ mesh nets was monomethyl mercury.

The scientists determined that mesoscale eddies — large, circular currents of water from the California current — are depositing dimethyl mercury into the fog, where acidic marine aerosols — particles left over from evaporated droplets of ocean spray — convert it into its monomethyl mercury form. That compound is then blown ashore in the fog, where it is deposited on the coastal landscape.

“Understanding the mechanism — a process that reaches into the ocean, pulls out a neurotoxin, then shuttles it ashore in fog — is very important,” Coale said. “This is a completely new pathway.”

Relatively small levels

Animals and plants in foggy regions may contain as much as 10 times the level of the mercury compound as life in less-foggy areas, according to the research that the scientists presented at last month’s meeting of the American Geophysical Union in San Francisco.

The mercury levels found in fog are relatively small, and “walking around in it and breathing it in doesn’t seem to be a health risk,” Coale said. But they do add to the mercury that humans introduce into the environment through coal burning, mining and metals processing, and contribute to the buildup in the plants that animals eat and the water they drink, researchers said.

For example, the scientists found that wolf spiders along the coast that they examined during foggy periods had levels of the mercury compound in their systems that exceeded the U.S. Food and Drug Administration’s 3 parts per million safety threshold. People won’t eat spiders, but the arachnids are part of a food chain that eventually lands on dinner plates.

In the long run, fog could become a bigger source of mercury in California’s coastal environment. A separate study by Clive Dorman, a research oceanographer from UC San Diego, found that fog occurrences along the Northern California coast increased by 7.4 percent from 1950 to 2007. Fog could increase further as global warming heats up inland areas, drawing marine air onto land.

Considering limits

As this mechanism continues, regulators may want to consider action to restrict human sources of mercury, the scientists said.

“I am hopeful that when all the exposure routes are added up, we will see that we are receiving quite a burden,” Coale said, “and that perhaps we should be considering limiting (mercury) emissions, particularly from coal-fired power plants — not just from a climate change perspective, but from an ecosystem health perspective as well.”