Reuters/Robert Galbraith

The city’s iconic fog is carrying the toxic element onto land and into the ecosystem.

Karl, the iconic fog that regularly blankets San Francisco, is somewhat of a personality among locals. He’s accumulated more than 100,000 Twitter fans over the past six years. He’s witty, sarcastic—and although he runs counter to the image of sunny California, Karl remains a charming and recognizable part of SF.

The latest news from researchers behind the FogNet project at the University of California, Santa Cruz, may just ruin Karl’s reputation. It turns out, San Francisco’s fog has been pulling toxic mercury from the ocean and distributing it onto the city’s coast. Fog, in essence, is made up of tiny droplets of water floating in the air, which is very effective in absorbing gases from the atmosphere.

Over the past two summers, the environmental microbiologist Peter Weiss-Penzias and his colleagues have been collecting fog samples along the coast of California.They found that the concentration of mono methyl mercury—a potent neurotoxin especially dangerous to children and pregnant women—is about 10 parts per trillion in SF, which is 20 times higher than that of rain there, according to the San Francisco Chronicle. To be clear, there’s always been mercury present in the atmosphere—in California, San Francisco, everywhere—says Weiss-Penzias, and the concentrations found in fog are nowhere high enough to prompt any health concerns. In fact, it remains 100,000 times lower than the safety threshold of what we eat.

“The question that we've been probing is … the methyl mercury that’s coming in fog [and] depositing on the landscape, could it be accumulating in the terrestrial food web?” Weiss-Penzias tells CityLab.

The problem is that mercury bioaccumulates, or builds up in organisms that become part of the food chain. The presence of mercury in our oceans can be traced back to human activities like coal burning and mining, which have long polluted the air and water. When the tiny microbes of mercury hit seawater, however, it can convert to its most toxic form. Once ingested by marine organisms, the mercury is transferred from predator to predator, and can eventually reach humans.

With fog carrying mercury from the ocean onto land, the levels of mercury among plants and animals may be up to 10 times higher than in less foggy places, reports the San Francisco Chronicle. “We started to look at plants and animals that eat plants, and insects that are at the top of the food chain and mammals that eat the herbivores, and we're starting to see a pattern,” says Weiss-Penzias. The researchers are still sifting through the data, so there are no definitive conclusion yet.

It's not necessarily a direct human impact, but is it impacting the ecosystem in ways that we can't predict?” he says. “Yeah, that's definitely a possibility."

All this must have come as a shock to Karl, who spoke out on Twitter in denial of the accusations:

Rising levels of mercury as a result of human activities is a serious problem around the globe that needs to be tackled. But San Francisco, at least this fog won’t turn you inside out.

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