As October wildfires blazed over much of Southern California, few people realized that a toxic metal was crackling out of the burning grass and trees, billowing up into the sky to travel the world on stratospheric air currents.

Forest fires in the United States may release about as much mercury into the atmosphere as coal-burning power plants — around 44 tons a year, with roughly 18 of those in the Western states — say scientists at the National Center for Atmospheric Research in Boulder, Colo. Their just-published research is now pushing regulatory agencies to include fire-emitted mercury in their annual mercury inventories – numbers that the researchers say have been substantially underestimated for years.

When NCAR scientists Hans Friedli and Christine Wiedinmyer began looking into mercury emitted from wildfires several years ago, they were surprised at how much of the volatile metal — which can cause brain damage and severe birth defects when it reaches humans through the aquatic food chain — was going up in smoke. Their current study, published Oct. 17 on the Environmental Science & Technology Web site, was the first attempt to estimate the emissions on a national basis. “We knew that mercury was being released by fires,” says Wiedinmyer. “The big question was how much, and where, and when.” Although the mercury levels in wildfire smoke are not high enough to directly harm people, says Wiedinmyer, the increased mercury in the air raises the likelihood that it will end up in waterways, where it can be converted to methyl mercury, the toxic form that accumulates in fish and other aquatic animals.

Wiedinmyer emphasizes the fact that forest fires don’t create mercury. Rather, they release the mercury that was already there -- previously spewed into the atmosphere by both humans and the earth. The heat of wildfires converts carbon-bound mercury in both plants and soil into its gaseous elemental form. Carl Lamborg, a mercury expert at the Woods Hole Oceanographic Institute, estimates that two-thirds of mercury in surface soils is deposited by human activities such as fossil fuel burning, power plants and factories, while the other third comes from natural sources including volcanic explosions and ocean vents. As particulate mercury lands in the forests, it is taken up by plants and tied up in the soil, essentially locking it away in a relatively harmless form.

“Forests, particularly those with thick organic soils, do a really good job of storing the mercury we’ve been pumping out into the environment,” says Merritt Turetsky, an ecologist at Michigan State University.

But as climate change and years of fire suppression lead to more frequent and intense fires, Turetsky and others are concerned that more mercury will be released — particularly from boreal forests that store large amounts of the metal in their thick soils. In the NCAR study, Friedli and Wiedinmyer found that wildfires in Alaska released an average of 12 tons of mercury a year — more than a third as much as wildfires in the Lower 48 states combined. Over the years, Turetsky says, northerly air currents have blanketed the Arctic with the chemical byproducts of industry, and deep, ancient soils such as peat and permafrost store more than their fair share of the metal. Until recently, Alaskan forests rarely went up in flames. “There’s a lot there to burn,” says Turetsky, “(If fires keep increasing), wildfire in Northern ecosystems could contribute a lot of mercury into the atmosphere.”

And the toxic element will not necessarily stay in one place. “It’s an extremely moveable metal,” explains Friedli. “If a fire is intense, it may go all the way up into the stratosphere, and once it’s up there, it may go all the way around the globe.”

The link between wildfires and mercury is adding to the perception of mercury pollution as a global rather than local issue. In 2005, the Environmental Protection Agency implemented the Clean Air Mercury Rule, which was the first federal attempt to regulate interstate mercury emissions, but the agency is only now revising its protocol to include mercury from wildfires. Meanwhile, Friedli is working with the United Nations’ Mercury Programme to add the wildfire data to its 2009 assessment. Because including mercury from fires will increase the EPA’s annual mercury emissions estimate by about 30 percent, reducing those emissions to acceptable levels might mean requiring even sharper cuts for industry.

The flow of mercury through the air, water and land is a vicious cycle, Friedli says. “It doesn’t go away. The only way mercury gets out of the biogeochemical cycle is when it sediments at the bottom of the ocean — and that’s rare.” As long as forests harbor the metal, fires will continue to release it, and there’s not much that anyone can do about it, says Turetsky: “The only way to reduce emissions is to reduce industrial emissions.”