Forest Fire Smoke Poses Mercury Danger

Where there’s smoke, apparently there’s mercury.

Researchers from Colorado have found appreciable amounts of mercury in smoke from wildfires in the West, a discovery that could change the way scientists study the distribution of the toxic metal.

Scientists caution that there is no acute health threat from the airborne mercury, which becomes more dangerous when it enters bodies of water and gets into the food chain.

The results, however, could add some complexity to how scientists view the source and distribution of mercury pollution worldwide.

Scientists from the National Center for Atmospheric Research conducted monitoring flights two weeks ago above forest fires in Washington and found mercury levels up to five times higher than usual.

The measurements confirmed earlier laboratory tests on underbrush from California and other locales in the West, and offer convincing evidence that forest fires act as recycling engines for mercury pollution.

“What wildfires are essentially doing is taking mercury that has been deposited and redepositing it in places where it can be dangerous,” said Hans Friedli, an atmospheric research center chemist who participated in the study. “The mercury that’s really of concern is mercury that ends up in water.”

The soluble form of mercury, or methylmercury, enters the food chain and is concentrated in fish eaten by humans. High levels of the known neurotoxin can be dangerous, particularly to children and pregnant women.

The immediate health threat near wildfires has not been quantified, though Friedli cautioned that preliminary measurements were only a few nanograms--a billionth of a gram--per cubic meter of air. The EPA has established an acceptable maximum level for airborne mercury of 300 nanograms per cubic meter.

Only a small percentage of the mercury would be deposited nearby as particles fall to Earth, while the vast majority drift to greater distances in the atmosphere, Friedli said.

Air quality officials from California were cautious about the test results Tuesday. Similar flights over Southern California wildfires four years ago failed to show any trace of mercury, said Richard Varenchik, a spokesman for the California Air Resources Board.

“I’m sure we’ll review the data when we get the chance to see the study,” Varenchik said.

At the very least, the experiments could change the way scientists think about mercury pollution. Friedli has estimated that as much as 800 tons of mercury may enter the atmosphere annually from burning vegetation worldwide--ranging from wildfires to farmers clearing underbrush.

Major man-made sources of airborne mercury include coal-fired power plants, solid waste burning, smelters and factories. The federal Environmental Protection Agency estimates that U.S. coal-fired power plants pump 41 tons of mercury into the atmosphere annually, contributing to the 6,500 tons circulating worldwide at any moment.

When atmospheric mercury falls to the ground in liquid form, it is absorbed by leaves and needles, where it stays, at least until fires send it wafting into the air again.

Adding thousands of fires worldwide to the mix of mercury sources potentially complicates scientific models for tracking the pollutant.

“If you’ve got multiple sources from fires, you’ve obviously got a lot more complicated situation,” said Peter Hobbs, a professor of atmospheric science at the University of Washington.

The center also has studied emissions from vegetation samples in a lab, where measurable traces of mercury led them to examine actual wildfires. “We were waiting for a good fire,” Friedli said.

Friedli and fellow researcher Larry Radke will refine their study by monitoring a controlled burn in Saskatchewan this month. Their study was sponsored by the National Science Foundation and the Electric Power Research Institute, a nonprofit industry group in Palo Alto.