Mercury is a neurological poison that people can be exposed to when they eat fish from contaminated waters -- and contaminated water is everywhere, since mercury -- like acid rain -- falls from the sky in tiny quantities in each rainstorm.
Mercury is found in rocks, so volcanic eruptions release it to the atmosphere. Burning coal releases it, as do certain other industrial processes.
And, according to new research out of the National Center for Atmospheric Research, wildfires release it. The research, funded by the National Science Foundation, the Environmental Protection Agency and the Electric Power Institute, was published in the journal Environmental Science and Technology.
The study -- admittedly preliminary and subject to a high degree of uncertainty -- estimate that fires release about 44 metric tons of mercury into the atmosphere every year, but that figure, and the state-by-state estimates it produced have a margin of error of 50%. U.S. industrial sources release 108 metric tons annually.
The map above shows the annual average of mercury (in metric tons) released by fires for every state except Hawaii. The estimates are based on fires from 2002 to 2006.
The mercury released by the fires originates in both natural mineral deposits in the soil, and the same industrial rain that contaminates lakes.
Mercury can hang up in the atmosphere and travel the world. Less than half of the mercury that rains down in the U.S. originates here, according to EPA estimates. The Bush EPA imposed the first ever mercury regulations on coal-fired power plants, the nation's largest source, and several states have enacted more strict requirements.
The EPA and Food and Drug Administration warn people -- particularly women who are or may become pregnant, women who are breast feeding, and children under the age of 15 -- to avoid eating many species of fish because of mercury content. Mercury can interrupt the normal development of the brain and nervous system.
The study raises a question about the interactions between global warming and pollution. Climate scientists expect more frequent and intense wildfires in the future, as heat waves and drought become more common, and winter snowpacks become less intense. This research suggests that those wildfires could degrade water quality and affect the fishing and eating habits of millions of Americans.
For more information, see this list of lakes, reservoirs and other bodies of water that have fishing advisories due to mercury and other contaminants.
Enter your city or zip code to get your local temperature and air quality and find local green food and recycling resources near you.