by Mary Caperton Morton Thursday, January 5, 2012
At the close of the Permian, 252 million years ago, conditions on Earth took a turn for the worse, nearly wiping out life on land and at sea in the planet’s most severe extinction event. Now, eons later, geologists are implicating a coal seam that dates to the “Great Dying” at the Permian-Triassic boundary in one of the modern world’s worst cancer epidemics.
Since the 1980s, epidemiologists have been scouring Xuan Wei County in southwestern China for the smoking gun responsible for the region’s astronomically high incidence of lung cancer. Home to 1.2 million Chinese, Xuan Wei has the some of the highest lung cancer mortality rates in the world, 20 times higher than the rest of China. A new study published in Environmental Science & Technology suggests that an unusually high concentration of fine-grained silicon dioxide, or silica, particles in the region’s coal could be to blame.
Two years ago, the International Agency for Research on Cancer, a division of the World Health Organization, declared silica a carcinogen. Fine particles, which can be easily inhaled into the lungs, are a particularly virulent threat. David Large, a geologist at the University of Nottingham in England and lead author of the new study, and his colleagues examined Xuan Wei’s bituminous coal, which is primarily burned in household stoves for heat. They found that it contains a high concentration of fine-grained silica particles.
“This was the first time that geologists looked at Xuan Wei’s coal in detail and we found it is quite unique and potentially more toxic than most,” Large says. “Epidemiologists have long suspected the cancer was linked to the coal, but they didn’t know why this particular coal from this region was so toxic.”
“This study goes a long way to answering our big question of ‘Why here?'” says Robert Chapman, an epidemiologist at Chulalongkorn University in Bangkok, Thailand who has studied the Xuan Wei cancer cluster for more than 25 years but was not involved in the new research. The study is not conclusive, he says, and more work is needed to establish the medical link between the coal's high silica content and this type of lung cancer. But, he says, “it wouldn’t surprise me at all if silica turned out to be one of the bad actors in Xuan Wei.”
That Xuan Wei’s uniquely toxic coal deposit occurs directly below the Permian-Triassic boundary is no coincidence, Large says. “Here we have this very toxic coal and right on top of it is one of the most extreme climatic and environmental events that has ever happened on Earth,” he says. The massive Siberian Traps volcanic flows that spewed carbon dioxide and sulfur dioxide into the atmosphere 250 million years ago would have created a very acidic atmosphere, which would have lead to enhanced weathering of the solidified lava, Large says, including the Emeishan basalt deposits local to Xuan Wei. Such weathering would produce silica-rich groundwater that could have settled in highly permeable peat bogs, impregnating the future coal seam with silica particles early in its diagenesis.
The Xuan Wei situation is something of a geologic perfect storm, Large says. “As far as we know, this is the only place in the world where people are burning coal from the Permian-Triassic boundary,” he says. Furthermore, because the silica content is so high, the coal’s carbon and volatile content is relatively low, so people have to burn more of it to produce adequate heat.
Now that Large and colleagues have identified what makes Xuan Wei’s coal unique, Chapman says, more research is needed from a medical perspective to determine whether the silica is solely to blame or whether other compounds, like polycyclic aromatic hydrocarbons, may be playing a role as well.
Currently, investigators from the U.S. National Cancer Institute (NCI), based in Bethesda, Md., are conducting a systematic study, trying to link exposure rates to lung cancer cases in Xuan Wei. “We are looking at all the possibilities, including exposure to silica, polycyclic aromatic hydrocarbons, heavy metals and radon,” says Nathaniel Rothman, a physician specializing in environmental medicine at the cancer institute and an investigator on the NCI study.
The NCI study is an important step because while coal has been linked to cancers and other serious illness for years, “we know surprisingly little about the number of components of coal-fire gas that are toxic and how much exposure will lead to illness,” says Glenn Stracher, an expert on coal fires at East Georgia College in Swainsboro, who was not involved in the new study. “Every coal fire is different and some produce more toxic or carcinogenic gas components than others.”
The NCI study will take several years to complete, Rothman says. In the meantime, Chinese authorities are encouraging the people of Xuan Wei to properly vent their stoves, which traditionally lacked chimneys, and to wear facemasks and gloves when disposing of ashes. A government subsidy program has provided funding and materials for people to build chimneys in their homes and most houses in Xuan Wei are now better vented, although Rothman says that exposure rates are still high.
Large and his colleagues plan to continue studying Xuan Wei’s coal, hoping to learn more about how it formed and how the volatiles and silica interact during combustion. “The idea that something that affected the planet’s health 250 million years ago is still having an impact on people today is tragic,” Large says. “It’s a fantastic geologic detective story that should be solved.”
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