Domestic use of coal containing high levels of silica could be the cause of higher cancer rates within rural Xuan Wei county, China.
David Large
A recent mining disaster in China, which claimed the lives of 108 people, has highlighted the potential dangers of coal. The incident was one of the deadliest tragedies in a series of coal mining accidents within the People's Republic.
While Chinese authorities are currently investigating the matter and working to improve safety practices, it appears that miners aren't the only ones at risk from coal, according to a new study.
Since at least the 1980s, health experts have wondered why non-smoking women in some parts of rural Xuan Wei county, China, die of lung cancer at higher rates than anywhere else in the world -- at rates up to 20 times higher than elsewhere in China.
A new study may have found the cause: domestic use of coal containing high levels of silica, a consequence of volcanic activity during the world's greatest mass extinction 250 million years ago.
In a paper published in Environmental Science and Technology, a research team led by David Large of the University of Nottingham in the United Kingdom took a geological view of the mystery, analyzing the composition of the coal from several sites in Xuan Wei and comparing it with samples from elsewhere in China and the United States.
For years, researchers have attempted to explain this cancer epidemic. Many have suspected polycyclic organic hydrocarbons (PAHs), large organic molecules that form when coal is burned. PAHs are often toxic and suspected to cause cancer.
However, PAH levels alone cannot account for the high incidence of cancer in Xuan Wei.
Coal-burning produces PAHs. Unventilated, indoor heating and cooking stoves, which provide a steady source of smoke in the home, were common throughout China until recently.
The coal used in Xuan Wei had a unique composition, Large found. "There is masses and masses of silica, and most of that silica is very fine-grained," he said.
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Silica alone may pose a health hazard -- fine silica was recently classified as a carcinogen -- but a preliminary analysis by Large's team suggests that it is combined exposure to silica and PAHs that cause the increased cancer risk.
They looked at the few existing samples where coal composition and mortality risk were available together. Their statistical tests showed that the presence silica and PAHs together was a better predictor of cancer deaths than either of them alone.
However, the researchers do not know how the two materials might interact to increase risk. It may be that silica irritates the lungs, making them more susceptible to damage by PAHs, or silica may carry PAHs into the lungs.
"The silica, in conjunction with the PAHs -- all of the fumes generated by the coal -- are acting together to get a much higher incidence of cancer mortality than you would expect," Large said. "That brings you to the question of, 'Where is the silica coming from?'"
The answer, Large believes, stems from the world's largest mass extinction event 250 million years ago, when about 95 percent of marine species and 70 percent of land vertebrates went extinct.
"The boundary lies right on top of that coal," Large said. "You have this coal which is highly toxic which is associated with the greatest extinction of all time."
"The paper is very thought provoking," said Robert Chapman of Chulalongkorn University in Bangkok, Thailand, who has studied the cancer cluster for more than 25 years. "They point out some things that are very intriguing. The main thing is that the silica content in this coal seems to be much higher than in U.S. coal, and the organic content is quite moderate. They ask the question we've been asking for 25 years, which is, 'Why there?'"
Chapman points out that this study raises an interesting hypothesis, but more research will be needed.
A study underway, led by Nathaniel Rothman and Qing Lan of the U.S. National Cancer Institute, should provide the data necessary to do just that. The study will include more than 1,000 individuals in the area with and without cancer, and it will collect detailed information on their actual exposure to PAHs and silica, rather than just the composition of the coal.
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