A group of scientists now say they have a possible explanation: the burning of coal formed during volcanic eruptions hundreds of millions of years ago.
Coal in that part of China contains high concentrations of silica, a suspected carcinogen, the scientists reported in a recent edition of the journal Environmental Science & Technology.
Like others in rural China, the families of Xuanwei County use coal for heat and for cooking. As the coal burns, particles of silica are released with the vapor and inhaled. Women, who do the cooking, face the greatest exposure.