The findings, from a study carried out in Guizhou province, contradict the previous belief that the disease was caused by people burning fluoride-rich coal in indoor stoves without chimneys.
Because of this belief, last year, the Chinese authorities invested 34 million yuan (US$4.2 million) to help farmers in Guizhou build stoves with outdoor chimneys.
But, in a paper published in the July-August issue of the Chinese Journal of Endemiology, researchers led by Zheng Baoshan of the Institute of Geochemistry in Guizhou province show that the average fluoride content of coal burnt in Guizhou is lower than in any other major coal-burning province in China.
Instead, they say that clay, which rural people in the province mix with coal powder, is to blame.
Mixing clay with coal is much cheaper than burning large blocks of coal. The combined fuel is used to dry corn before storing it. Corn, however, absorbs more smoke than other grains, such as rice or wheat.
Zheng's team found that only 15 per cent of the fluoride released when the coal and clay mixture is burnt comes from the coal.
The fluoride that contaminates the food and air comes mostly from the clay, which is taken from fluoride-rich soil, Zheng told SciDev.Net.
Simply building outdoor chimneys will not be enough, he adds, though this will reduce the threat of arsenic poisoning, which is mainly caused by burning coal indoors.
He hopes his team's findings will help researchers develop solutions ?such as substitutes for clay ?to solve the problem of fluoride poisoning in southwestern China.
But Zheng admits this will be very difficult, as any available substitute would be more expensive than clay. Incomes in Guizhou are among the lowest in China.
According to official figures, in 20 00, nearly 18 million Chinese people, mainly in rural areas, suffered dental fluorosis and nearly 1.5 million had the skeletal form of the disease.
People get fluoride poisoning from consuming too much fluoride. It makes their teeth discoloured and fragile, and in severe cases can distort bones, stiffen joints and lead to paralysis.
In Guizhou province, where the problem is most serious, there were 10.5 million cases of dental fluorosis and just over half a million cases of skeletal fluorosis in 2001.
Reference: Chinese Journal of Endemiology 24, 468 (2005