"My knees just hurt too much," Zhang told AFP outside his grey-walled concrete room subsidised by the government.

He also receives 50 yuan (6.45 US dollars) a month for food, but his brother-in-law, Li Xiaoxin, also a farmer, said it was not nearly enough.

"I work and live one day and then take care of him the next," said Li, 65.

Zhang is one of 42 million people in China with fluorosis, a condition caused by ingesting too much fluoride, which corrodes the teeth, pitting them and turning them a rusty brown, in cases of excessive, prolonged exposure.

In severe cases like Zhang's, the mineral also weakens the bones and stiffens the joint ligaments, eventually making movement difficult and painful, much like osteoarthritis does.

In many developed countries, fluoride, which is often naturally occurring in water and also coal, is added to toothpaste and drinking water.

Since the 1960s many medical experts have believed that small amounts of fluoride helps prevent tooth decay and strengthen bones. However, the claimed benefits have increasingly drawn fire from medical experts who say that there are no tangible health advantages to the human body.

Several European countries over the last five years have stopped public water fluoridation programmes amid questions whether the mineral does more harm than good.

At its most severe, fluorosis results in the type of paralysis Zhang now has, and can even lead to cancer and brain damage. The sickness is incurable.

In China, where coal is still a primary cooking and heating fuel in rural areas, the disease is especially prevalent because the fluoride-carrying fumes from coal burnt inside homes pollute the air and the food.

According to Zhijin health bureau officials, the condition is particularly widespread in Guizhou because two regional food staples, corn and red peppers, are dried inside. The rising coal smoke then laces the food with fluoride.

Wang Jianjun, a vice director of the local health bureau, said extended educational campaigns have helped raise awareness of the disease, which can easily be avoided through the use of other cooking fuels. But the cheapness and availability of coal in rural China has meant that old practices die hard, Wang said.

"We educate the villagers through television, radio and posters, let them know the cause and harm of the disease," he said. "It's very hard to change people's habits."

Wang added that it was not until 1978 that people knew what caused the disease, so China's older generation stood no chance of avoiding contamination.

Ma Wenbo, director of the health bureau, said that one partially successful solution was getting residents to install chimneys in their homes. Another measure was the use of gas made from manure.

However, the disease is still endemic and in Guizhou, a stunning 92 percent of the province's 37 million people are estimated to have some form of fluorosis.

While only about 4,000 people have severe skeletal fluorosis, said Ma, a look around in Zhijin reveals that residents as young as six have decaying teeth that are severely discoloured and other signs of the disease.

Nationwide more than 2.8 million have some form of skeletal fluorosis, according to data published in 2005 by China's national health ministry.

The condition is so common in rural China, where 800 million of the nation's 1.3 billion people live, that the health ministry estimates that 100 million are at risk.

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