In ancient China, it was considered impolite for well-bred ladies to show their teeth while smiling.

Today, some young people purse their lips while smiling - not because they are following any tradition but because they do not want to expose their discolored molars.

Statistics from the National Plan for Prevention and Control of Major Regional Diseases from 2004 to 2010, jointly issued by Ministry of Health, National Development and Reform Commission and Ministry of Finance in last October, show that 40 million people's smiles are affected by fluorosis - one of the most serious regional diseases in China, Xinhua News Agency reported today.

That is to say, one in every 30 Chinese people has the disease.

The ingestion of excess fluoride, most common in drinking water, can cause fluorosis, which affects the teeth and bones, World Health Organization report said.

By the end of 2003, 38.7 million Chinese people were suffering from dental fluorosis, and 2.84 million people were affected by skeletal fluorosis, according to the statistics.

Dental fluorosis is characterized by the staining and pitting of the teeth, with some severe cases in which all the enamel may be damaged.

In skeletal fluorosis, fluoride accumulates in the bone progressively over many years, with symptoms such as stiffness and pain in the joins.

The bone structure may change and ligaments may calcify, resulting in the impairment of muscles, and pain.

In China, the disease is found to be most severe in southwest China's Guizhou Province, where coal is the major fuel for cooking.

Statistics in 2003 revealed that the province has more than 10 million patients affected by dental fluorosis, or 85 percent of its population. Of them, 1.1 million are children.

Zheng Baoshan, a researcher from the Chinese Academy of Sciences, who has studied fluorosis for more than two decades, discovered that the local clay in the coal is rich in fluoride, which is released when it is burnt.

(China Daily August 22, 2005)