A 20-square-mile corner of the Lin Xian valley in eastern China was a tragic pocket of cancer for at least 2,000 years. One in every four persons died of cancer of the esophagus, the tube leading to the stomach - a rate 250 times greater than the Chinese average. But in one of the few instances in which a cancer cause has been unmistakably identified, Chinese scientists traced the disease to its origin and began to control the ancient terror of Lin Xian.

The program started in 1958 with attempts to isolate the cause. Local residents blamed the cancer on a habit of eating steaming hot food. Food, it turned out, was involved, but not its temperature. The soil of Lin Xian is low in the element molybdenum. Without it, crops accumulate nitrite compounds; normally an enzyme containing molybdenum limits nitrites in living organisms. The plants also produce less vitamin C, which helps eliminate nitrites. Hence the people of Lin Xian were building up high nitrite concentrations. At the same time, the foods they dried or pickled grew mold, considered a delicacy. The molds contain substances known as amines, which combine with nitrites to form nitrosamines. And nitrosamines cause esophageal cancer.

In early stages the cancer can be cured by surgery. In China train technicians were assigned to screen residents of Lin Xian for treatment. And an educational campaign promoted preventive steps. It will be years before results can be gauged in lives saved, but villagers quickly began to treat fields with molybdenum and improve preserving techniques - measures to block the nitrosamines that had cursed Lin Xian with cancer.

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