Arsenic is also a chemical element that occurs naturally in rocks and soils all over the world. It enters into water supplies when the minerals in these formations dissolve, or when they are released by mining or other industry. The first arsenic standard for drinking water was set in 1942 at 50 parts per billion (ppb), or five teaspoons per 1.3 million gallons of water. But that was before its potency was widely understood. According to a September 2001 study by the National Academy of Sciences, even trace amounts can cause bladder and lung cancer. Other studies have linked arsenic to diabetes, respiratory and cardiovascular ailments, and birth defects. The Environmental Protection Agency -- criticized for its failure to act sooner -- has finally reduced the standard to 10 ppb. Even at that level, the lifetime cancer risk from arsenic is 30 times greater than that for any other carcinogen regulated by the EPA. "But if you lowered the limit to 3 or 5 ppb," says Skip Wolfe, of Kinetico, a water treatment company in Newbury, Ohio, "the cost would rise exponentially because so many more communities would be affected" (see map).