Arsenic has long been regarded as a dangerous poison and an environmental contaminant. But in the 1980s the focus on arsenic changed dramatically when approximately 3 million tube wells in Bangladesh and West Bengal, India, were found to be contaminated with that highly reactive chemical agent. By 2003 public health authorities estimated that as much as 40 million persons were being exposed to varying concentrations of the chemical in Bangladesh, plus another 3 million in West Bengal. The source of the arsenic came as a surprise to the toxic substance community in that the contamination was so widespread and came not from any industrial source but from rocks and sediment in the region's natural geological formations.

Arsenic is one of the most ubiquitous and paradoxical substances on Earth. In very small amounts, it is essential to life. In large amounts it is poisonous. While its inorganic forms are toxic, its organic forms...