As soldiers stood guard in the northern Italian town of Seveso, hundreds of villagers last week loaded into their cars or hand-drawn carts the few belongings they were allowed to take, then fled southward. Behind them they left the bodies of scores of animals in a desolated area now sealed off by barbed wire. The cause of the exodus: a cloud of toxic gas caused by an explosion at a chemical plant in Meda, twelve miles north of Milan.
Seveso's nightmare originated at Icmesa, a chemical plant that makes trichlorophenol, which is used in manufacturing disinfectant soaps and deodorants. The process can...