Accidents in which large quantities of dangerous chemicals are released into the environment are almost inevitable in the modern world. Toxic chemicals are produced in such large volumes today that it would be a surprise if such accidents were never to occur. One of the most infamous accidents of this kind occurred at Seveso, Italy, a town near Milan, on July 10, 1976.

The Swiss manufacturing firm of Hoffman-LaRoche operated a plant at Seveso for the production of hexachlorophene, a widely used disinfectant. One of the raw materials used in this process is 2,4,5-trichlorophenol (2,4,5-TCP). At one point in the operation, a vessel containing 2,4,5-TCP exploded, releasing the chemical into the atmosphere. A cloud 100–160 ft (30–50 m) high escaped from the plant and then drifted downwind. It eventually covered an area about 2,300 ft (700 m) wide and 1.2 m (2 km) long.

Although 2,4,5-TCP is a skin irritant, it was not this...