The Death-fogs; The Meuse Valley, 1930
During the first week of December, 1930, all of Belgium was blanketed by dense fog. In addition, there was a temperature inversion in the Meuse Valley, which served as a lid to prevent the upward escape of gases.

In a 15 mile stretch of the Valley, with hills of 250 to 350 feet on each side, some 6,000 people became violently ill and, on the third and fourth days 60 died. Many cattle were also killed.

An official investigating committee declared (42) that the symptoms were those of fluorine poisoning; but that only one plant was emitting fluorine and the amounts produced could not have caused the trouble. They said it must have been the sulphur dioxide and sulphuric acid.

However, van Leeuwen (43), Fenner (44), Flury (45), Teleky (46), and Schwartz (47) all disagreed. For one thing, windows and light bulbs showed etching by fluoride. Flury gave figures to show that toxic amounts of fluorine were present and Schwartz pointed out that soluble gases such as HF and SO, can become enriched in fog particles and produce acute poisoning even if the initial concentration is very small.

The evidence was carefully investigated by Roholm who was, at the time, the world's greatest authority on fluorine poisoning. He said (48) that he was convinced by the symptoms and the details of the disaster that the malady was acute fluorine intoxication. Of the 27 factories in the region, 15 either used raw products containing fluorine (superphosphate works, zinc works) or add fluorine compounds to the raw materials (steel works, iron works, glass works), involving the possibility of passing gaseous fluorine compounds to the materials (steel works, iron works, glass works), involving the possibility of passing gaseous fluorine compounds (SiF4,k HF) into the chimney smoke.

Moreover, 20 years later the vegetation in the region contained enough fluorine to indicate tht fluorine pollution was high, and had probably been high in 1930 (49).