Meuse Valley was the site of many factories including chemical plants, zinc smelters, and coke ovens, all of which belched a constant stream of pollutants into the air.
On December 1, residents noticed a strange, dense, cold fog had settled over the valley (due to temperature inversion).
Visibility was poor, barely a few feet. Residents continued their business as usual, although the fog irritated their eyes and made it difficult to breathe.
By the time the fog lifted five days after it began, between 60-75 people were dead. Thousands were ill or suffered permanent injury.
Two years later, Belgian authorities investigating the mystery officially reported that the fog had been trapped over the valley due to freak weather conditions, and that toxic gasses from the factories caused the deaths.
