At the end of World War II, US and British scientists were dismayed to learn that the Germans were far ahead in the development of chemical and biological warfare agents. German scientists had developed artillery shells capable of delivering highly concentrated sarin, tabun, and other nerve agents to a battlefield, without endangering their own troops. Of even greater concern to the western powers was the knowledge that the Soviet Union – an ally with which relations were rapidly deteriorating – now had access to the same technology.
The Germans had developed the means of delivering chemical agents via artillery, rocket, and from aircraft. Once the toxic agent so delivered was suspended in the atmosphere, wind currents allowed it to drift across large areas. As the toxic cloud drifted its toxicity decreased due to being spread out in a less concentrated form.
The Germans had been unable to collect much data on the effect of this dilution due to the pressing issues of dealing with the Russians on the one hand and the Western Allies on the other. Yet the Germans never used their chemical capability. The Nazi hierarchy refused to allow the use of gas or chemical against troops on either the Eastern or Western front during the hostilities.
Motivated by the fear of the Soviets gaining superiority in the possession and deployment of chemical weapons, the UK and the United States invested huge sums studying the potential use of such weapons. The United States Army re-designated its chemical weapons unit (which had existed since the First World War and focused on defense against chemical weapons attack) as the “US Army Chemical Weapons Corps.” The mission of the Corps shifted from defense against chemical agents to delivery of chemical weapons against an enemy.
As the United States fought a conventional war in Korea, the Chemical Weapons Corps studied ways and means of using toxic agents on the battlefield and against North Korean, Chinese, and Soviet bases and cities. They did this by simulating attacks on American cities, which possessed geographic, meteorological, and demographic similarities to those in enemy nations. St. Louis was one such city.
The Chemical Corps would never have considered using an actual biological toxin against an American city. A substitute material was required. It came in the form of Zinc Cadmium Sulfide, which had been used in similar dispersal tests along the Georgia and South Carolina coastlines, where it drifted harmlessly off to sea. The material was further treated in some cases with Radium FP226, a fluorescent substance believed by the Army to be harmless, which more readily enabled tracking of the material as it was dispersed in the atmosphere.
Some of the zinc cadmium sulfide was dispersed in the air via US Air Force C-119 cargo airplanes. The majority of the substance was distributed in St. Louis via rooftop fans and roving vehicles on the ground. To reassure residents the Army announced that the project was testing “smoke screens” as part of developing plans to defend St. Louis from an air attack. Large amounts of the material was distributed from the rooftops of the Pruitt-Igoe housing complex, an urban housing project completed in 1956. The complex exhibited density of population similar to areas targeted in recently rebuilt Soviet and East German cities. More than 70% of the residents were children under the age of 11, also reflective of the population of Soviet housing projects.