Military Chemical Experiments in St Louis
In the mid-1950s, and again a decade later, the Army used motorized blowers atop a low-income housing high-rise, at schools and from the backs of station wagons to send a potentially dangerous compound into the already-hazy air in predominantly black areas of St. Louis.
Local officials were told at the time that the government was testing a smoke screen that could shield St. Louis from aerial observation in case the Russians attacked.
But in 1994, the government said the tests were part of a biological weapons program and St. Louis was chosen because it bore some resemblance to Russian cities that the U.S. might attack. The material being sprayed was zinc cadmium sulfide, a fine fluorescent powder.

https://clinicalnews.org/2014/01/19/revealed-army-scientists-secretly-sprayed-st-louis-with-radioactive-particles-for-years-to-test-chemical-warfare-technology-2/