“I am deeply troubled in light of recent reports that the U.S. Army may have been involved in exposing thousands of unsuspecting Missourians to a chemical called zinc cadmium sulfide during the Cold War,” Blunt wrote. “The idea that thousands of Missourians were unwillingly exposed to harmful materials in order to determine their health effects is absolutely shocking.”

Research was released this week that showed in the 1950s and 1960s aerosol testing by the Army was conducted with fluorescent particles of zinc cadmium sulfide, in order to track dispersal patterns, which were studied for the U.S. Biological Warfare Program.

"Given the nature of these experiments, it's not surprising that Missouri citizens still have questions and concerns about what exactly occurred and if there may have been any negative health effects," McCaskill said Thursday. "The National Research Council recommended that additional studies should be conducted and it's my goal to find out whether or not they were."

McCaskill also wrote McHugh demanding answers. She pointed out that in the 1990s, a study found that the spraying would not generally cause non-cancer health effects or lung cancer. At the time Congress asked the Army to review the study.

“It is also unclear whether all documentation about how the program was carried out has been released,” McCaskill wrote. “I also ask you to review whether any and all pertinent information related to the Army's Cold War-era chemical testing in St. Louis has been made public. Transparency regarding this episode is critical to providing the impacted communities final resolution.”