Hazardous materials teams rushed to the hospitals after patients entered complaining of exposure to an mysterious powder, and the teams began decontamination of the patients and dozens of people quarantined after coming in contact with the patients.
"Several patients came to St. Anthony's Emergency Department from a site in East St. Louis, Illinois with an unknown chemical exposure," St. Anthony's Medical Center spokeswoman Mary Jo Wich said in a statement on the hospital's website.
"As a precaution for our patients, visitors and employees, access to the entire hospital campus initially was restricted," she said.
Restrictions on the hospital's non-emergency areas were subsequently lifted, but the emergency department "will remain closed until the nature and extent of the exposure is determined," Wich said.
Mehlville Fire Protection chief Jim Silvernail said the powder was believed to be a hazardous material and caused respiratory problems among those exposed, the St. Louis Post Dispatch reported on its website.
Jamie Newell, a spokeswoman for SSM DePaul Health Center in Bridgeton, Missouri near St. Louis, said three men who "came in contact with a dangerous substance" were admitted to the facility Saturday afternoon and that the emergency department was put on lockdown shortly threafter.
The men were in fair condition and recovering, she said.
"Fifteen employees and one patient had to be decontaminated as a precaution... but we don't believe that anybody else has been harmed," Newell told AFP.
She said officials had identified the substance, and though they had not publicly named it Newell said it was not viral -- meaning a widespread airborne contamination was not likely.
Local television station KMOV reported that two patients were in critical condition at SSM DePaul Health Center but Newell denied the report.
KMOV reported that the cases were likely linked to people who were exposed to a substance while rooting through a dumpster in East St. Louis.
News of the exposures emerged as Republican presidential hopeful John McCain was preparing to fly to St. Louis ahead of a rally on Sunday in nearby O'Fallon, Missouri.
There is "no change of plans at this point" McCain spokeswoman Kimmie Lipscomb told AFP.