The incident happened around 7:30 p.m. Saturday at the Monaca Walmart, the Pittsburgh Post-Gazette reported.
Spokeswoman Dianna Gee said the store was evacuated after 10 people became ill, mostly in the snack aisle of the grocery section. Three people were treated at a local hospital, while others were treated at the store.
The Center Township Volunteer Fire Department and hazmat crews responded to the incident.
"They assessed the situation and once the air was clear we were given the OK to reopen," Gee said. "We're just really glad everyone is OK and we appreciate the quick response."
The store opened again around midnight.
The cause of illness has yet to be determined, Gee said.