Pooler Fire Chief Wade Simmons said the combination of chemicals used to clean the restaurant's bathrooms is thought to be the cause of the fumes than resulted in one woman ending up in critical condition, the Savannah (Ga.) Morning News reported.
Two women were found unconscious in a bathroom of the restaurant Wednesday, Simmons said.
The two women were taken to a hospital, and one remains in critical condition.
Two other customers, an employee and three firefighters were sent to the hospital.
Pooler firefighters and Savannah Fire's hazardous materials crew decontaminated all customers and employees of the McDonald's, hosing down sick people and taking contaminated clothing.
"We weren't exactly sure what it was," Simmons said. "We decontaminated them and bagged their clothing, just to limit any possible risk."
Two people were treated at the scene of the incident, but were not taken to the hospital.