An ammonia leak at a Tyson Foods poultry processing plant in Rogers, Arkansas on Wednesday sent 18 employees to an area hospital for evaluation and treatment, officials said.
Rogers fire chief Tom Jenkins said workers complained of difficulty breathing and swallowing.
Tyson Foods said the incident occurred during maintenance on the plant's roof and that ammonia fumes were brought into the facility by its air conditioning system.
The company said in a statement it immediately evacuated about 250 employees as a precaution and the plant's emergency response team quickly sealed the leak.
"Initial readings inside the plant indicate that ammonia was not detected in the environment," it said.
A fire department spokesman said no fumes escaped the building.
Rogers is about 210 miles (340 km) northwest of Little Rock.