Offices of the Environmental Protection Agency in Research Triangle Park were briefly evacuated Wednesday afternoon after a release of chlorine dioxide.
No injuries or illnesses were reported.
A spokeswoman for the agency said 31 contractors and five employees were in the High Bay building when it was evacuated about 11:30 a.m. after a monitoring system detected a gas leak in a laboratory and sounded an alarm.
The workers were allowed back inside after the Durham Fire Department investigated and said the building was safe.
The spokeswoman said the EPA is conducting research on the decontamination capabilities of chlorine dioxide to help the United States respond to emergencies that involve hazardous chemicals.