1 : a safeguarding method used to prevent unplanned releases of toxic or hazardous compounds into uncontrolled work areas. 2 : level of containment that is external to and separate from primary containment . Secondary containment is a method of safeguarding used to prevent unauthorized releases of toxic or hazardous gases into uncontrolled work areas. Secondary containment means those methods or facilities in addition to the primary containmentsystem. [SEMI F6-93]