Airport security personnel is a blanket term that describes all workers who protect the safety of passengers and staff in the nation's airports and aircraft. One of the largest group of personnel in this line of work is transportation security officers (more commonly called security screeners), who are responsible for identifying dangerous objects or hazardous materials in baggage, cargo, or on traveling passengers and preventing these objects and their carriers from boarding planes. Also included in this group of workers are air marshals, who act as on-board security agents, protecting passengers, pilots, and other airline staff in the case of any emergencies while in the air. Security directors oversee security operations and staff for entire airports. Approximately 45,790 security screeners are employed in the United States.