Exemption— An employee who the head of the employing agency determines serves the interest of law enforcement, public safety, or the national security of the United States, or who is essential to respond to an emergency declared under section 501 of the Robert T. Stafford Disaster Relief and Emergency Assistance Act (
42 U.S.C. 5191) (as determined by that agency head), shall not apply for the purposes of determining the number of employees employed by that agency under subparagraph (A) or (B) of paragraph (1).