I. Notwithstanding the provisions of RSA 281-A:2, XI and XIII, 16 and 27, there shall exist a prima facie presumption that heart or lung disease in a regular, call, volunteer or retired member of a fire department is occupationally related. However:
(a) A call or volunteer firefighter shall have the benefit of this prima facie presumption only if there is on record reasonable medical evidence that such firefighter was free of such disease at the beginning of his or her employment. It shall be the duty of the employer of a call or volunteer firefighter to provide that reasonable medical evidence. If the employer fails to do so, the call or volunteer firefighter shall have the benefit of the prima facie presumption regardless of the absence of the reasonable medical evidence.