Upon application by the member or the chief of the fire department, any member who has become totally and permanently incapacitated for duty as the natural and proximate result of an accident occurring while in the actual performance of duty or exposure while in the actual performance of duty in response to an emergency call shall be retired by the board of trustees, if the medical board shall certify that the member is mentally or physically incapacitated for further performance of duty, that such incapacity is likely to be permanent and that the member should be retired. If the accident occurred prior to the beginning of the member’s twentieth year of service, application for benefits must be made before this time, except that the interval between the date of accident and of application may be at least one year.