Apparatus at one Fairfax County fire station was out of service on six occasions yesterday because not enough firefighters were assigned to the station, an executive of the firefighters' union charged.

The charge came two days after the union. Local 2068 of the International Association of Firefighters, is claimed a "very critical and dangerous" manpower shortage existed in the department and asked for hiring of 150 more firemen.

Glenn Benarick, a county fireman and an officer of the union local, said that on six occasions yesterday all five firefighters assigned to Fire Station 18 in Jefferson Village were sent out on calls, leaving some of the four pieces of equipment in the station unmanned.

The equipment based at the station consists of a two vehicle engine coman ambulance, he said.

At one point he said that five firefighters took the engines to three-alarm fire that caused extensive damage at an apartment complex at 7602 Broadway Dr. in the Merrifield section of the county.

The rescue squad and ambulance were left unmanned, Benarick said.

Three firemen suffered slight injuries in the fire, the cause of which was not immediately known.

Benarick said the rescue squad might have been used to ventilate the building and facilitate work of the firefighters. He said another rescue squad was summoned from a station farther away.

George H. Alexander, the county fire chief, has agreed that there are many days when not enough firemen are on duty to "appropriately" do their job.

Alexander said yesterday he could not comment on the incidents Benarick said had occurred because he was not familiar with them.

County Executive Leonard Whorton has said he thinks the department provides an "effective" level of manpower.