Nearly a third of the District's basic life support ambulances are out of service for repairs, a situation that couldn't come at a worse time, union officials said.

The shortage of transport units comes as a heat advisory has been issued and the fire department is reporting an unusually high number of calls.

The downed ambulances mean longer transport times for patients and could be the difference between life and death, said Ed Smith, head of the D.C. fire and emergency medical services labor union.

"The welfare of the public is at risk," Smith said.

As of Tuesday morning, seven of the 25 basic life support units were out of service because of mechanical problems, Smith said. He was told that most of the units were out because of the air conditioner in the patient compartment.