"If a person doesn't have insurance, in most cases they do not pay," Assistant Commissioner Steve Rush testified at a council budget hearing.
"We offer to those patients charitable payments, which is a reduced fee, but most of them elect not to take advantage of it. There's only so far you can go."
Rush added that the department had tried to sell the mountain of debt to collection agencies but found no takers.
"They are no longer interested," Rush said. "It's a very difficult debt to collect on."
The unpaid ambulance bills represent 32 percent of the $380 million that it costs taxpayers annually to run the fleet of ambulances.