The new respirators seemed like just the thing for firefighters to wear to any blazes in Washington's subway tunnels, and the District fire department bought 25 in 1979 at $1,200 each. Now they sit in storage as evidence in a possible lawsuit.

The "Biopak 60" units, made by Rexnord Safety Products Inc., formerly Biomarine Industries, of Malvern, Pa., are among the most sophisticated on the market.

They have a small compressed oxygen tank, a full face mask and an optional hood. Exhaled air is circulated through a chemical cartridge that cleans it of carbon dioxide, and new oxygen comes in from the tank. A firefighter could breathe for 60 minutes or so in any kind of gas or smoke with that, the manufacturers promised.

"It was very comfortable and very lightweight, about 24 pounds," recalled Capt. H. L. Schneider, an instructor at the fire department's training center.

The only problem was that when burning material was put near the face seals during routine tests on a breathing mannequin, the lower part of the mask burned off, along with part of the dummy's face.

It seems the oxygen didn't stay inside the mask but formed a small cloud just outside, Schneider said.

After some negotiations, Biomarine sent the District free auxiliary hoods for the firefighters to wear. The hooded versions survived the first tests, so last year the department tried them in one of the fires it stages at its training facility near Blue Plains.

"The air we were breathing became very warm," related Schneider, who took part in the test. "About 110 degrees with a spike to 150." He compared it to breathing in an oven. "You didn't want to inhale." It seems that the chemical filters were cooking the air instead of cooling it, he said.

Rexnord sales director Frank Robertson said the District's tests were faulty. A burning face mask is "a remote possibility" that occurs only if coals actually touch the mask, he said, and not at all with the new hood. The overheated air was measured not in the nose and mouth cup but at the cheek area, where heat from the fire was responsible, Robertson said. The breathing air may have reached 120 degrees, he said, but noted that the air in a sauna is about that hot.

The Boston and San Francisco fire departments have also halted use of their Biopak 60s after conducting their own tests. The District unit, giving up, went out and bought 30 respirators of another brand. Legal action remains a possibility, Schneider said.