BALTIMORE (AP) _ Asbestos manufacturers knew the insulation material was dangerous in large quantities but for decades thought it was being used safely, a defense attorney said today at the nation's largest asbestos injury trial.

Edward Houff, an attorney for five of 13 defendant companies, said in opening arguments that asbestos had been used since the beginning of the century because of its heat-insulating qualities, durability and adaptability.

''It was intended to keep a person from something harmful,'' Houff said. ''For many years, asbestos products were seen as safety products.''

Most companies stopped mining or using asbestos in the 1970s after it was linked to disabling and sometimes fatal lung ailments.

Houff said the trial unfairly lumped the companies together. He told jurors a decision to award punitive damages would have enormous consequences for the companies involved.

Damages in the trial could exceed $1 billion, if all 8,555 plaintiffs were awarded $70,000 in compensatory damages and an equal amount in punitive damages, he said.

The trial consolidates cases filed in Baltimore and four Maryland counties. It is expected to last four months.

Jurors must decide whether six plaintiffs whose medical histories will be profiled had or have asbestos-related diseases. If the jury awards punitive damages to the six plaintiffs, damages for the rest of the cases will be decided at minitrials.

Two of the plaintiffs - steamfitters Lawrence Leaf and Ira Russell, both of suburban Baltimore - died from asbestos-related diseases, their attorneys said. One of the six is hospitalized and the remaining three were in court Tuesday.

The jury also must determine whether the companies knew asbestos was dangerous, neglected to warn users and whether the companies must pay punitive damages to those who became ill.

Peter Angelos, the lawyer for about 90 percent of the plaintiffs, on Tuesday accused the companies of knowingly disregarding worker health.

The plaintiffs are seeking damages for illnesses they claim can be attributed to occupational exposure to asbestos. It has been linked to many ailments, including cancer.

Fibreboard Corp. settled out of court Friday and Owens-Illinois Inc. settled with most of the plaintiffs.

The other defendants are AC&S Inc., Armstrong World Industries Inc., GAF Corp., Keene Corp., National Gypsum Co., MCIC, Owens-Corning Fiberglass Corp., Pittsburgh Corning Corp., Porter-Hayden Co., Quigley Co., U.S. Gypsum Co., W.R. Grace & Co.