When air quality officials recommended in January that hydrofluoric acid be banned from use in Los Angeles County, oil and chemical industry spokesmen attacked their report as “unrealistic,” saying it was predicated on such a large release of the chemical that no accident of that magnitude is likely to occur.

However, The Times has learned that an even larger release of the acutely hazardous substance occurred in 1980 in Matamoros, Mexico, killing two workers and injuring 11 others.

In the Matamoros accident, which is believed to be the largest release of hydrofluoric acid on record, the chemical poured out for 20 minutes from a ruptured valve at a rate of 500 gallons a minute, creating a dense cloud of toxic fumes.

The report by the South Coast Air Quality Management District was based on a release of 70 gallons a minute for 100 minutes. Its computer study was based on a 1987 accident at a refinery in Texas. The Times has also learned of two other major releases from an Australian refinery in 1976 and 1977. There were no serious injuries from either of the Australian accidents.

Ed Camarena, deputy executive director of the AQMD, said last week that information about the Matamoros accident, as well as the two Australian releases, refutes industry criticism that the air quality staff is too pessimistic about the potential for accidents involving the chemical.

Camarena said his staff was not aware of the three incidents — each involving the release of thousands of gallons of hydrofluoric acid — until The Times made inquiries about them.

He said the information obtained by The Times shows that “significantly larger quantities can, and have, escaped from these facilities (using hydrofluoric acid) . . . ”

Hydrofluoric acid is so toxic that the release of a teaspoonful in a 500-square-foot room would create an immediate danger to life and health, according to state health officials.

Four oil refineries in Los Angeles County and an Allied-Signal plant in El Segundo each typically have acid inventories ranging from 7,000 to 29,000 gallons. The oil refineries are Mobil in Torrance, Ultramar in Wilmington and Powerine and Golden West in Santa Fe Springs.

The hazards of hydrofluoric acid became an issue in the South Bay after about 12 gallons were released during an explosion and fire at Mobil’s Torrance refinery on on Nov. 24, 1987. One worker at the plant has alleged in a lawsuit that he suffered lung damage from breathing acid fumes.

Federal safety officials later concluded that Mobil would have avoided the accident if refinery officials had followed their own written safety instructions.

Torrance voters go to the polls Tuesday to vote on a ballot measure that would effectively ban the use of hydrofluoric acid at the refinery.

Representatives from Ultramar, Mobil and Allied-Signal attacked the AQMD in January when staffers recommended banning the substance after studying the probable consequences of a major release in the Los Angeles area.

One of the critics, William E. Hague, an Allied-Signal engineering expert, said last week that he had been aware of the Mexican and Australian releases and that he remains critical of the AQMD study because it was predicated on a rupture in a four-inch valve; Allied-Signal’s plant only uses one-inch valves.

Another critic of the AQMD, Ultramar refinery manager Marshall A. (Bud) Bell, said he was unaware of the three releases. However, he challenged the relevance of the information.

“It is not logical to say that just because something happened at some facility, that it is going to happen at my facility,” he said. “While I can say, ‘That is interesting. That is amazing,’ I am not really worried about it because I know it can’t happen here.”

Mobil spokesman Tom Collins said late Friday that the oil company had no comment on the information about the three major spills.

Camarena said, however, that the information would be helpful in his agency’s continuing evaluation of the hazards of hydrofluoric acid and “does indeed verify” that the AQMD did not use “overly pessimistic assumptions” in analyzing the consequences of a release from local facilities.

The AQMD used computerized estimation techniques to predict the consequences of an accident here that would be similar to a 1987 release at the Marathon Oil refinery in Texas City, Tex.

The Texas City accident was caused when construction workers dropped a 30-ton piece of equipment on pipelines connected to a tank containing 35,700 gallons of hydrofluoric acid. The resulting leak released an estimated 6,548 gallons of hydrofluoric acid, forcing the evacuation of 4,000 residents, sending 1,037 to area hospitals, and defoliating trees for miles. No one died.

The AQMD’s main finding was that even with efforts to control such a release, a cloud of acid fumes would threaten the lives of nearby residents. The report also said that a similar release of less volatile sulfuric acid, which is being considered as an alternative in refineries, would not produce a toxic gas cloud.

The Matamoros accident occurred at the Quimica Fluor plant, which manufactures hydrofluoric acid. The two employees who died were trapped in a building engulfed by fumes, plant manager Javier Villarreal said in a telephone interview.

An employee eventually donned a protective suit and shut off a valve to stop the release. A small community near the plant, which is in a rural area 4 1/2 miles outside Matamoros, was spared because the wind was blowing the other direction, Villarreal said.

After the accident, Villarreal said, metallurgical analysis determined that the studs holding the valve cover had been weakened by hydrofluoric acid. He said that more resistant alloys now are used and that no similar accident has occurred.

He added that the 11 employees who were injured have suffered no long-term consequences and that the plant now receives awards for its safety program.

Dr. Ronald Koopman, a scientist at the Lawrence Livermore National Laboratory who has studied the hazards of hydrofluoric acid since the mid-1980s, told The Times about two major releases from the Ampol refinery in Brisbane, Australia. His information was confirmed by Ampol refinery general manager Barry Woods and Australian safety officials.

The first Ampol release, which occurred Dec. 22, 1976, was caused when a heavy steel bar fell onto a heat exchanger during a construction project.

The heat exchanger was connected to a tank containing 28 tons, or about 7,000 gallons, of hydrofluoric acid, all of which was released.

Ultramar’s Bell and Koopman said Ampol used poor judgment in undertaking a construction project near the tank without draining it of its dangerous contents .

“That’s pretty foolish,” Bell said. “. . . It is certainly contrary to what we would do. You can’t condone that.”

Said Koopman: “They never should have worked on the tower without draining the unit. It is inconceivable, but, of course, Marathon (in Texas) was doing the same damn thing.”

Ampol’s Woods said in a statement that the release took about 2 1/2 hours, for an average rate of about 50 gallons a minute.

Woods said that refinery workers controlled the release with fire sprays and fog nozzles and that some of the gas was vented to a relief system where it could be neutralized.

“There is no record of any employee being exposed to HF vapor to the extent which required any medical or even first-aid treatment,” Woods said.

The second Ampol release took place on July 8, 1977, and was caused by the failure of a weld. It again led to a release of 7,000 gallons but no estimate of the rate is available.

Woods reported that one worker inhaled toxic fumes, was sent to a hospital for “precautionary checks and was then sent home.” Woods said the employee remains at the refinery and “is in the best of health.”

He added, “Both incidents were handled professionally by the refinery work force with no outside assistance being required. There were no significant injuries to refinery personnel. There was no impact beyond the refinery perimeter. There have been no similar incidents in the elapsed 13 years.”

Camarena said that AQMD staff members now are trying to obtain full reports of the three accidents from local authorities and company officials.

In Torrance, clouds of hydrofluoric acid escaped the Mobil refinery grounds and led Torrance authorities to close Crenshaw Boulevard for several hours on Nov. 25 and Nov. 29, 1987, according to Torrance Fire Department reports.

The release on Nov. 29 was caused when an employee accidentally opened a valve to a pipe containing acid vapors, according to the Fire Department.

The report said refinery workers were able to shut the valve within five minutes. An estimate of the rate of the release or the amount released was not available. The Fire Department did not list a cause of the Nov. 25 release, and no injuries were reported in either case.

The most recent incident involving hydrofluoric acid in Los Angeles was a release Jan. 13 at the Powerine Oil Co. in Santa Fe Springs, which sent seven refinery workers to the hospital. The incident occurred when a corroded pipe holding the acid ruptured, creating a vapor cloud that employees diluted with water and stopped from drifting outside the refinery, according to oil company officials.