While officials of Union Carbide Corp. say the company's plant at nearby Institute has a 17-year record of safely manufacturing the deadly poison methyl isocyanate (MIC), records show that 13 workers were treated at the plant's dispensary and one was sent to a hospital for observation after an MIC spill six years ago.

The circumstances of that spill and other accidents involving MIC, as well as conflicting reports about the amount of MIC emitted into the air here, are among the topics that Union Carbide Chairman Warren Anderson and other officials of the beleaguered chemical giant are expected to be questioned about at a congressional hearing here Friday.

The House Environment and Commerce subcommitee on health and environment is investigating the plant after an MIC leak at a similar Union Carbide plant in Bhopal, India, killed more than 2,000 people.

An aide to Rep. Henry A. Waxman (D-Calif.), the subcommittee chairman, said records uncovered here this week "indicate that the chemical is not handled quite as carefully as Union Carbide had indicated."

The aide added that "a series of changing numbers" about how much MIC gets into the air here "also concerns us."

Carl G. Beard II, director of the West Virginia Air Pollution Control Commission, said today that on Wednesday Union Carbide sent him a corrected copy of a 1981 emissions inventory that said 78 pounds of MIC, rather than the previously reported 354 pounds, were emitted into the air that year. Beard said company officials told him that had a computer had failed to consider how much of the MIC is eliminated by scrubbers and flares before it reaches the atmosphere. There is no indication that either amount posed a health threat to nearby residents.

Dick Henderson, Union Carbide's spokesman at the plant eight miles from here -- the only place in the United States where MIC is manufactured -- said today that he had not heard of the incident involving treated workers at the plant, where he has worked 35 years.

Henderson said there had been a phosgene spill in May 1978 in which "one person was treated at the dispensary and a large number of employes were sent to the hospital for chest X-rays." According to records at the state air pollution control commission, the MIC spill occurred on Aug. 24, 1978, at 5:55 p.m.

Commission director Beard said he recalled that about 20 pounds of MIC spilled after a hose was punctured. Beard said there were "no complaints from outside the plant."

Company official N.B. (Nate) Martin, in a Aug. 31, 1978, letter to Beard, said workers had just finished loading two tank cars with MIC when a half-inch rubber nitrogen line ruptured, releasing nitrogen and MIC vapors into the air.

"Although the line was valved off in less than five minutes, a number of construction employes in a stucture directly downwind of the leak were exposed," Martin wrote.

He added that no one was seriously injured, and that the hospitalized man was released within 48 hours.

Martin said, "Patrols sent to the plant perimeter during the incident could detect no sign of MIC vapors outside the fence line. The total amount of MIC discharged was estimated to be less than two gallons."

Martin said at the time that inspection of the hose revealed a 2- to 3-inch slit. He said that as a result of the accident, various alternatives are being evaluated to prevent recurrence of a similar incident.

Meanwhile, Union Carbide said from its headquarters in Danbury, Conn., that it has hired the international law firm of Kelley, Drye & Warren to help its lawyers defend the numerous lawsuits that have been filed against the company in the past few days.

[The company said: "The defense is principally directed toward those cases that seek unjust compensation. The team is also assisting the management of Union Carbide Corp. in addressing the issues raised in structuring fair and equitable compensation."]