Union Carbide Corp., after a preliminary investigation, today blamed pressure buildup and a leaky valve for spilling 500 gallons of a nontoxic hydraulic fluid Tuesday from its South Charleston plant.

The leak occurred two days after toxic aldicarb oxime, a pesticide ingredient, escaped from Carbide's nearby plant in Institute, sickening 142 people. It came eight months after a leak of another pesticide ingredient, methyl isocyanate (MIC), from the firm's plant in Bhopal, India, killed more than 2,000 people.

No injuries were reported in Tuesday's leak, although one South Charleston resident was treated for eye irritation. State environmental officials said an undetermined amount of the fluid, a mixture of a metal lubricant and isoproponal, ran into the Kanawha River but posed no threat to fish or other aquatic life.

Meanwhile, state officials said they have been talking to the company for a month about emissions from its new Silicones II unit in South Charleston. Carl Beard, director the state Air Pollution Control Commission, said emissions of methyl chloride are up to 10 times higher than the unit's permit allows.

Environmental Protection Agency spokesman Al Heier said methyl chloride, used as a solvent or a refrigerant or a propellant for foam products, is considered to be of "low toxicity." In sufficient concentrations it can cause kidney and liver damage, according to Janet Luffy of the EPA's regional office in Philadelphia.

Beard said the 18-month-old unit is emitting methyl chloride at a rate of 130 tons annually -- 10 times more than the 13 tons set by the permit. "We don't know what concentration those emissions" leave in the air, Beard said, adding that there have been no complaints from area residents.

He said the colorless gas has a faint, sweet odor. "In the concentrations it's being discharged and at the heights it's being discharged, people probably wouldn't notice it," he said "But that doesn't mean we don't consider it serious."

Plant spokesman Mike Lipscomb said the company is aware of the state's concern and has "outlined a plan" to take care of the problem. Beard said that Carbide has been cooperative, proposing a $7.5 million incinerator to eliminate the excess gas, but that the commission is pushing for a quicker solution.

The unit where Tuesday's leak occurred has been shut down, Lipscomb said. The tank held a mixture of polyalkaline glycol, a lubricant sold under the trade name UCON; isoproponal, a thinning agent and the main ingredient in househould rubbing alcohol; and trace amounts of sulfuric acid and other materials.

Lipscomb said the fluid escaped as vapor when a pressure buildup released a safety valve and as liquid from a second valve with a hole "about the size of a pinky finger."

Charleston city officials said they received about 50 calls, beginning about 8 p.m. Tuesday, from residents who said they had noticed an unusual odor, according to a spokesman for Mayor Mike Roark. After being told by state and company officials that the fluid was not dangerous, emergency services officials did not evacuate residents but urged them to stay indoors with the windows closed and ventilation systems turned off.

Roark, who had criticized Union Carbide for not informing him promptly of Sunday's leak in Institute, said he was satisfied with the company's response to the South Charleston spill.

Several residents who live about five blocks north of the plant said they had noticed the odor but did not worry about it.

"I smelled something last night; I thought something was burning," said Sandra Chapman. "But we always smell the smells from Carbide. It's nothing unusual to me."City officials suggested that the many calls were provoked less by the spill's actual effects than by lingering concern over Sunday's incident. "There's certainly a heightened sensitivity to smells now," Roark said.

The maker of the computerized chemical tracking system installed early this year at the Institute plant said today that the system was not programed to predict the path of aldicarb oxime, which is mixed with MIC to produce the pesticide aldicarb, sold under the trade name Temik.

Gary Gelinas, president of Safer Emergency Systems Inc., said in a telephone interview from his Westlake, Calif., office that the computer has been programed to track only three chemicals -- chlorine; phosgene, a nerve gas; and MIC.

Gelinas said the system was installed at the Institute plant as a precaution after the Bhopal incident. The $80,000 safety package allowed Carbide to obtain the tracking software for 10 chemicals at no added cost. Carbide recently asked for the software for the remaining seven chemicals, but aldicarb oxime was not among them, Gelinas said.

He said additional software packages cost about $500 each, with no limit on the number that can be purchased. Of the 40 computer tracking systems the company has in operation in the United States, Gelinas said, several have been programed to track 30 to 40 chemicals.

Asked why the computer was not programed for aldicarb oxime, company spokesman Thad Epps said, "I can't answer why aldicarb oxime wasn't considered a priority chemical." He added that there are hundreds of toxic chemicals at the plant.

Epps said during Sunday's leak, the shift coordinator on duty "made a visual observation about the direction of the chemical cloud and his visual observation was confirmed by the Safer system." He said the computer operator used data for a chemical "that was the closest" to aldicarb oxime.

The tracking system uses a weather tower on the premises to monitor the temperature, wind speed and direction with continuing updates. When the computer operator feeds in the name of the chemical and the location and amount of the leak, the computer can predict the path of the chemical cloud and the speed with which it will travel.