Carbide Leakage Called a Nervous System Toxin : It Is Described as Mostly Methylene Chloride, a Suspected Cause of Cancer in Lab Animals

WASHINGTON — Federal investigators have concluded that the gas cloud that escaped Sunday from a Union Carbide Corp. plant in Institute, W. Va., was not primarily aldicarb oxime but rather was about two-thirds methylene chloride, a nervous system toxin and a suspected cause of cancer in laboratory animals, The Times has learned.

The chemical, a solvent widely used as a paint remover and liquid “carrier” for other chemicals, was placed under special review by the Environmental Protection Agency in May after laboratory tests linked it to unusual numbers of malignant lung and liver tumors in mice. The review will establish whether the substance poses a significant health risk to humans.

No ‘Serious Concerns’

The EPA’s Superfund office said the 135 persons hospitalized after Sunday’s incident will be regularly checked by the federal Centers for Disease Control for evidence of chronic or latent health problems.

But while such effects cannot be ruled out, one federal official said, residents have no reason for “really serious concerns.”

Scientists at Allied Corp., which manufactures aldicarb oxime for Union Carbide, said the chemical properties of aldicarb oxime “preclude it from responsibility for what happened” in Institute.

Mid-1970s tests showed that even high air concentrations of aldicarb oxime produced only “transitory” effects in animals, said James A. Hathaway, medical services director for Allied’s chemical sector.

The chemical’s boiling point of 410 degrees Fahrenheit makes it “physically impossible to generate a cloud of this (gas) and have it travel very far. When it hit the atmosphere, it would condense out and drop like rain,” he said.

Methylene chloride, however, boils into a gas easily, making it more likely to drift outside the Institute plant. When inhaled in large doses, it can cause nervous system and brain disorders; eye, skin and respiratory irritation; pulmonary edema, and the destruction of red blood cells.

The sudden emergence of methylene chloride’s key role in Institute adds a potentially controversial twist to an investigation already marked by verbal back steps and wrong turns.

Investigators had centered on aldicarb oxime, the pesticide ingredient that Union Carbide publicly calls a “severe eye irritant” but privately rates as highly toxic, as the likely cause of the eye and lung problems that have been the gas leak’s major effects.

According to Union Carbide and EPA tests, however, the gaseous mixture that burst from the Institute reactor tank was 65% methylene chloride and only 35% aldicarb oxime, Rick Horner, a chemical engineer with the Superfund office, said Thursday.

Traces of sulfur dioxide and mercaptans--sulfur compounds that give the “rotten eggs” smell to natural gas--probably caused many of the effects suffered by nearby residents, Horner said.

Union Carbide has described the tank’s contents as “500 gallons of aldicarb oxime/dichloromethane mixture,” or 3,000 to 4,000 pounds, but its news releases have centered on the effects of aldicarb oxime. Dichloromethane is a chemical synonym for methylene chloride.

Chronic exposure to methylene chloride has caused liver and kidney damage in animals. Although regular, large doses of the gas might cause human health problems, Horner said, the chances of lasting effects from the single Institute release are probably low.

“I don’t know what concentrations people were exposed to,” he said. “I’ve been exposed to it myself in a laboratory situation, and I don’t think they have anything to worry about unless it keeps happening.”

In 1981, Union Carbide’s Institute plant reported venting about 2,500 pounds of methylene chloride into the atmosphere as a pollutant.

The EPA’s special review of the chemical--in effect, a speeded-up health study ordered because of unusual scientific concern--was triggered by National Toxicology Program experiments that suggest the substance causes cancer in mice.

The chemical industry says no link to cancer has been proved and other tests have shown no cause for concern.

The methylene chloride that escaped the Institute plant was not an ingredient in any other product, Union Carbide officials said Thursday, but was a “carrier” solution for aldicarb oxime, which is combined with methyl isocyanate to make the highly potent pesticide Temik.

Meanwhile, EPA and Union Carbide officials narrowed their inquiry into what caused the leak to a chemical tank that was overfilled, then partly emptied into a reactor vessel that leaked. The probe has implicated a broken tank meter and a malfunctioning--or accidentally opened--steam system.

That explanation is “company speculation which our people on the scene concur with,” Horner said. The Institute plant’s aldicarb production line will not reopen until the accident’s cause is found, he said.

Company officials have refused so far to speculate on the cause of the accident, except to acknowledge that the liquid mixture was improperly kept in a “reactor vessel” normally used to make products instead of to store them.

Horner said, however, that company and EPA investigators believe a large amount of aldicarb oxime and methylene chloride were pumped into the vessel when a nearby tank--part of a new Temik production line--was overfilled. A broken meter may be responsible, Horner said.

Workers later drained the vessel of most of the overflow, but apparently did not empty about 500 gallons of the liquid. A “steam jacket” surrounding the vessel, designed to heat the tank to cause chemical reactions, was “either accidentally opened or the steam leaked into it,” Horner said--and the resulting hot gas burst through gaskets and into the air.