After Bhopal: tracing causes and effects.

Shortly after midnight on Dec. 3 last year, a cloud of deadly methyl isocyanate vapor escaped from a storage tank at a Union Carbide chemical plant in Bhopal, India. Within hours, more than 2,000 people died and tens of thousands were injured. Since then, two questions have dominated investigations of the Bhopal tragedy: Why did it happen, and could such as disaster occur in the United States?

The answer to the first question is slowly emerging, although findings so far are incomplete and controversial. Last week, Union Carbide Corp., based in Danbury, Conn., reported the results of its investigation. A team of seven engineers and scientists did about 500 experiments in trying to match the chemical residues in the leaking storage tank in order to reconstruct the events at Bhopal. They conclude: "This incident was the result of a unique combination of unusual events."

The study suggests that somehow a large volume of water--between 120 and 240 gallons--was "inadvertently or deliberately" pumped ito one of three tanks storing liquid methyl isocyanate. The investigators did not rule out sabotage. The presence of water triggered a heat-generating chemical reaction. The high temperature allowed chloroform, a solvent contaminating the methyl isocyanate, to decompose. The resulting chloride ions corroded the stainless steel tank, releasing iron, which catalyzed another "runaway" reaction. At some point, the tank could no longer withstand the steadily increasing temperature and pressure, and in the end about 50,000 pounds of methyl isocyanate escaped.

The Union Carbide report notes that several "critical" violations of company safety procedures also contributed to the disastrous leak. A refrigeration system that was supposed to keep methyl isocyanate cool and relatively unreactive had been shut down five months before the accident. A flare tower designed to burn off vented gases was not operating. An alarm meant to warn of rapid temperature rises did not sound at the time of the accident.

Responsibility for safety lies chiefly with local plant managers. Says Union Carbide Chairman Warren M. Anderson: "That plant should not have been operating."

Partly because the Indian government denied Union Carbide investigators access to important documents and to plant employees, uncertainty still surrounds the events in Bhopal. A spokesman for the Indian Embassy in Washington, D.C., protested that Union Carbide's implication that jobs properly was "unjustified and unacceptable." The Indian government is conducting two inquires of its own into the causes of the Bhopal disaster.

In addition, S. Varadarajan, India's chief scientist and leader of a technical team studying the accident, stands by his team's conclusion that only a small amount of water entered the storage tank, initiating a somewhat different but equally devastating sequence of chemical reactions. Indian scientists are preparing to open the storage tank for a more complete study.

Nevertheless, says Anderson, "We can say with a great deal of confidence what went wrong technically at Bhopal." Adds Jackson B. Browning, Union Carbide's vice president for health, safety and environmental affairs, "Now, after the investigation, . . . We can confidently say: It can't happen here."

Meanwhile, Union Carbide has taken steps to reduce the risk of problems at its recently closed plant in Institute, W. Va., in preparation for reopening the plant as early as next week. These steps include installation of a computerized chemical vapor warning system, the use of chloroform instead of salt water for storage tank cooling, the reduction of methyl isocyanate inventories and louder sirens to warn the community.

However, some U.S. critics are not satisfied that the Union Carbide scenario accounts for the Bhopal leak. If a different sequence of reactions had taken place, as the Indian scientists suggest, then it still isn't clear that a similar problem can't occur in the United States.

"Union Carbide Doesn't really have a basis for making the kind of judgments as to what exactly happened," says A. Karim Ahmed of the Natural Resources Defense Council, an environmental group based in New York. "We need an independent assessment of whether the plant in West Virginia is, in fact, as safe as Union Carbide officials are claiming it to be."

The Bhopal tragedy has also prompted the U.S. chemical industry to look more closely at its safety practices. This week, the Washington, D.C.-based Chemical Manufacturers Association announced a program that focuses on integrating chemical plant emergency procedures with a community's emergency-response plans. At the same time, the program is designed to increase public access to information about hazardous chemicals.

In response to the Bhopal disaster, Congress is collecting information from the chemical industry on the hazards that various chemicals present to communities where plants are located.