Bayer officials have repeatedly issued statements saying that no MIC was released during the Aug. 18, 2008, explosion and fire that killed two workers.
But during an interview last week, plant officials were unable to explain exactly how they knew that to be true.
"We're still investigating whether, if we had a release of MIC, it would have detected that and reported it to us," said Michael Wey, the plant's environmental and safety director.
Plant manager Nick Crosby downplayed the importance of the monitors, and said he is still confident no MIC was released.
But Wey also confirmed that the two monitors mounted at the plant's eastern and southern fence line -- toward West Virginia State University and the Kanawha River -- were not calibrated the night of the explosion to specifically detect MIC.
Wey said he didn't know if they could be adjusted to do so. "I don't know if they're physically capable of it," he said.
Questions about plant air monitors were among the few questions about the August explosion that Bayer would answer during media tours of the parts of the facility last week. Bayer officials scheduled the tours as part of what they called a "new day" in plant relations with the Kanawha Valley community.
Investigators from the U.S. Chemical Safety Board have been looking into the Bayer plant's monitoring systems, both inside the Methomyl pesticide unit that blew up and around the perimeter of the Institute facility.
"All critical alarms and safety equipment should be properly inspected and tested to verify operability prior to startup," said Daniel Horowitz, a spokesman for the Chemical Safety Board. "We do not have information to indicate why the monitors weren't functioning, but we are looking into it."
In a preliminary report, CSB investigators said last month that significant safety lapses by Bayer led to the explosion inside a tank used to decompose waste Methomyl to be burned in the plant powerhouse.
The explosion blew the 5,000-pound tank into the air and across the plant. CSB investigators say that only "random chance" sent it in the opposite direction from the MIC "day tank" located just 80 feet away.
Bayer stores more than 240,000 pounds of MIC, the chemical that killed thousands of people when it leaked from a Union Carbide plant in Bhopal, India, in December 1984.
Most of the MIC is kept in an underground storage tank on the eastern end of the plant. But some amount is transferred each evening by pipeline across the plant to the Methomyl-Larvin area, where a tank can hold up to 37,000 pounds of it for use in those units.
John Bresland, a former chemical plant manager who heads the CSB, said the August incident "was potentially a serious near miss, the results of which might have been catastrophic for workers, responders and the public." Congressional investigators concluded that the incident "could have eclipsed" the Bhopal disaster.
Last week, Crosby repeated Bayer's position that "there was no MIC involved in this incident."
In the days after the explosion, Bayer issued several statements that said, "Monitoring of the air in and around the site by our industrial hygiene professionals showed no chemical exposure from this incident either on or off site."
At a company-sponsored public meeting in October, Crosby said, "there were no harmful chemical releases" during the explosion and subsequent fire.
Bresland testified during an April 21 congressional hearing that such statements by Bayer were "clearly incorrect."