Behind Laos's yellow rain and tears

A controversial Radiolab episode opens old wounds and raises countless questions for Minnesota's Hmong

Chester J. Mirocha settles into an armchair in the living room of his St. Paul home with a cup of tea and a freshly baked chocolate chip cookie. His white beard is neatly trimmed, and he wears a bird-embroidered denim shirt tucked into denim pants.

He doesn't look much like the kind of man to bring chemical warfare to light.

For more than 30 years, Mirocha was a plant pathology professor at the University of Minnesota's St. Paul campus. He was also one of the world's leading experts in mycotoxins, the kind of poison the government suspected was carried in yellow rain. He had been working with these specific toxins, known as trichothecenes, for over a decade.

Eng Yang grew up near what he remembers as "hundreds" of honeybee hives. One recent day, he sketched on a napkin the approximate sizes of several species local to Laos and Thailand, to illustrate his familiarity with bees (and their byproducts).
courtesy of Eng Yang
Eng Yang grew up near what he remembers as "hundreds" of honeybee hives. One recent day, he sketched on a napkin the approximate sizes of several species local to Laos and Thailand, to illustrate his familiarity with bees (and their byproducts).
Narrative embroidery tells the story of the Hmong escape, including, at left, yellow rain
collection of the Center for Hmong Studies, Concordia University
Narrative embroidery tells the story of the Hmong escape, including, at left, yellow rain

"We had done most of the work on trichothecene toxins," Mirocha recalls, adding that his lab had worked on major government contracts for several years. During that time, he had not only developed new analysis methods, but had "kind of pioneered" a particularly precise method, known as mass spectrometry.

Mirocha remembers his lab's accomplishments carefully, with the measured words of a scientist. "I can only speak for my lab," he says, "but I think we were rather good."

Mirocha wasn't just a leading expert in mycotoxins: He was an expert in Russian mycotoxins. In the early 1970s, he had traveled to Russia twice to study problems the Soviets were having with these kinds of poisons.

Following World War II, Russia had suffered a serious natural outbreak of a particular type of trichothecene, known as T2 toxin, after citizens ate wheat that had been left to mold under snow. Mirocha obtained samples of the toxins from the Soviets, studied them, and published his results. He suspects that the Russians would have been able to weaponize T2.

"The Soviets had an excellent background in toxicology," Mirocha says. "And they had stockpiles of a lot of biological weapons."

In 1981, Mirocha was sent a leaf sample and asked to test for that same substance, T2 toxin. He didn't know where the sample was from or who was sending it, but he performed the test as usual. He found several toxins, reported the result, then flew off to Cairo to teach at a mycotoxins workshop.

In Egypt, Mirocha received a phone call. There was a reporter on the other end, but the connection was fuzzy, and all he could hear were questions about his analyses — something about secret research.

While Mirocha had been overseas, the world had been waking up to yellow rain. On September 13, 1981, Secretary of State Alexander Haig gave a dramatic speech in West Berlin: "We now have physical evidence from Southeast Asia which has been analyzed and found to contain abnormally high levels of three potent mycotoxins — poisonous substances not indigenous to the region and which are highly toxic to man and animals."

The analyses Haig referenced were — by all declassified accounts — Mirocha's.

When Mirocha got back to St. Paul, his name was in the papers. On September 28, 1981 — just 15 days after Haig's speech — a two-inch headline on the front page of the St. Paul Dispatch read, "U professor made secret tests for biological warfare agents."

The paper argued that Mirocha had conducted secret government research, in violation of university policy. The university, however, recognized that he hadn't been doing anything clandestine, just standard tests that were part of his job.

His name was quickly cleared. But it was his first taste of yellow rain's political baggage.

"I was a celebrity for a while," Mirocha remembers. "These people were coming in with these big cameras and things, and taking up all the space in the hallways. It was kind of hard to take."

On top of the reporters, FBI and CIA agents arrived to question him and inspect his lab procedures. Mirocha testified twice before Congress, and took a trip to the Pentagon to listen to the government's theories.

He went on a six-mile run every day to manage the stress.

"My priority was our work in mycotoxins," he says. "But in terms of politics, that was hijacked, because politics became more important to other people."


Matthew Meselson first became suspicious of the official explanation for yellow rain in November 1982.

He was reading through State Department briefings on the new threat, when he noticed that the yellow samples had a high pollen content. One of the defense scientists speculated that the Russians had added pollen to the toxin to aerosolize it.

To Meselson, a Harvard biochemist and expert in chemical biological weapons, this was nonsense.

"When I read that, I knew they had set their foot down a blind alley," he says.

So he got to work forming a hypothesis of his own. Meselson organized a brainstorming meeting in Cambridge, and later called a honeybee expert at Yale by the name of Tom Seeley. Meselson described the yellow spots to Seeley — their size, their color, and how they were loaded with bee pollen.

"'The State Department explanation is not parsimonious,'" Seeley said, according to Meselson's recollection. "'It's bee —.'" Meselson breaks off, laughing. "And then he used a four-letter word."

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