Behind Laos's yellow rain and tears

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

On the morning of May 16, Eng Yang rose early. He read notes he had taken 30 years earlier in books that had survived the trek out of the mountains of Laos, across the Mekong River, through the refugee camps of Thailand, and beyond the Pacific, all the way to his Brooklyn Park home.

He put on a white button-up and a sweater and set out fruits and juices. Just before 10 a.m., his niece, the author Kao Kalia Yang, arrived with a sound engineer. The three connected their phone to a studio at WNYC, New York City's public radio station, and got on the line with a producer and co-host of Radiolab, a popular science show that boasts more than four million monthly listeners via downloads or streaming, and even more who catch broadcasts on over 300 radio stations nationwide.

For the next two hours, Eng, with Kalia translating, told the producer, Pat Walters, and the host, Robert Krulwich, what he remembered. He talked about where he was born, about the Laotian village where he grew up. He talked about how his people, the Hmong, had fought along with the Americans during the Vietnam War and the Secret War in Laos, and how, after the U.S. pulled out of the region, the Vietnamese and the Lao retaliated.

Eng Yang and his niece, Kao Kalia Yang, at St. Paul's Hmongtown Marketplace
Mark N. Kartarik
Eng Yang and his niece, Kao Kalia Yang, at St. Paul's Hmongtown Marketplace
This Hmong boy reported that his stomach rash was a result of yellow rain
courtesy of Fred
This Hmong boy reported that his stomach rash was a result of yellow rain
A team of Hmong resistance fighters returned to Laos to look for samples of yellow rain. Here, they present their findings to a Ban Vinai camp leader, the man in the blue blazer. Eng Yang stands to the right.
courtesy of Fred
A team of Hmong resistance fighters returned to Laos to look for samples of yellow rain. Here, they present their findings to a Ban Vinai camp leader, the man in the blue blazer. Eng Yang stands to the right.
Fred inspects slides of his photos from Ban Vinai Refugee Camp in the 1980s
Olivia LaVecchia
Fred inspects slides of his photos from Ban Vinai Refugee Camp in the 1980s
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

He talked about how their Communist militias used bombs and guns, and something else. There were planes, he remembered, that sprayed some kind of substance, a gas or a powder. The Hmong who had seen it described it as pink, blue, and green, but most often as yellow. Like a yellow rain.

Months later, on Monday, September 24, Radiolab released a podcast of its segment on yellow rain. The episode, titled "The Fact of the Matter," was supposed to be about the nature of truth.

The middle segment of the hour-long show explored the story of yellow rain. As Radiolab described it, with the help of a former CIA agent and two leading scientists, the belief that yellow rain was a chemical weapon almost single-handedly re-escalated the Cold War.

After walking through what the stuff was, how the government came to believe it was toxic (and to blame the Soviets), and how it led to the U.S. producing its own chemical weapons for the first time in 20 years, the show threw listeners a curveball.

The two scientists explained how their work led them to hypothesize that yellow rain wasn't a manufactured chemical at all. It was honeybee droppings.

Earlier in the segment, Radiolab had introduced the Yangs, and after unpacking the honeybee theory, the show returned to them.

"At a certain point in our conversation," related Walters, the producer, "we explained that the evidence they'd been attacked by chemical weapons seems a little shaky."

Eng disagreed. "How do you explain the kids dying?" he asked. "That where there is this yellow thing, where there are no bees, whole villages die?"

Walters conceded that the Hmong had definitely died. "They were malnourished and drinking from contaminated streams; diseases like dysentery and cholera were rampant," he said in the podcast. "And the way a lot of people see it is that they may have misattributed some of these mysterious deaths to this cloud of bee poop that looked like it could have been a chemical weapon."

Kalia began to feel that the inquiry had become an inquisition. "There's a sad lack of justice," she said, "that the word of a man who survived this thing must be pitted against a professor from Harvard."

Krulwich didn't let up. "But as far as I can tell, your uncle didn't see the bee pollen fall," he argued. "Your uncle didn't see a plane. All of this is hearsay."

When Kalia answered, her voice cracked and she started to sob through her words. "We have lost too much heart and too many people," she finally concluded. "I think the interview is done."

Once the podcast hit, listeners started commenting in swarms. Radiolab has since amended the episode. One of the show's hosts, Abumrad, issued a response. The other, Krulwich, apologized both in writing and at the end of the altered podcast.

But as the hosts tried to mitigate the damage, audience comments continued to spiral into the hundreds. For David Shih, a professor of English at the University of Wisconsin, it was a textbook example of media bullying.

"This was a racialized event," Shih says. "This was members of an advantaged group speaking for members of a targeted group and saying, 'You don't know what really happened to you. We do.'"


Eng Yang has a wide, friendly face, flecked with sun spots and laugh lines. But when he remembers his life in Laos in the 1970s, that face becomes serious, concentrated. He wants to tell the story carefully.

In the aftermath of the American wars, when the Vietnamese and the Lao started attacking his people, Yang became a leader of the Hmong resistance. By the mid-1970s, he had fled his village and gone to live in a rebel hideout in the caves of Phu Bia mountain.

The Thai government supplied the outmatched Hmong fighters with some supplies, Yang remembers, like medicine. They also sent a radio.

Yang was tasked with operating this radio and using it to report to Thai officials. So one day, around March 1978, when he heard from the resistance headquarters that three Hmong camps had been attacked, he went to investigate.

"I had been trained to be a reporter and a recorder of what was happening to my people," Yang says. "The rule was for me to be as diligent as I could."

When Yang arrived at the first village, it had been a day or two since the attack, but he could still see yellow spots on the leaves. The largest were the size of corn kernels, and the smallest were like round rice grains.

Yang started interviewing people, and they told him that this yellow stuff on the leaves was what was making them sick. He remembers them vomiting, and washing their skin with opium to dull the pain.

Yang had been trained as a medic during the war and through the years treated case after case of dysentery and cholera. What he was seeing in these people wasn't that, he was sure.

"The first time I saw people suffering, I knew it was different from anything I had seen before," Yang says. "These symptoms together was all new for us."

So he looked more closely at the yellow drops. "It was clear to me that it wasn't part of a liquid explosion from some bullet," he remembers. "It wasn't bee poop, either."

When he reported what he had seen to the Thai, they told him to cover his mouth and his nose with wet cloths on future investigations.

Not long after the first few incidents, "so many reports started coming in," Yang says. He guesses that he reported about two attacks per month for the next year. Yang trusted the people he was interviewing when they said they got sick following the yellow rain attacks. But he couldn't be certain a chemical was poisoning his people.

"I believed," Yang says, "but I didn't have the ability to test, so I couldn't say for sure."

In 1979, Yang finally fled the jungle, swimming across the Mekong River to seek refuge in Thailand. Once over the border, he was eventually sent to Ban Vinai Refugee Camp.

It was the largest refugee camp in Thailand, a teeming city of more than 40,000 that had sprouted up in 1975. The grounds were packed with newly minted refugees beginning the process of creating a new life for themselves, as well as Thai and U.S. officials, United Nations workers, and aid groups of all kinds.

When Yang first got there, the camp had no sanitation or public health. But a year later, an American volunteer, Fred (not his real name) arrived. Fred had served as a medic in Vietnam, and soon returned to the region. He never left. Now, decades later, he continues to work there, and so prefers to use an alias when discussing yellow rain.

In 1980, as Fred went about procuring a pump truck to empty latrines that had been full for two years, organizing the camp's bamboo heath clinics, and generally trying to improve operations, he started hearing about something the Hmong called chemi — yellow rain.

By 1982, after a brief stint back home in Minnesota, Fred returned to the camp. The Chemical Biological Weapons Information Project hired him to interview Hmong who said they had experienced yellow rain attacks. Over the next two years, Fred reported on over 100 cases.

New groups of refugees arrived once or twice per month, and if they knew of yellow rain, camp leaders referred them to Fred. He split them up, questioned them individually, recorded their responses on a form, and filed it.

"They were shy, sometimes afraid, to talk to a Westerner," Fred remembers. "Sometimes they'd never seen one before."

Many of them described nausea, vomiting, diarrhea, and in severe cases, hemorrhages and death. Some of them could recall the particular fixed-wing planes they had seen just prior to the attack, and when Fred showed them pictures of several aircraft models, he says that a majority pointed at the same one.

Fred was no doctor, but he had seen his share of diseases — both as a medic during the war and later when setting up Ban Vinai's public health operation. When the people he was interviewing told him their symptoms, though, the accounts didn't match any illness with which Fred was familiar.

The only signs he could see for himself were lingering rashes, which victims blamed on the chemi. Fred remembers these as odd.

"They were unusual-looking, and startling," he says. "You couldn't diagnose it quickly. It wasn't scabies, even infected scabies."

Fred never interviewed Eng Yang, but the two met and became friendly. One day, Yang remembers, an American woman gave out a pamphlet with a person in a gas mask on the front cover. Inside were photos of people with rashes. Their skin looked exactly like what Yang had seen on his neighbors' bodies back in Laos.

"That's when I knew it was yellow rain, a chemical," Yang recalls. "My heart had always believed, but now I had no more questions."


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.

"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."

Seeley told Meselson how, after hibernating, North American bees take "cleansing flights," where they swarm from the hive and defecate en masse. But Seeley wasn't sure if Southeast Asian honeybees did the same thing. So in March 1984, Meselson and Seeley took a trip to Thailand to try to investigate the phenomenon.

Fred, the public health guy from Ban Vinai, joined them for part of the trip as a guide and translator. One day, he remembers, the team spread out sheets of paper in concentric circles around a hive in Khao Yai National Park, a forest north of Bangkok.

"Sure enough, in the morning, the bees pooped en masse, and I watched it land on the sheets," Fred says. "I stood there and watched yellow rain."

But then he catches himself. "At least what they were saying was yellow rain."

Meselson recalls two other incidents during the trip when the team got caught in feces showers. During one, they were driving when a hive in front of them "kind of changed color," Meselson says. Seeley had his bee gear on and watched, but Meselson took cover in the car.

"We could actually hear these spots hitting the metal roof of the Land Rover," Meselson says.

The scientists had proven that Southeast Asian bees also take cleansing flights. They didn't think the bees were pooping out toxins, though. They thought that Mirocha had been wrong.

Meselson speculated that Mirocha could have easily contaminated the samples. His lab at the U trafficked in high quantities of these same toxins, and he was testing for tiny amounts of it.

"To do that work you have to be sure that your lab is very, very clean," Meselson says. "And preferably that it's a lab that has never seen those very same substances."

As yellow rain became an increasingly bigger issue, more and more labs around the world began testing samples. And none of them found any poison.

Other pieces of the case had started breaking down too. Deeper analyses of the pollen showed that it was from plants native to Southeast Asia, and from a wide variety of them — meaning that it was almost certainly natural in origin.

As Meselson looked into it more, he discovered that a similar scenario had unfolded before, in a province in China. In 1974, the area had suffered a large earthquake, and in its wake, Chinese villagers reported mysterious yellow spots, which they thought were poisonous. But when a professor took samples of these spots, he discovered that they were full of pollen.

"They were frightened by this and simply didn't recognize what it was, even though they had lived there for generations," Meselson says. "If you have a population under stress, you can understand how they can attribute their illness to something like this happening that they don't understand."

Meselson believes that's what happened to the Hmong.

"There's not a single shred of objective evidence," Meselson says today. "Deep inside the United States government I think they knew it was a mistake, but unfortunately they have never admitted it."


Mirocha knows Meselson and the honeybee theory well. But he disagrees that it debunks the stories of toxic yellow rain.

"The honeybee fecal matter explanation is a red herring," Mirocha says. "If the Hmong people were told that they were not really the victims of chemical warfare, then that is nonsense."

Mirocha stands by his initial findings, and insists that his lab procedures were excellent. Two years after the first analysis, his lab participated in a trichothecene testing trial, and was one of the few labs involved in the experiment that produced fully accurate results.

Furthermore, recently declassified government data confirms that all of the environmental controls Mirocha tested came back negative — in other words, he didn't find false positives in the toxin-free samples.

Mirocha's analyses and methodology had been "put through the wringer" and come out intact, said Gary Crocker, the State Department's senior intelligence officer at the time, in a 1991 New Yorker article on yellow rain. Crocker added, "One thing I can say about Dr. Mirocha is that he is not involved in politics. He's a pure scientist."

Crocker also readily admitted that "an awful lot of our environmental samples were nothing but bee spots," which is in fact what Meselson showed.

But not all of them were. Recent research by professors from Princeton and George Washington University, who concluded that chemical weapons were likely used against the Hmong, found that some of the government's environmental samples didn't contain any pollen — meaning they weren't bee droppings.

Mirocha sees an additional oversight. Meselson and Seeley "did very good work" with their honeybee research, he says. "That article came out as the first account of mass honeybee defecation."

Then Mirocha shakes his head. "But the natives had known this for hundreds of years!"

From Mirocha's perspective, this lack of consideration of local knowledge was just one way the original controversy was unfair to the Hmong.

"I believed what was going on over there. It would be stupid for me not to, because it's just a no-brainer," Mirocha says. "People are being killed. People are dying. And they're not lying."

Mirocha recalls being "disgusted" by how the Hmong testimonies were treated, as the scientific and political narratives spiraled out of control.

"I remember reading an editorial that said something like, 'Well how can these Hmong people identify what planes were dropping the chemicals, they have no knowledge of planes,'" Mirocha says. "But we trained some of the Hmong people as pilots! They fought on our side. They might not have been educated, but they're intelligent."

Fred, who worked both with yellow rain victims and with Meselson and Seeley, agrees that bees take the cleansing flight, and that it can appear as rain. He also believes that some of the samples really were bee crap — especially later, as word got out that Americans were looking for the stuff, and people got less rigorous about what they were collecting.

"I'm pretty open-minded, because of my experience with both sides, frankly," Fred says. "But that doesn't mean that some of the other samples weren't chemicals. I can't imagine anyone saying that 100 percent."


As Jonathan B. Tucker, a chemical weapons expert, wrote in a definitive 2001 account on the science of yellow rain, "Whether or not toxin warfare agents were used in Laos and Cambodia between 1975 and 1983, and if so which ones, remains a mystery."

Both sides of the issue hold fast to their beliefs. Both the U.S. and the Russian governments still keep their separate secrets. On one side, information about the former Soviet Union's weapons program continues to emerge, and on the other, the State Department maintains that it has further research that proves once and for all that yellow rain was a chemical weapon. This evidence, however, remains classified.

When asked what that smoking gun might be, Mirocha says that to him, yellow rain has already been proven. The exact combination of toxin, or toxins, used in an aerial spray remains unclear, according to Mirocha. Not all of the symptoms the Hmong experienced can be explained. But, he says, "Indeed Hmong people were killed by some aerial spray."

"What proof would be enough?" he asks. "Well to me, the personal, first-hand information from the Hmong people is enough."

Through a spokesperson, Radiolab and WNYC declined to comment on this story. Dean Cappello, WNYC's chief creative officer, has written that Walters, the producer, spent "months reviewing nearly 20 years' worth of academic papers and media reports on Yellow Rain," and "completed an in-depth examination of competing theories to the 'bee feces' hypothesis." He says that, based on this research, the show "strongly believes...that the accumulation of evidence would not have served the story or Mr. Yang's version of events."

Thomas Seeley, the honeybee expert, is sympathetic to the show's aims. "You have to look at the body of evidence, not individual opinion," he says.

But Paul Hillmer, a history professor at Concordia University, is one of the people not buying it. In his view, even though Radiolab's segment was only 25 minutes long, it should have made more clear that what Cappello calls the "bee feces hypothesis" was just one theory, not fact.

"People for the most part are outraged that Robert Krulwich was rude, right?" Hillmer says. "When the real sin is that the bee dung theory is not open-and-shut, and they didn't tell their audience that."

Kao Kalia Yang isn't just upset because the show aired her breakdown, or even because it dismissed parts of her uncle's story and the Hmong experience. She's upset about the facts of the matter.

"They made it seem as though they were telling the truth, and we just couldn't handle it," Yang says. "But none of what they were telling us was new information. Really, it was that I knew what they were presenting as truth was not the full truth."

Yang received an email from Cappello two weeks after the podcast. Although Krulwich made a public apology, Yang has not heard from him directly. She says her emails to Krulwich and Walters, and her reply to Cappello, have gone unanswered.

"To be honest, I feel exploited," she says. "To be used as a pawn in a political debate — that's what happened in the war, and it happened again on the show." 

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