“Yellow Rain” Hearing Summary

  • Last updated on November 10, 2022

The U.S. government alleged that the Laotian and Vietnamese military, under Soviet supervision, used fungal toxins against Hmong tribespeople, but the charges were later disproven. In the long term, the yellow-rain affair damaged the credibility of the United States in the worldwide scientific community.

Summary of Event

The “yellow rain” hearing before the Subcommittee on Arms Control, Oceans, International Operations, and Environment of the Committee on Foreign Relations took place on November 10, 1981. Tales of atrocities abound in all wartime situations, but the U.S. government had begun checking for Soviet use of chemical warfare in Southeast Asia in 1979. The term “yellow rain” had first appeared in publication in the Baltimore Sun in an interview of a Hmong soldier. The soldier reported seeing a poisonous yellow spray delivered by an aircraft over Laos. Chemical weapons;accusations Weapons;chemical [kw]"Yellow Rain" Hearing (Nov. 10, 1981) [kw]Hearing, “Yellow Rain” (Nov. 10, 1981) Yellow rain Chemical weapons;accusations Weapons;chemical [g]North America;Nov. 10, 1981: “Yellow Rain” Hearing[04690] [g]Southeast Asia;Nov. 10, 1981: “Yellow Rain” Hearing[04690] [g]United States;Nov. 10, 1981: “Yellow Rain” Hearing[04690] [g]Laos;Nov. 10, 1981: “Yellow Rain” Hearing[04690] [g]Cambodia;Nov. 10, 1981: “Yellow Rain” Hearing[04690] [c]Terrorism, atrocities, and war crimes;Nov. 10, 1981: “Yellow Rain” Hearing[04690] [c]Diplomacy and international relations;Nov. 10, 1981: “Yellow Rain” Hearing[04690] Evans, Grant Haig, Alexander M. Shultz, George P.

In March, 1982, the U.S. secretary of state, Alexander M. Haig, reported to Congress and charged that air attacks had been used in the embattled area since the fall of 1978 to spray a yellowish substance on villages and crops. By November, 1982, the new secretary of state, George P. Shultz, repeated the charges before Congress, detailing that the yellow rain contained trichothecene mycotoxins and other poisonous substances. The wide range of severe symptoms attributed to this chemical or biological agent included vomiting, skin lesions, bleeding, and blistering.

Four types of evidence were provided to support these serious charges: a large number of interviews with alleged witnesses to yellow rain, a very limited number of samples of leaves, stems, and rocks with the spotty yellow material, lab reports of trichothecene toxins in the samples of yellow rain, and secret intelligence reports associating reported yellow-rain flyovers with classified surveillance. Intelligence reports remained unavailable to scientists or the public for independent evaluation, and the interview testimony was subject to eyewitness interpretation. Items of physical evidence, however, were subject to scientific analysis, and the respected journal Science reported the U.S. case alleging mycotoxin weapon use as “persuasive” by July, 1982. Beginning with a series of editorials in the The Wall Street Journal in November, 1981, the news media continued to advocate and popularize the unfolding yellow-rain scenario. By mid-1982, an intelligent reader would assume the case was proven and would have little basis to question or dismiss the charges that had been brought forth with considerable political authority.

The eagerness with which the allegations about yellow rain were accepted by the American public can be understood by an examination of U.S. history in Southeast Asia. The United States had lost the war with Vietnam, and Vietnam had Soviet backing. In late 1978, Vietnam had invaded Kampuchea (known as Cambodia in the U.S. press). China, which had firmed diplomatic ties with the United States, invaded Vietnam in February, 1979. With Ronald Reagan’s landslide victory in 1980, the views of the conservative Right were gaining more legitimacy. The Cold War with the Soviet Union had been intensified, and the Soviet intervention in Afghanistan and continued Soviet support of Vietnam was seen as proof of Soviet expansionist plans.

The apparent victims of the yellow rain were the Hmong tribesmen who had formerly been U.S. allies in battles against the Vietnamese and communist Laotian forces. In Laos in 1968, the U.S. Central Intelligence Agency Central Intelligence Agency (CIA) had backed the secret army of Major General Vang Pao Vang Pao to fight the communist Pathet Lao. About twenty thousand of Pao’s troops were Hmong tribesmen from the highlands of Laos, and Vang Pao relied heavily on CIA support to supply his mountain bases. With the formation of a coalition government in 1974, the United States pulled out of Laos; a year later, Vang Pao fled to Thailand and abandoned the remnants of his secret army. Displaced during hard economic times, some Hmong refugees succeeded in crossing into Thailand to reach refugee camps, but most Hmong remained in Laos. Some continued the resistance to the new government. In this climate, it was reasonable to portray yellow rain as a vicious retaliation against the Hmong.

The charges about yellow rain, however, began to unravel soon after they were released in 1982. On May 20, the Soviets sent a nineteen-page reply titled “Chemical and Bacteriological (Biological) Weapons” to the secretary-general of the United Nations. The Soviets noted that the spotty occurrence of mycotoxins indicated natural fungal growth in contrast to the widespread dispersal that would occur from an effective aerosol weapon. They also pointed out the very low toxicity of mycotoxins and the fact that the Hmong symptoms were not specific to mycotoxin poisoning but were related to a wide array of naturally occurring ailments, including tuberculosis and malaria. The last pages of the Soviet reply digressed to promote an alternative explanation of mycotoxin poisoning based on another fungus in the genus Fusarium. The Soviets contended that Fusarium proliferated on the elephant grass that took over following the massive defoliation from earlier U.S. spraying with herbicides and the elimination of native microorganisms from the use of napalm. This speculation was as weak as the U.S. evidence for yellow rain and diverted many Western observers away from an otherwise correct assessment of the issue.

Meanwhile, sociologist and journalist Grant Evans entered Laos and reinterviewed Hmong tribespeople. He found an array of weaknesses in the testimonies that allegedly supported accounts of yellow rain. In spite of the fact that refugees were dispersed among many camps, eyewitnesses came from a few select camps where the social systems promoted the stories about yellow rain and gassing. The original interview protocol had been designed to gather details on a chemical warfare attack and assumed it had occurred. No attempt had been made to reinterview witnesses, nor were efforts made to cross-check stories among people who had been at the same attack site to verify if it had indeed occurred.

Victims varied widely in their reports of how the yellow rain was delivered from jets to propeller planes to helicopters and from sacks to rockets to shells yet no fragments of shells or contaminated munitions were ever produced. Moreover, Hmong concepts related to both gases and the causes of disease varied from Western concepts. Thus individuals would associate a plane flyover with events that occurred hours afterward or would describe a skin condition as being the result of a gas attack and would later ask for scabies (mite) medication. The Hmong concept of time was based on seasons with little recognition of months, weeks, days, and hours. This brought into question the U.S. assertion that timing of the attacks could be independently confirmed by classified intelligence surveillance systems.

In January, 1982, by simple microscopic observation, a British scientist discovered that a sample of yellow rain was nearly 100 percent pollen. This was soon confirmed by independent researchers in Canada and Thailand. U.S. Army specialists discounted the discovery, claiming that the sample was a clever mixture that would allow the dried dust to be inhaled or was an agent that would retain the toxin in the victim’s body, a contention that even Western doctors found unlikely.

Yale scientist Thomas Seeley Seeley, Thomas suspected that the yellow rain was a natural phenomenon the feces of wild bees and presented this theory at the May, 1983, meeting of the American Association for the Advancement of Science. Bees;yellow rain Honeybees;yellow rain While U.S. government officials scoffed at the idea, pollen experts in France, Great Britain, and the Smithsonian Institution in Washington, D.C., were narrowing the possibilities of pollen to plants native to Southeast Asia. The scientific evidence was converging to indicate that this substance was exactly what would appear as excrement of wild honeybees in Laotian and Kampuchean forests. By March, 1984, American scientists and Thai workers witnessed wild bees leaving the nest and, at an altitude high enough for the bee swarm to be invisible without the use of binoculars, the bees produced a shower of yellow rain. The researchers not only collected samples the density of yellow spots was 209 per square meter on the surface of their parked Land Rover but also were themselves caught in a fecal shower.

Meanwhile, international laboratories were failing to find positive results from tests for trichothecene, the fungal toxin that was originally proposed to be the active biological warfare agent. Using a rigorous gas chromatography and mass spectrometry technique, more than one hundred samples tested negative at British, French, and Swedish laboratories. The earlier five positive tests on which the U.S. charges were based were viewed as false positives attributable to less rigorous precautions.

Finally, U.S. reasoning was based, in part, on negative evidence. For example, certain fungi and mycotoxins had not been collected from Southeast Asia. Their presence in Southeast Asia was thought to be a result of being transported in from elsewhere. With time and research, it became clear that low levels of natural mycotoxins did occur there naturally, and such negative evidence had never been evidence at all. This demonstrated that the level of understanding of scientific principles was limited among administrators as well as the general public.

With the meager physical evidence for yellow rain clearly understood in the scientific press as a phenomenon of nature, and as a result of the weakness of the Hmong testimony exposed by Evans and others, the credibility of the U.S. charges faded in the scientific community by 1985 and in the international political community by 1987.


The allegations regarding yellow rain held legitimacy in the public eye from 1982 to 1984, although the testimony of eyewitnesses was confusing and inconsistent and the scant physical evidence suggested a very mild toxin that had little or no destructive capability. Despite the insubstantial nature of the U.S. allegations against the Soviet Union, there is no evidence that the charges were deliberate U.S. propaganda. The imprecise manner in which the early testimonials were taken and the lab evidence was interpreted suggest, rather, an overzealous attempt to portray the Soviet Union as a violator of international convention. During this time, a general U.S. public perception of the Soviet Union as an outlaw nation that violated international conventions, in the view of many, provided support for the U.S. decision to manufacture binary nerve gas weapons. Yellow rain

In the long term, the yellow-rain affair damaged the credibility of the United States in the worldwide scientific community. By the end of the 1980’s, the U.S. charges were generally viewed as premature and irresponsible. Although the charge concerning yellow rain was not initially politically motivated, the U.S. government’s tenacity in defending its allegations, even as evidence to the contrary rapidly mounted, caused concern among scientists, who developed a new appreciation for the extent to which science can be misused by politics. Yellow rain Chemical weapons;accusations Weapons;chemical

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Evans, Grant. The Yellow Rainmakers: Are Chemical Weapons Being Used in Southeast Asia? London: Thetford Press, 1983. After two months of research in Southeast Asia, an Australian sociologist clarifies the weakness of the U.S. allegations and the lack of conclusive physical evidence. Evans’s thorough reanalysis of Hmong refugee testimony, in a cultural and historical context, explained how the mythology of gas attacks had been so widespread and so easily accepted by gullible officials.
  • citation-type="booksimple"

    xlink:type="simple">Guillemin, Jeanne. Biological Weapons: From the Invention of State-Sponsored Programs to Contemporary Bioterrorism. New York: Columbia University Press, 2004. Chronicles the proliferation of and policies regarding biological weapons by nation-states from the early twentieth century to the early twenty-first century. Chapter 7, “The Soviet Biological Weapons Program,” addresses the U.S. allegation that the Soviets had conspired to use yellow rain in Southeast Asia.
  • citation-type="booksimple"

    xlink:type="simple">Katz, Rebecca Lynn. Yellow Rain Revisited: Lessons Learned for the Investigation of Chemical and Biological Weapons Allegations (Vietnam, Cambodia, Laos, Afghanistan, Russia). Ann Arbor, Mich.: Proquest, 2005. Dissertation uses yellow rain as a case study for examining the investigation of biological and chemical weapons allegations. Argues that there is sufficient evidence to conclude that yellow rain was used against the Hmong in Laos, the Khmer Rouge in Cambodia, and the mujahideen in Afghanistan. Draws from recently released U.S. government documents and interviews with people involved in previous investigations.
  • citation-type="booksimple"

    xlink:type="simple">Robinson, Julian, Jeanne Guillemin, and Matthew Meselson. “Yellow Rain: The Story Collapses.” Foreign Policy (Fall, 1987): 100-117. Summarizes the history of the charges, the research that failed to confirm substantial levels of toxins, the confirmation of the rain droplets as bee excrement, and the fallibility of the Hmong testimony. Few articles on the topic appeared after this.
  • citation-type="booksimple"

    xlink:type="simple">Seagrave, Sterling. Yellow Rain: A Journey Through the Terror of Chemical Warfare. New York: M. Evans, 1981. The earliest book on the subject of yellow rain, this noncritical narrative sensationalizes early testimony. An example of the extent to which weak testimony can be misconstrued as hard evidence. Places the chemical and biological warfare and related conventions in historical perspective.
  • citation-type="booksimple"

    xlink:type="simple">Seeley, Thomas D., Joan W. Nowicke, Matthew Meselson, Jeanne Guillemin, and Pongthep Akratanakul. “Yellow Rain.” Scientific American 253 (September, 1985): 128-137. This clear account of the nature of the yellow substance found on rocks and leaves in Southeast Asia and the documentation of actual showers of bee feces decisively ended speculation of the nature of the physical evidence for yellow rain.
  • citation-type="booksimple"

    xlink:type="simple">Spiers, Edward M. Chemical Warfare. Urbana: University of Illinois Press, 1986. An excellent overview of the topic. Numerous footnotes, good bibliography.

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