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Conflicting Evidence Revives "Yellow Rain" Controversy

Jonathan B. Tucker
Aug 01, 2002

Two conflicting pieces of evidence--a declassified CIA report and a yellow-green shower in India--may revive the 20-year-old debate over Yellow Rain, which remains a lingering mystery of the Cold War. On September 13, 1981, then-U.S. Secretary of State Alexander Haig accused the Soviet Union of supplying trichothecene mycotoxins (poisonous compounds made by fungal molds that infect grain), popularly known as Yellow Rain, to the Communist regimes in Vietnam and Laos for use in counterinsurgency warfare. Leading American scientists challenged the U.S. government's evidence for these allegations, however, and the controversy was never fully resolved. [See Jonathan B. Tucker, "The 'Yellow Rain' Controversy: Lessons for Arms Control Compliance," (PDF format) Nonproliferation Review, Spring 2001.]


After the Communist victories in Vietnam and Laos in 1975, the two governments launched a "pacification" campaign against the Hmong tribesmen in northern Laos who had fought on the U.S. side during the Vietnam War and still resisted Communist rule. During the summer of 1975, reports began trickling out of Laos that government forces were using Soviet-supplied chemical weapons to terrorize the Hmong and drive them from their mountain sanctuaries. Refugees described a variety of toxic agents delivered by low-flying aircraft; about 70 percent of the reports involved an oily yellow liquid that made a sound like rain when it struck leaves or roofs, causing the Hmong to call it "Yellow Rain."

Eyewitnesses claimed that Yellow Rain smelled like gunpowder and left a residue of sticky yellow spots on leaves, rocks, and rooftops. Exposure to high doses reportedly caused heavy bleeding from the nose and gums, blindness, tremors, seizures, other neurological symptoms, and death. Similar incidents were reported in Democratic Kampuchea (Cambodia) in 1978, after the Vietnamese Army invaded that country to topple the dictator Pol Pot and his murderous Khmer Rouge regime.

Initially, U.S. chemical warfare (CW) experts were mystified by the alleged attacks because the symptoms described by refugees did not match the effects of any known CW agents. In July 1981, however, U.S. Army toxicologist Dr. Sharon Watson noted a striking similarity between the reported symptoms and those resulting from exposure to fungal poisons called trichothecene mycotoxins. When samples of Yellow Rain from an alleged attack site in Laos were analyzed by a laboratory at the University of Minnesota, three different trichothecenes were detected in concentrations and mixtures not known to occur in nature. Another piece of evidence was the fact that consumption of trichothecene-contaminated grain had long been a serious public health problem in the Soviet Union, leading to intensive research on mycotoxin poisoning. The U.S. intelligence community speculated that this research had caused the Soviets to recognize the military potential of trichothecenes and develop them as a weapon.

Based on these preliminary findings, Secretary Haig made his dramatic allegation in September 1981. "For some time now," he said, "the international community has been alarmed by continuing reports that the Soviet Union and its allies have been using lethal chemical weapons in Laos, Kampuchea, and Afghanistan. . . . We have now found 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."

Subsequently, a group of academic scientists led by Harvard molecular biologist Matthew Meselson questioned the validity of the evidence on which the U.S. government had based its allegations of toxin warfare. The skeptics argued that trichothecene mycotoxins occur naturally in Southeast Asia and that the alleged victims had confused chemical attacks with harmless showers of yellow feces released by swarms of giant Asian honeybees. The scientific critics also raised doubts about the reliability of the refugee testimony and the laboratory analyses. Although the general public assumed the Yellow Rain allegations had been laid to rest, the U.S. government never retracted them. More than 20 years later, the controversy has yet to be fully resolved. One reason is that much information supporting the U.S. government's case has not been publicly released because of the need to protect sensitive intelligence sources and methods.

Now, two conflicting pieces of evidence may revive the Yellow Rain controversy. On the one hand, a declassified CIA intelligence report provides some support for the U.S. government's allegations; on the other hand, the report of a recent "yellow rain" attack in India lends credence to the scientific critics.

Positive Evidence: Declassified Intelligence Report

A declassified CIA intelligence document, written in 1983, suggests that the Soviet Union developed weapons based on trichothecene mycotoxins as early as 1941 and may have tested them on political prisoners. Titled "Annex B: Soviet Development of Toxins," this document is part of a Special National Intelligence Estimate (SNIE) on "Implications of Soviet Use of Chemical and Toxin Weapons for US Security Interests." The SNIE, classified SECRET/NOFORN/NOCONTRACT/ORCON [no distribution to foreigners or contractors, distribution controlled by originating agency], was declassified by the CIA's Historical Review Program and is available at the U.S. National Archives in College Park, Maryland. Although the CIA report does not provide direct evidence for the use of trichothecene mycotoxins in Southeast Asia, it contains new details about the Soviet effort to develop these compounds into weapons. Note that each paragraph of the report is classified separately using the following abbreviations: U = Unclassified, C = Confidential, S = Secret, S NF = Secret/No Foreigners.


1. The use of a variety of lethal chemical agents in Laos, Kampuchea, and Afghanistan has been largely overshadowed by the discovery of a single new agent--trichothecene mycotoxins--a component of "yellow rain." (U)

2. [. . .] Recent intelligence attention to Soviet toxin research has brought to light some additional information that raises our concern about the threat we face. (S)

3. Until recently, US intelligence on toxic agents of interest to the Warsaw Pact has emphasized those agents known to exist during and shortly after World War II, such as the mustards and nerve agents. Evidence exists, however, that the use of toxins as combat weapons is not a newly developed or experimental Warsaw Pact concept, but that the trichothecenes may have been part of the Soviet arsenal for decades. (C)

4. A 1951 intelligence report written by a captured German chemical warfare expert, Dr. Walter Hirsch, contained detailed information on Soviet chemical R&D programs from 1939 to 1945, resulting from his Soviet POW interrogations. Among the new war gases under development in the Soviet Union during that period was a "powdery, yellow-brown" agent called lebeda. The word lebeda in Russian refers to a millet-like feed extender, an indirect reference, no doubt, to the trichothecene-contaminated millet that caused the devastating disease outbreaks in Orenberg in the Soviet Union during and after World War II. Beginning in 1941 and continuing until Hirsch's capture, the new agent lebeda was mentioned repeatedly by Soviet prisoners of war who had technical training or connections with Soviet CW schools. Hirsch was not able to identify the agent on the basis of its described properties, but noted an array of symptoms that bear striking similarity to those observed in yellow rain victims. Interestingly, the agent was also described as being disseminated in munitions or as an aircraft spray. (S NF)

5. A prominent scientist who left the Soviet Union in 1958 has provided additional insight into Soviet trichothecene R&D. As an investigator of the Orenburg outbreak, he traced the origin of the epidemic to natural contamination of grain sources by toxin-producing Fusaria [fungal molds]. Having identified optimal conditions for toxin production by the fungi, he was ordered to supply large amounts of toxic culture extracts to other Soviet scientists for classified research projects. Subsequent Soviet toxicity studies in humans involved addition of various doses of the toxic material to ground meat which was then fed to political prisoners, and the course of development of toxic effects was monitored. Inhalation experiments were also conducted using monkeys. Techniques for enhancement of toxic effects by combining toxins of different types were also investigated. Extensive debriefings of this source have led us to conclude that his technical bona fides are impeccable and that the striking claims he continues to recall and support are highly credible. (S)

6. The Soviet Union has maintained active research projects in all aspects of natural toxin research on a scale many times more extensive than one would expect solely on the basis of agrotechnological or epidemiological R&D. The research is well supported, involves both military and civilian investigators, and in many cases has been linked with facilities associated with CBW [chemical and biological weapons] research and development. . . .

[1] Annex B, Special National Intelligence Estimate (SNIE) 11-17-83, Implications of Soviet Use of Chemical and Toxin Weapons for US Security Interests, November 17, 1983, U.S. National Archives at College Park, MD.

Negative Evidence: Recent "Yellow Rain" Incident in India

The scientific critics' view of Yellow Rain also received some support recently. In mid-June 2002, a yellow-green rain fell from the sky on the town of Sangrampur, near Calcutta, India. Rumors spread that the rain might be contaminated with toxins or chemical warfare agents. Shortly after the "attack," however, Deepak Chakraborty, chief pollution scientist for the Indian state of West Bengal, reported that the yellow-green droplets were in fact bee feces containing pollen from local mangoes and coconuts. He concluded that the colored rain may have been caused by the migration of a giant swarm of Asian honeybees, which are known to produce "golden showers."[1]

These two conflicting pieces of evidence suggest that the mystery of Yellow Rain is unlikely to be resolved soon. Given growing concerns over the proliferation and use of toxin weapons, however, clarifying this historical controversy--ideally, through the declassification of additional U.S. intelligence documents--is of more than academic interest.

[1] Fred Pearce, "Green rain over India evokes memories of cold war paranoia," New Scientist, vol. 174 (June 22, 2002), p. 13.

Contacts for more information about the "Yellow Rain" Controversy:

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Lisa Nagy M. D.
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