Investigation
C. J. Mirocha at the University of Minnesota conducted a biochemical investigation, looking for the presence of trichothecene mycotoxins, including T-2 toxin, diacetoxyscirpenol (DAS), and deoxynivalenol (DON) (10). This included chemical analyses of blood, urine, and tissue of alleged victims of chemical attacks in February 1982 in Laos and Kampuchea. "The finding of T-2, HT-2, and DAS toxins in blood, urine, and body tissues of alleged victims of chemical warfare in Southeast Asia provides compelling proof of the use of trichothecenes as nonconventional warfare agents....Additional significant findings lie in the trichothecenes found in the leaf samples (T-2, DON, nivalenol) and yellow powder (T-2, DAS)....The most compelling evidence is the presence of T-2 and DAS in the yellow powder. Both toxins are infrequently found in nature and rarely occur together. In our experience, copious producers of T-2 toxin (F. tricinctum) do not produce DAS, and conversely, good producers of DAS (F. roseum 'Gibbosum') do not produce T-2." (10) In 1983, these charges were disputed by Harvard biologist and biological weapons opponent Matthew Meselson and his team, who traveled to Laos and conducted a separate investigation. Meselson's team noted that trichothecene mycotoxins occur naturally in the region and questioned the witness testimony. He suggested an alternate hypothesis that the yellow rain was the harmless fecal matter of honeybees. The Meselson team offered the following as evidence: separate "yellow rain drops" which occurred on the same leaf, and which were "accepted as authentic", consisted largely of pollen; each drop contained a different mix of pollen grains, as one would expect if they came from different bees, and the grains showed properties characteristic of pollen digested by bees (the protein inside the pollen grain was gone, while the outer indigestible shell remained). Further, the pollen mix came from plant species typical of the area where a drop was collected.
The US government responded to these findings by arguing that the pollen was added deliberately, in order to make a substance that could be easily inhaled and "ensure the retention of toxins in the human body". Meselson responded to this idea by stating that it was rather far-fetched to imagine that somebody would produce a chemical weapon by "gathering pollen predigested by honeybees." Meselson's work was described in an independent medical review as providing "compelling evidence that yellow rain might have a benign natural explanation".
Analyses of putative "yellow rain" samples by the British, French and Swedish governments confirmed the presence of pollen and failed to find any trace of mycotoxins. Toxicology studies questioned the reliability of reports stating that mycotoxins had been detected in alleged victims up to two months after exposure, since these compounds are unstable in the body and are cleared from the blood in just a few hours. Surveys also showed that both mycotoxin-producing fungi and mycotoxin contamination were common in Southeast Asia, casting doubt on the assertion that detecting these compounds was an unusual occurrence. For example, a Canadian military laboratory found mycotoxins in the blood of five people from the area who had never been exposed to yellow rain, out of 270 tested, but none in the blood of ten alleged victims, and a 1988 paper reported that illnesses from mycotoxin exposure may pose a serious threat to public health in Malaysia. It is now recognized that mycotoxin contamination of foods such as wheat and maize is a common problem, particularly in temperate regions of the world. As noted in a 2003 medical review, "The government research highlighted, if nothing else, that natural mycotoxicoses were an important health hazard in Southeast Asia."
in 1987 the New York Times reported that later freedom of information requests showed that field investigations in 1983-85 by US government teams had produced no evidence to substantiate the initial allegations and instead cast doubt on the reliability of the initial reports, however The Times noted that these critical reports were not released to the public. A 1989 analysis of the initial reports gathered from Hmong refugees that was published in the Journal of the American Medical Association noted "marked inconsistencies that greatly compromised the validity of the testimony" and criticized the methods used in interviews by the US Army medical team that gathered this information. These issues included the US Army team only interviewing those people who claimed to have knowledge of attacks with chemical weapons and the investigators asking leading questions during interviews. The authors noted that individuals' stories changed over time, were inconsistent with other accounts, and that the people who claimed to have been eyewitnesses when first interviewed later stated that they had been relaying the accounts of others.
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