Yellow rain is an airborne substance that clings to exposed outdoor surfaces. Human exposure can result in a variety of debilitating symptoms and sometimes death. First noticed in Southeast Asia during the 1970s, it initially was thought to be caused by a chemical or biological weapon. The preponderance of evidence now suggests that it was honeybee feces.
In 1975, Hmong tribesmen came under attack by the military forces of Laos and Vietnam. Refugee tribesmen reported that an oily, yellow aerosol was being dispersed from low-flying aircraft. Those exposed claimed life-threatening physical and neurological symptoms. This unknown substance became commonly referred to as yellow rain. Reports of similar incidents followed the Vietnamese invasion of Cambodia in 1978.
In 1981, United States Secretary of State Alexander Haig accused the Soviet Union of supplying client states with chemical weapons and biological agents, including T-2 mycotoxins, for use in counterinsurgency warfare. These accusations were consistent with a U.S. Army Medical Department report attributing thousands of deaths in Vietnam, Cambodia and Afghanistan to toxins delivered by multiple systems. The Soviet Union denied the accusations.
A 1982 investigation by C. J. Mirocha of the University of Minnesota into alleged chemical attacks in Southeast Asia claimed to have found T-2 and other mycotoxins in the blood, urine and tissue samples of victims. Dr. Mirocha also asserted that the toxins in question are found infrequently in nature and rarely ever together, because different processes produce them. This was seen as compelling evidence that the toxins were being artificially produced and used in yellow rain as a chemical warfare agent.
Later investigations began to cast doubt on the proposition that yellow rain was a chemical weapon. Follow-up investigations by U.S. government agencies produced no evidence to verify earlier claims. It was questioned how the Mirocha study could find evidence of toxins in samples taken weeks after exposure when these toxins are eliminated from the human body within few hours. This would seem to indicate a naturally occurring source for the contamination.
A 1983 investigation by biologist Matthew Meselson found that evidence considered to be authentic consisted largely of pollen previously digested by bees. This confirmed a prior analysis by the Australian Ministry of Defense. Traces of each significant toxin were detected, though at levels far below that necessary to suggest use as a weapon. Further research showed that fungi feeding on the bee droppings produced the toxins naturally.
Studies by Canadian and Malaysian biologists demonstrated that Asian honeybees undertake mass defecation flights in order to lower their body temperature. This is done to lower the colony temperature and protect developing larvae. It also provides a natural delivery mechanism for the widespread deposit of predigested pollen. Yellow rain, it seems, is no more than Asian honeybee feces.
The same conclusion was reported in Chinese scientific journals as early as 1977. Investigation into a yellow rain incident in Jiangsu province in September 1976 found that the contamination consisted mainly of pollen. Chinese scientists concluded at the time that it was bee excrement.
The fungi that produce the toxins in question infest not only yellow rain but also the region's food supply. Adverse reactions seem to be a naturally occurring consequence of this fact. The potential for these toxins to be used as a weapon exists, however. So long as that potential exists, one cannot state with absolute certainty that they have never been used as a weapon.