Introduction

The anthrax attacks of 2001 have increased concerns that "weapons grade" biological agents can be obtained or manufactured and disseminated by terrorists. In order to assist in planning for future attacks, bioterrorist attack scenarios have been envisioned that involve the use of aerosol-delivery technologies to target large civilian populations by air, inside buildings, or in mass transit systems. However, there is relatively little unclassified data on which biodefense planners can base their understanding of the potential consequences of a large-scale bioterrorist attack. A 1970 World Health Organization (WHO) study[1] estimated that 50 kg of Bacillus anthracis released over an urban population of 5 million would sicken 250,000 and kill 100,000 people, and a 1993 Office of Technology Assessment (OTA) study[2] estimated that between 130,000 and 3 million deaths would follow the release of 100 kg of B. anthracis. However, neither of these analyses employed empirical data.

While aerosol technologies for large-scale dissemination have been developed and tested by the United States during the existence of its biological weapons program, and by the former Soviet Union[3] and Iraq,[4] few details of those tests are available to civilian planners. A declassified report of a 1960s U.S. experiment performed near Johnston Atoll in the South Pacific reported that a plane "sprayed a 32-mile long line of agent that traveled for more then 60 miles before it lost its infectiousness" to nonhuman primates.[5] However, there are no known unclassified data on human exposure to large-scale aerosol releases of biological weapons. The only known aerosol release of B. anthracis spores resulting in large numbers of human deaths occurred in 1979, when anthrax spores were accidentally released from a military facility in Sverdlovsk, in the former Soviet Union.[6] Ninety-six people were reportedly sickened and 68 people died, although the death toll may have been as high as 105.[3]

Some analysts have questioned whether the B. anthracis spores such as that used in the 2001 attacks could be produced and deployed effectively by terrorist groups without the support of a nation-state. Some have asserted that to be used effectively as a biological weapon, B. anthracis would have to be in a dry powdered form, highly concentrated, of uniform particle size, low electrostatic charge, and treated to reduce clumping in order for the bacteria to penetrate the spaces of the deep lung. Some analysts have also argued that foggers and crop dusters would not be effective ways to disseminate B. anthracis, because the use of liquid formulations would require a high level of purity to prevent plugging of nozzles and would create "globs" which would harmlessly fall to the ground rather than staying suspended in the air. Finally, it has been argued by others that the use of an aircraft to attack a large city with B. anthracis would be ineffective because most of an urban population is inside buildings at any given time, offering some level of protection against breathing air contaminated by spores.

In the absence of empirical data, it is difficult to confirm or refute these assertions. Experiments designed to examine human exposure to biological weapons in civilian populations obviously cannot be conducted. Yet valuable lessons can be taken from the world of biological control of defoliating insect pests. Over the past two decades, bacteria, fungi, and viruses have been proven to be useful and effective tools for such purposes.[7,8,9,10] Biological agents are used routinely in forestry and agriculture, and they are often used near densely populated urban areas. In this article, we summarize the empirical data from the human exposure that followed the outdoor spraying of the biological control agent Bacillus thuringiensis, a bacteria that is closely related to B. anthracis. Detailed results of this scientific assessment have been published in an earlier paper; however, the implications of this data for biodefense are the focus of this article.[11,12] The data from this report and from the larger body of information related to the biological control of insects suggest that a liquid biological agent can indeed be disseminated on a large scale using crop-dusting airplanes. Biodefense planners should take this into account as they seek to prevent the use of biological weapons and to plan to reduce the consequences of such potential attacks.