The type of processing done before an agent is used as a weapon can alter how hazardous it is to humans and its persistence in the environment. This processing might be termed weaponization if it increases the ability of the agent to cause harm by making the agent more stable, more infectious, or better able to penetrate the human body. For agents that cause harm via inhalation, the size of the particles is crucial. Particle size also affects the ability of the agent to be aerosolized or reaerosolized.

Knowing the particle size of the pathogenic agent is critical in determining its potential for dispersal, reaerosolization, and infectivity—especially if the agent is released and spread as an aerosol. The particle size distribution depends on the agent (e.g., spore, vegetative cell, viron), the degree of weaponization sophistication (e.g., electrically neutralized, finely milled, encapsulated), and aerosol transport mechanism (e.g., dry cells, wet aerosol). A crudely weaponized agent is likely to have a large particle size distribution that varies from single particles of 0.2-2 µm to clumps of many particles or liquid droplets as large as 30 µm. Variola major virons can have complex shapes from 0.2-0.4 µm, Y. pestis cells are rod shaped and range from 0.5 × 1 µm to 1 × 2 µm, B. anthacis vegetative cells are rod shaped from 0.25 × 1 µm, and B. anthracis spores are spherical and 1-1.5 µm. There are many routes for hazardous insult by the threat agent ranging from contact with eyes or broken skin to inhalation into the respiratory tract. Infection is promoted by the growth of threat agent cells in local macrophages or by the proliferation of cells into the bloodstream. Most morbid infections stem from inhalation of aerosols though the nose or mouth. Large particle clumps or droplets (10-20 µm) can lodge in the mucosa of the nasal cavity or the pharynx, causing infection by local macrophages or gastrointestinal infection by ingestion. Particle clumps or droplets in the range 5-15 µm can lodge in the trachea. The most dangerous infections are caused by 0.1-10 µm particles lodged in the lungs, where they may be retained in the upper bronchiole region (5-10 µm particles) or in the lower alveolar region (0.1-5 µm particles).

The dynamics of particle size retention depend on the flow rate, mass impaction, diffusion, and gravitational settling which are, in turn, related to the activity of the person, tidal volume, and oral versus nasal inhalation. Several modeling efforts have helped to explain those dynamics. Calculations by Yu and Diu (1983) for spherical uncharged particles in the lung showed good agreement with experimental data. Yeh and Schum (1980) performed detailed in vitro measurements on lung molds created from human cadavers to validate deposition equations, again with spheres. Harvey and Hamby (2002) presented a model for deposition differences by age and sex. Generally the experiments show a retention rate of about 20-30% for 0.1-0.2 µm diameter spheres, which drops to about 10% for spheres in the 0.3-0.5 µm range and then rises to 90% or more at diameters 6 µm and greater. All of the models and data clearly reflect the partial clearance (exhala-