The importance of particle size in such aerosols has been thoroughly demonstrated. The natural anatomical and physiological defensive features of the upper respiratory tract, such as the turbinates of the nose and the cilia of the trachea and larger bronchi, are capable of impinging out the larger particles to which we are ordinarily exposed in our daily existence. Very small particles, however, in a size range of 1 to 4 microns in diameter are capable of passing these impinging barriers and entering the alveolar bed of the lungs. This area is highly susceptible to infection. The entrance and retention of infectious particles in the alveoli amounts almost to an intra tissue inoculation. The relationship between particle size and infectious dose is illustrated in Table 1.

In considering BW defense, it must be recognized that a number of critical meteorological parameters must be met for an aerosol to exhibit optimum effect. For example, bright sunlight is rapidly destructive for living microorganisms suspended in air. There are optimal humidity requirements for various agents when airborne. Neutral or inversion meteorological conditions are necessary for a cloud to travel along the surface. It will rise during lapse conditions. There are, of course, certain times during the 24 -- hour daily cycle when most of these conditions will be met.

Certain other properties of small particles, in addition to those already mentioned in connection with penetration of the respiratory tract, are noteworthy in defense considerations. The smaller the particle the further it will travel downwind before settling out. An aerosol of such small particles. moreover, diffuses through structures in much the same manner as a gas. There may be a number of secondary effects resulting from diffusion through buildings such as widespread contamination of kitchens, restaurants, food stores, hospitals, etc.. Depending on the organism, there may be multiplication in some food or beverage products, i.e., in milk for example. The secondary consequences from this could be very serious and must be taken into consideration in planning for defense.