such as by heat, radiation and chemical agents. Specific properties of spores are responsible for

their resistance, including low water content in the core and saturation of the spore

deoxyribonucleic acid (DNA) with a group of small, acid-soluble spore proteins (SASP) of the

a/P-type (Popham et al. 1995; Tennen et al. 2000). Thus, bacterial spores have been classified as

a group of bioagents for which treatment and disinfection are specially challenging.

 Viruses are the smallest biological agents; a single naked virus ranges from 20 300 nm.

However, in the natural environment, they are not typically present as a single naked virus due to

aggregation of several single viruses or attachment to non-biological particles (e.g., dust) in the

air that result in several unique properties of their own (Hinds 1999a). Because of the

surrounding outer layer of viruses, the inner viruses of an aggregate can be protected from

inactivation treatments. This "shielding effect" of viral aggregates from inactivation treatment

was already observed in water (Galasso and Sharp 1965). When viruses are attached or enclosed

to the surrounding substances in the air, their infectivity can also be preserved by reduced contact

with inactivation agents, i.e., "an encasement effect". Second, the viral aggregates can be

present in the ultrafine size range. Ultrafine particles are of great concern because respiratory

deposition of a significant number of these particles is easily achieved. Behaviors in contrast to

larger sized particles include translocation to extrapulmonary sites and migration to other target

organs by a different transfer route (Oberdorster et al. 2005). Inhalation of aggregated ultrafine

biological agents can result in fatal consequences, because only minute amounts of viruses are

needed to cause disease (Hinds 1999a). For example, the infectious dose of smallpox is 10-100

viruses and that of viral hemorrhagic fever is 1-10 viruses (Pien et al. 2006). The low dose is

estimated and true the infectious dose of many virus agents is unknown. Furthermore, a small

viral particle can be suspended in air for a long time and travel considerable distances by itself or