range compared to the PSD generated from tryptone solution due to the increased volume

fraction of solid materials in the nebulizer suspension. The PSD of infectious viruses followed a

lower order dependence on dimension, between number and area distributions, as shown in

Table 4-3. The results at other RHs show a similar pattern and are presented in Appendix D.

 Figure 4-7 shows NPFU as a function of particle size at three RHs. Compared to the MS2

aerosols generated from tryptone solution and sterile DI water, there was less increment as

particle size increased at three RHs. There are two possible reasons for this phenomenon: (1)

negligible shielding effect of bigger particles due to insufficient amount of MS2 virus to be

aggregated, and (2) adverse effect of saliva components on viral aerosols. In terms of the

amount of MS2 viruses, the NRNA values for MS2 generated from artificial saliva are similar to

those for tryptone solution, as shown in Tables 4-7 and 4-9. Since the latter presented a shielding

effect, MS2 aerosols generated from artificial saliva should have sufficient NRNA to present a

shielding effect of aggregates. The fact that NPFU is low implies that MS2 virions in aerosols do

not aggregate well to achieve a shielding effect. To address this issue, one should recall that the

artificial saliva used in this study is a mucin-containing medium. Mucin has an oligosaccharide

chain containing numerous hydrophobic regions, which are responsible for its sticky property

(Mehrotra et al. 1998; Zalewska et al. 2000). In a later study (Habte et al. 2006), it was observed

that mucin aggregates HIV-1 (human immunodeficiency virus type 1) leading to an enhanced

filtration through 0.45-[m pore size cellulose acetate filters. Therefore, it can be inferred that

mucin induces hydrophobic interaction with MS2 protein, thus reducing MS2 aggregation by

itself. The lack of shielding effect of aggregates is verified by the lower slope value shown in

Table 4-3.