by attachment to a non-biological particle resulting in a higher potential to spread disease. The

other concern is that virions and even viral aggregates are in the size range of minimum filtration

efficiency in air (usually 0.05 0.5 [tm) (Hinds 1999b). Although the nominal MPPS (most

penetrating particle size) designated by the NIOSH filter certification protocol is 0.3 |tm, the

reported MPPS of viral aerosols through N95 and N99 facepiece respirators was < 0.1 |tm

(Eninger et al. 2008b). Small particle size along with low infectious dose, high penetration

through filter media, possible shielding effect of aggregation, and encasement effect of foreign

substances are all challenges that anti-bioterrorism and public health workers need to overcome.

 Stability of Viral Aerosols

 In addition to the above properties, a key factor in determining the spread of disease by

viral aerosols is their ability to survive and maintain infectivity, i.e., the stability of viral aerosols

(Cox 1995). The stability is influenced by compounds in the spray medium and environmental

conditions such as temperature, relative humidity (RH), oxygen, and pollutant (Ehrlich et al.

1964; Songer 1967; Benbough 1971; Trouwborst and de Jong 1973; Schaffer et al. 1976; Ijaz et

al. 1985; Hermann et al. 2007). Benbough (1969) investigated the effect of various compounds

including NaC1, KC1, glucose, inositol, raffinose, glycerol, and bovine serum albumin on the

survival of Semliki Forest virus (a group A arbovirus) and observed no affect of these

compounds except NaC1. The removal of NaCl from the spray suspension led to better survival

of viruses especially at high RH (HRH). In a following study (Benbough 1971), he observed that

polyhydroxy-compounds protected arbovirus aerosols from virucidal effects of NaC1. The

protective effect of polyols was also reported in another study of influenza A viral aerosols

(Schaffer et al. 1976). Similar studies were conducted for human rotavirus by using tryptose

phosphate broth and fecal matter; stability of viral aerosols at mid-range RH was observed (Ijaz