is a non-enveloped, icosahedron-shaped, single-stranded RNA with a single-capsid size of 27.5

nm, and it infects only male Escherichia coli (Prescott et al. 2002). MS2 has been used as a

surrogate for small RNA enteroviruses pathogenic to humans because they both have no lipid

component surrounding the protein coat and are considered to have similar resistance (Aranha-

Creado and Brandwein 1999; Brion and Silverstein 1999). Freeze-dried MS2 was suspended

with filtered deionized (DI) water to a concentration of 108-109 plaque forming units (PFU)/mL

as the virus stock suspension and stored at 4 C.

Experimental System and Conditions

 The experimental set-up for testing the removal efficiency of filters is shown in Figure 3-

1. Seven Lpm (liters per minute) of dry, filtered compressed air was passed though a six-jet

Collison nebulizer (Model CN25, BGI Inc., Waltham, MA, USA) to aerosolize the viral

suspension. The virus concentration in the Collison nebulizer was 105-106 PFU/mL and was

prepared by diluting 0.10 or 0.20 mL of virus stock suspension in 50 mL of sterile DI water. The

aerosolized particles were dried with filtered compressed air in a 2.3-L glass dilution dryer. A

flow rate of 8 Lpm, which corresponds to a face velocity of 14.2 cm/s, was used for each stream

(i.e., control and experimental) and controlled by a calibrated rotameter. Based on the velocity,

flight time through the 1-mm filter is estimated to be 0.007 seconds. This face velocity, used by

Safe Life Corp., corresponds to certification testing of 100 cm2 media (Di Ionno and Messier

2004) against the 85-Lpm flow rate specified by the National Institute for Occupational Safety

and Health (NIOSH 2005). Pressure drop across each filter was monitored with a Magnehelic

gauge measuring 0-2491 Pa and recorded every 30 minutes. The viral aerosols entering and

penetrating the test and control filters were collected in an AGI-30 impinger (Ace Glass Inc.,

Vineland, NJ, USA) containing 20 mL of sterile phosphate buffered saline (PBS). The collection