COVID-19 Filter Testing

quick lit review

SARS-CoV-2 characteristics

• single stranded RNA virus, beta-coronavirus
• "Therefore, virus transmission via respiratory secretions in the form of droplets (>5 microns) or aerosols (<5 microns) appears to be likely. " ¹
• nonsocomial spread of SARS-CoV-1 was linked to aerosol-generating medical procedures ¹
• "The virus aerosol deposition on protective apparel or floor surface and their subsequent resuspension is a potential transmission pathway and effective sanitization is critical in minimizing aerosol transmission of SARS-CoV-2." ²
• airborne SARS-CoV-2 found at 0.25-0.5um and <2.5um, but not coexisting so potentially different formation mechanisms ²
  • air samples in various locations collected and filtered through cascade impactor then tested for presence of SARS-CoV-2
  • ²
• "We believe one direct source of the high SARS-CoV-2 aerosol concentration may be the resuspension of virus-laden aerosol from the surface of medical staff protective apparel while they are being removed. . . We hypothesize the submicron aerosol may come from the resuspension of virus-laden aerosol from staff apparel due to its higher mobility while the supermicron virus-laden aerosol may come from the resuspension of dust particles from the floors or other hard surfaces" ²

so we're targeting filter material able to block particles with minimum ~0.25um diameter and high flow rate for comfortable breathing

mask characterization metrics

• filtration efficiency: percent efficiency of upstream colony-forming units (cfu) to downstream cfu ³
• fit factor: ratio of concentration of microscopic particles outside to particles that leaked in ³
• microbial penetration value: number of microbes passing through the filter per 10 million microbes in the challenge ⁴
• measuring virus quantity:
  • plaque assay: plaque forming units (pfu) - virus infected cell will lyse and infect other cells, then form the plaque which can be counted manually visually or by optical microscope, takes days for virus to infect (time depending on virus)
  • focus forming assay (FFA) - similar to pfu/mL but instead of waiting for cell to lyse add reagents to highlight infected cells, less time but more costly with equipment
  • fifty-percent tissue culture infective dose (TCID_50) - amount of virus required to kill 50% of infected hosts. 0.69 pfu = 1 TCID bc poisson distribution
• measuring aerosol particles:
  • MMAD: mean mass aerodynamic diameter
  • CMAD: count median aerodynamic diameter

conventional test methods

• Henderson apparatus: generates closed circuit microbial aerosols from a Collison nebulizer at a controlled humidity
  • Collison nebulizer - generally a pneumatic atomizer ⁵
  • 
• mobile henderson appartus 1968 ⁶
• challenge particles
  • Bacillus atrophaeus : 0.95-1.25um, 10^7 cfu in challenge ³
  • Bacteriophage MS2 MCIMB10108: single strange RNA ~20nm diameter, 10^9 pfu in challenge ³
  • E. Coli phi174 ⁷

performance difference between pleated hydrophobic filters and electrostatic filters??

Resources

needed for testing apparatus

• nebulizer for aerosolization ⁸
  • three-jet Collison nebulizer?
• impinger for collection
• wet and dry thermometer / hygrometer for humidity
• andersen sampler?? could be nice to bin different size particles - six-stage viable particle cascade impactor