Effect of aerosol generation conditions on the particle filtration efficiency measured for meltblown electret mask filter media

The particle filtration efficiency (PFE) of mask filter media is measured with a test aerosol generated by an atomizer. Test standards specify this aerosol but not how the atomizer is operated. Six generation variables (atomizing pressure, solute, solution concentration, challenge concentration, charge neutralization and reservoir liquid level) were varied on one platform for five meltblown electret media and a glass-fiber high-efficiency particulate air (HEPA) reference. Across twelve charge-neutralized conditions, the measured efficiency of the electret media spanned 2.9 to 19.0 percentage points (pp), depending on the medium, against 0.30 pp for the HEPA medium. Without charge neutralization, efficiency was higher in 47 of 48 paired tests, by 2.38 pp on average; this gain comes from particle charge left by atomization, not from better filtration. Lowering the generator output reduced the count median diameter from 72.1 to 45.3 nm, outside the 42 CFR 84 specification. The uncharged HEPA medium also responded, so electrostatic capture amplified the effect but did not cause it. Within one laboratory, the generation conditions produced 1.6 to 12.6 times the between-laboratory variability reported in ASTM F2299 and changed the 95% pass/fail outcome for two media. The delivered aerosol should therefore be measured and reported with each result.