Hello Taylor — Thanks for critically reading! This is exactly the type of discussion we are hoping to spark from our articles, please keep it up.

Thanks for highlighting a point of confusion. It seems that from your understanding, a ‘liquid droplet’ could imply a particle of larger diameter than an ‘aerosolized particle’.

From our intended definition though, an ‘aerosolized particle’ = ‘a very small liquid droplet that is airborne’.

How are aerosolized particles generated? By coughing, sneezing, and ventilators.

How large are aerosolized particles? We have not seen good data for how large aerosolized particles from ventilators are, but as shown in this paper that “measured the amount and size of aerosol particles containing influenza virus that were produced by coughing,” “thirty-five percent of the influenza RNA was contained in particles >4 µm in aerodynamic diameter, while 23% was in particles 1 to 4 µm and 42% in particles <1 µm.”

So how do each of the ASTM/CDC test methods stack up in measuring filtration of aerosolized particles of those sizes?

The way that filtration efficiency is measured in the N95 test method is by aerosolizing 0.075µm NaCl particles. The way that filtration efficiency is measured in the ASTM standard is by aerosolizing 0.1µm Latex Spheres.

Both test methods use test media that is small enough to measure robustness to aerosolized particles, according to the influenza virus paper. Both the ASTM (surgical mask material) and the CDC (N95s) certifications ensure filtration efficiency of ≥95% of their respective particle sizes.

Does that make sense?