Continuing the discussion from If you (individual or country) are still masking, why will you ever stop?:
I think you’re missing my point. It is presumably true that N95 masks are subjected to flow tests before being certified, and that they pass that test. While an appeal to authority is not irrefutable truth, the burden lies upon you to prove that the masks do not actually filter particulate matter. To convince me, you would need to show me an actual flow test, not simply note with a microscope that you can see holes the virus might slip through.
I’m all for skepticism, but I don’t have the knowledge in aerosol physics to come to any conclusion based on your microscope observations alone. For all I know, the filter could be thick enough and dense enough to be relatively opaque to aerosols that come across it.
As a counter-cite, here is a study from 1998 when the N95 standard was new - forgive me if I quote the wrong parts:
Qian Y, Willeke K, Grinshpun SA, Donnelly J, Coffey CC. Performance of N95 respirators: filtration efficiency for airborne microbial and inert particles. Am Ind Hyg Assoc J. 1998 Feb;59(2):128-32. doi:10.1080/15428119891010389. PMID: 9487666.
Excerpts from study (Click to show/hide)
In this study N95 respirator filters were tested with bacteria, NaCl, and polystyrene latex (PSL) particles, and procedures similar to those used previously were applied.(6–9) The N95 respirators were also tested at a flow rate lower than the certification flow rate of 85 L/min to examine their performance at a breathing rate that is typical for health care workers.
First, the size-fractionating aerosol generator was removed because no liquid particles were used in this study; all three types of solid test particles were generated by a six-nozzle Collison nebulizer (BGI Inc., Waltham, Mass.), which was supplied with clean compressed air at a pressure of 1.05 kg/cm2 (15 psi). Second, to measure particles in the most penetrating size range, a laser aerosol size spectrometer that measures in the 0.1 to 3.0 μm size range (LAS-X, Particle Measuring Systems, Inc., Boulder, Colo.) was operated in parallel with the previously used aerodynamic size spectrometer (Aerosizer, Amherst Process Instruments Inc., Hadley, Mass.). The smallest size at which the Aerosizer can properly measure aerosol concentrations is approximately 0.5 μm in aerodynamic diameter.(13,14) Both aerosol spectrometers measure number concentrations. For a polydisperse aerosol with a mean size at or near the most penetrating particle size, the respirator efficiency by count is equal to or less than the efficiency by mass. Thus, the minimum efficiency by mass of 95% is satisfied, if the count efficiency is 95% or higher.
The nebulized test particles were mixed with clean dilution air to attain an aerosol concentration of about 80 to 120 per cm3 in the test chamber. The aerosol was passed through a 10 mCi 85 Kr electrical charge neutralizer (TSI Inc., St. Paul, Minn.) before entering the test chamber. The N95 respirators, randomly selected from the boxes in which they were supplied, were sealed to a head form and tested by measuring the aerosol concentrations inside and outside the respirators with both particle-size spectrometers. Since particle deposition in the human respiratory tract depends on the aerodynamic particle size, all data are presented as a function of the aerodynamic equivalent diameter. While the Aerosizer data are recorded as a function of aerodynamic diameter, the LAS-X size spectrometer data are recorded as a function of optical equivalent diameter. The latter was converted to aerodynamic diameter through consideration of the particle density (2.2 g/cm3 for NaCl). The optical equivalent diameter of the LAS-X size spectrometer is based on the instrument’s calibration with PSL particles. The size and index of reflection differences between the NaCl and PSL particles have been neglected in the size conversions.
Each data set was repeated five times. The results of the measurements are presented by their means and standard deviations. All tests were conducted at a temperature of 25 6 3°C and a relative humidity of 20 6 2%. All respirators were equilibrated at the test conditions for 24 hours or more before experiments were performed with them.
The minimum efficiencies at the most penetrating particle size are about 96% for the N95 respirator, 92% for the DFM respirator, 82% for the DM respirator, and 71% for the surgical mask.
[…] As seen, the electrically charged polymer fiber filters of Companies A and B have a minimum efficiency of about 96.2% at the most penetrating particle size when tested at 85 L/min, and the charged fiber filters of Company C have a minimum efficiency of 95% (i.e., the filter materials of all three companies pass the certification requirement).
The performance differences between the respirators of different companies have been interpreted as follows. In the design of respirators the efficiency level for the filtration material is set by the thickness of the filter material and by several filter characteristics, such as filter diameter and packing density. To keep the pressure drop low for maximum breathing comfort and the efficiency above the certification level, a compromise is selected among these parameters. As pointed out in the Experimental Materials and Methods section, the filtration materials of the tested respirators differed in thickness and packing density, and, apparently, also in fiber size and the degree of electrical charge embedded in the fibers. For particle sizes above 0.75 μm, the filtration efficiencies of all tested respirators are 99.5% or higher […]
In these experiments the mean aerodynamic diameters of the B. subtilis and B. megatherium bacteria were measured to be about 0.8 and 1.2 μm, respectively. For both bacteria, the filtration efficiencies of the N95 respirator are 99.5% or higher, similar to the data for NaCl and PSL particles. Therefore, the filtration efficiencies of the tested N95 respirators can be regarded as 99.5% or higher for M. tuberculosis. If TB bacteria are contained in droplets of sizes larger than 1 μm and there is a good face seal, the filtration efficiency is also 99.5% or higher.