COVID: sterilized N95 masks??

A recent analysis suggested that using irradiation to sterilize N95 masks led to dramatically decreased effectiveness, despite their still passing the “Fit test” (see covid N95 irradiation jama2020 in dropbox, or doi:10.1001/jamanetworkopen.2020.9961)

 

Details:

-- viral inactivation of SARS-CoV-2 has been reported at doses of 10 kGy of radiation

-- 3 different  N95 masks were irradiated using a cobalt-60 irradiator, with 3 masks of each type receiving 0 kGy (control), 10 kGy and 50 kGy of gamma irradiation

-- all 3 masks were subjected to the standard OSHA Fit test 50

-- another set of masks were tested for their particulate single-pass filtration efficiency, with particle sized 0.3, 0.5, and 1 µm (the actual diameter of the SARS-CoV-2 virus is 0.1 µm)

 

Results:

-- all 9 of the tested control and irradiated N95 masks, when donned properly, passed the qualitative Fit test

-- there was marked degradation of filtration efficiency for all treated masks. The following graph compares the control group (0 kGy) with different radiation doses, and for 3 different particle sizes. Of note, there was not much difference between low and high radiation dose 

 

 

Commentary:

--pretty dramatic decreases in efficiency of the masks, independent of the intensity of the gamma irradiation

--this study is important mostly because masks are being sterilized and reused, and that what apparently may seem okay to us (ie, the mask fits, passes the “Fit test”, and seems like it would work) may be quite wrong. It’s sort of like doing a bone mineral density to assess fracture risk: a lot of bone strength  comes from the internal substructure of the bone and not the quantity of calcium overall. People with fragility fractures are at high risk of another, independent of BMD; and very dense bone from high dose fluoride treatment has a pretty dramatic increase in BMD, yet fractures more easily than some osteoporotic bone.  In the case of the masks, there may be very important changes in the flexibility or other internal structures of the fibers that really matter

--for example, a recent material science article came out assessing the effective barrier of handmade masks, noting that combinations of materials (eg cotton and silk) improved the filtration efficiency, but that it appeared that this efficiency was related to both the mechanical effects (higher weave densities decrease the passage of larger particles) as we might anticipate but also to electrostatic-based filtration (electrostatic attraction is especially important in removing low mass particles which are attracted to and bind to fibers). See covid mask material nano2020 in dropbox, or doi.org/10.1021/acsnano.0c03252

--a nonprofit scientific consortium assessing N95 respirator decontamination notes the importance of the electrically-charged flitration layer in the mask, and notes: "best practice is to use new N95s, Decontamination does not solve the PPE shortage crisis, and is an emergency practice to be considered during the COVID-19 pandemic. Efficacy and safety of N95 decontamination has not been fully characterized", per publication on 4/27/20. (see https://www.n95decon.org/ )

Limitations of the study:

-- the test used is not one that is approved by NIOSH

-- they only studied 3 types of N95 masks, limited by the supply available at that time

 

So, the bottom line:

-- it is likely true, as with many things, that our assumption may be wrong: we may well think that a mask that looks normal after sterilization, and acts normal in that the "Fit test" is normal, is in fact normal. But, there may be significant, undetected changes in the mask substructure or electrostatic properties that dramatically change its efficiency in filtering out particles

-- so, before accepting any apparently effective sterilization process for N95 reuse, we should make sure that the masks have in fact been appropriately tested with actual virus particles...

geoff

 

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