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Tue May 26, 2020, 07:01 PM

Steven Chu and Yi Cui: Can N95 Respirators Be Reused after Disinfection? How Many Times?

The paper to which I will refer is co-authored by President Obama's first Secretary of Energy (the best Secretary of Energy, by the way), Nobel Laureate Steven Chu and Dr. Yi Cui, who is among the most cited authors in Materials Science.

This is the paper: Can N95 Respirators Be Reused after Disinfection? How Many Times? (Lei Liao, Wang Xiao, Mervin Zhao , Xuanze Yu, Haotian Wang, Qiqi Wang, Steven Chu, and Yi Cui* ACS Nano 2020, 14, 5, 6348–6356.

The paper is fully open sourced, and anyone can read it. It's not overly technical, but has a few words that might strike a non-technical person as obscure, like say, "triboelectric," which means "rubbing together to generate static charges, something most everyone has done.

Some excerpts:

...Although the precise mode SARS-CoV-2’s viral transmission is not known, a primary transmission mode in viruses such as SARS and influenza is through short-range aerosols and droplets.(11) When a person infected with a virus breathes, speaks, sings, coughs, or sneezes, micron-sized aerosols containing the virus are released into the air. Data gathered from influenza patients suggest that these aerosols are typically fine (<5 μm) or coarse (>5 μm).(11−13) Coarse particles can settle due to gravity within 1 h. Fine particles, however, especially those smaller than 1 μm, can essentially stay in the air nearly indefinitely. Droplets, or particles >10 μm, settle rapidly and are not typically deposited in the respiratory tract through means of aerosol inhalation. Particles larger than 5 μm typically only reach the upper respiratory tract, whereas fine particles <5 μm are critically able to reach the lower respiratory tract, similar to harmful particulate matter pollution (Figure 1). Although coughing and sneezing provide many aerosols, the size distribution and number of particles emitted during normal speech serve as a significant viral transmitter.(12) Singing has been found to be comparable to continuous coughing in the transmission of airborne pathogens,(14) which was demonstrated during a choir practice on March 10, 2020 in Washington state. Although the choir members did not touch each other or share music during the rehearsal, 45 out of the 60 members of the Skagit Valley Choir were diagnosed with the virus 3 weeks later, and two had died...

...The N95 grade is determined by the CDC’s National Institute of Occupational Safety and Health (NIOSH) (document 42 CFR Part 84), which designates a minimum filtration efficiency of 95% for 0.3 μm (aerodynamic mass mean diameter) of sodium chloride aerosols. In addition to N95, there are N99 and N100 standards, which correspond to filtration efficiencies of 99% and 99.97%, respectively. For oil-based aerosols (DOP), NIOSH also has created grades R and P (with filtration efficiencies 95–99.97%). Elsewhere around the globe, the equivalent filtration grades to N95 are FFP2 (European Union), KN95 (China), DS/DL2 (Japan), and KF94 (South Korea). Although the actual SARS-CoV-2 virus is ca. 150 nm,(18) commonly found N95 respirators can offer protection against particles as small as 80 nm with 95% filtration efficiency (initial testing, not loaded).(19) With the actual viral aerosols in the ∼1 μm range, the N95 FFRs’ filtration efficiency should be sufficient for personal protection...


...The COVID-19 pandemic has led to a significant shortage of N95 FFRs,(25) especially among healthcare providers. Although the virus will eventually become inactive on the mask surface and it is unlikely to penetrate fully to the user’s intake side, a recent study shows that 72 h were required for the concentration of SARS-CoV-1 and SARS-CoV-2 viruses on plastic surfaces (40% RH and 21–23 °C) to be reduced by 3 orders of magnitude (from 103.7 to 100.6 TCID50 per mL of medium).(26) Assuming a similar longevity on FFR surfaces, it is important to develop procedures for the safe and frequent reuse of FFRs without reducing the filtration efficiency. The CDC has recommended many disinfection or sterilization methods, typically involving chemical, radiative, or temperature treatments.(27) In brief, the mechanisms of disinfection or sterilization of bacteria and viruses include protein denaturation (alcohols, heat), DNA/RNA disruption (UV, peroxides, oxidizers), and cellular disruption (phenolics, chlorides, aldehydes). Although none of these methods have been extensively evaluated for SARS-CoV-2 inactivation specifically, we tested methods that can be easily deployed within a hospital setting, and possibly accessible for the general population, with relatively high throughput for FFR reuse...




The caption:

Figure 2. Meltblown fabrics in N95 FFRs. (A) Peeling apart a representative N95 FFR reveals multiple layers of nonwoven materials. (B) Scanning electron microscope (SEM) cross-section image reveals the middle meltblown layer has thinner fibers with thickness around 300 μm. (C) SEM image of meltblown fibers reveals a complicated randomly oriented network of fibers, with diameters in the range of ∼1–10 μm. (D) Schematic illustration of meltblown fibers (left) without and (right) with electret charging. In the left figure, smaller particles are able to pass through to the user, but particles are electrostatically captured in the case of an electret (right).


I will skip my usual long winded commentary. Again, it's open sourced.

The paper is worth a look in my view.

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Reply Steven Chu and Yi Cui: Can N95 Respirators Be Reused after Disinfection? How Many Times? (Original post)
NNadir May 2020 OP
DonaldsRump May 2020 #1
eppur_se_muova May 2020 #2
NNadir May 2020 #3

Response to NNadir (Original post)

Tue May 26, 2020, 07:15 PM

1. I put mine out in the Northern California sun for a 1/2 day

It's nearly 90 degrees in the Bay Area this week, and I turn the mask both ways to face the sun. I usually go several days before wearing the mask again. I am hoping, perhaps naively, that this is enough.

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Response to NNadir (Original post)

Wed May 27, 2020, 09:58 AM

2. Electrets as a filter element ? That's a new one to me. Ingenious, of course. nt

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Response to eppur_se_muova (Reply #2)

Wed May 27, 2020, 01:24 PM

3. It's new to me as well, but apparently well known.

I just took a quick look on Google Scholar. There are hundreds of patents and papers on the subject apparently in just the last few years.

It is pretty cool, I think. Maybe air filters should include capacitors. Maybe they do, I don't know.

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