Cleaning N95 by heat; (also, sunlight vs aerosols) - Printable Version
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Cleaning N95 by heat; (also, sunlight vs aerosols) - M T - 08-11-2020
Quite a while back, we discussed some of the aspects of decontaminating SARS-COV-2 from N95 masks.
One way (mentioned in the CARDboard) is to just rotate among a set of masks so that you don't use the same mask for enough time for the virus to become inactive. This works on any kind of mask. You do want to avoid touching the mask while it is waiting its turn again. Putting it in a plastic bag is one way to achieve that (but not if it is damp). I tend to do this as I wear a mask maybe once a week.
Another way is to inactivate the virus through UV, sunlight, heat, etc. Back in April, a study showed the virus decreased by >6 log 10 (ie, > 1,000,000x) with 15 minutes at 92 degrees C. I expect others have repeated such tests, but I haven't looked for them.
A recent study indicates that a 3M-1860 N95 mask can go through 20 cycles of 50 minutes of dry, 100 degrees C heat, and still retain fit & adequate filtration. (I don't see why they stopped at 20 cycles. At that point, filtration was at 97%.) They also showed that 4 different viruses (AdV, RV, TV, TGEV; not including any Coronavirus) would be inactivated by at least 4 log 10 in the 50 minute cycle. The inactivation was greater for the inside layer of the mask than for the the outside layer, presumably due to different materials. The heating was done in a Faberware pressure cooker.
I would be hesitant to assume that other masks would survive the same number of cycles of heating and reuse. For instance, I use a foldable 5-layer KN95 mask. The material and straps are different than the rigid N95 mask. Because it is flexible rather than rigid, I don't think the fit would change much unless the ear straps degraded. (Actually, I don't know if any of the layers might shrink relative to the others.)
My concern is whether filtration would be reduced, especially if the mask was used long enough for moisture to build up in the fabric.
Some consumer-grade cloth masks might have components that shrink due to the heat, which may impact the fit. Any elastic may degrade and stretch or become weak.
I don't know if anyone is using the masks with exhaust valves, but that wasn't tested. I'd have questions about leakage after heating cycles.
I finally got around to finding the Department of Homeland Security's web page on the estimated effect of sunlight on AEROSOLS (see below for "surfaces") (This was the discussed in a press conference by the President.) NOTE this does NOT speak to inactivating the virus on a surface (say, of a mask) exposed to the sun.
If you remember, an earlier (April) study (first author from NIH) reported an estimate of the half life of the virus in an aerosol (this study is often misquoted as saying the virus survives up to 3 hours in air). At 21-23 degrees C and 65% RH, they reported median estimates of the half-life around 1.1 hours, and 95% credible intervals of [0.64, 2.64] hours for HCoV-19.
The DHS web page, when given parameters of UV Index of 0 (ie, indoors), temperature 72F, and 65%, estimates a half-life of 0.42 hours. I wish I understood which is more credible. That's a significant difference.
To achieve a 3 log 10 reduction(1000x, or 99.9% reduction) in the virus in an aerosol, one needs about 10 half-lives, or about 11 hours per the NIH study, or only 4.2 hours per DHS. To achieve a 6 log 10 reduction, one needs about 20 half-lives, or about 22 hours per NIH study, or 8.4 hours per DHS.
At noon in the Bay Area today (Aug 10), the UV index was about 9, temperature was 75F, RH was 65%. Per the DHS web page, the half-life of SARS-COV-2 in an aerosol would be 2.32 minutes. At 8PM, it would have been about 25 minutes (the calculator doesn't handle RH > 70%). at 7AM, it would have been 14 minutes. Remember to multiply by 10 to get a meaningful reduction.
The DHS also gives a one-size-fits-all estimate for the half-life of SARS-COV-2 on surfaces for temperature and humidity. The nature of the surface is not described. There is no mention of any solar radiation involved.
The NIH study gave half-lives on surfaces between 0.77 hours to 6.8 hours, depending on the surface at 22C and 40%RH.
The DHS calculator's formula gives a single half-life of 12.79 hours, at 22C and 40%RH.
Again, very different numbers. By the DHS formula, 3 log 10 reduction of the virus on a surface is 128 hours = 5.3 days. 6 log 10 reduction takes 10.6 days.
RE: Cleaning N95 by heat; (also, sunlight vs aerosols) - BostonCard - 08-11-2020
In an outdoor setting, settling, dilution, dispersion are probably the dominant modes of decontamination. The droplets fall to the ground, and the aerosols are carried by the wind and mixed with fresh air to the point where the concentration of viral particles per unit of volume drops by several log orders.
BC
RE: Cleaning N95 by heat; (also, sunlight vs aerosols) - DocSavage87 - 08-11-2020
(08-11-2020, 08:13 AM)BostonCard Wrote: In an outdoor setting, settling, dilution, dispersion are probably the dominant modes of decontamination. The droplets fall to the ground, and the aerosols are carried by the wind and mixed with fresh air to the point where the concentration of viral particles per unit of volume drops by several log orders.Whatever is left is taken care of by natural sunlight. Here's a paper on inactivation by sunlight:
BC
https://academic.oup.com/jid/article/222/2/214/5841129
I think this is from the biowarfare team in MD that suggested very rapid inactivation in direct sunlight, like below a minute originally, but these results suggest longer exposure is necessary.
RE: Cleaning N95 by heat; (also, sunlight vs aerosols) - BostonCard - 09-24-2020
Apparently 70% of KN95 masks don't meet the criteria for N95 filtration (that doesn't mean throw yours out; it is probably doing something).
https://www.usatoday.com/story/news/health/2020/09/22/covid-kn-95-masks-imported-china-dont-meet-standards-ecri/5806327002/
BC