If someone is sneezing or coughing, even a cloth mask will sharply curtail the ejection of virus-laden droplets into the air. But, otherwise, cloth masks are virtually worthless in deterring contagion, with mask pores typically being more than 100 times the size of a virus particle.
Cloth masks are unlikely to give much protection from airborne virus particles that are not clinging to sneezed or coughed moisture droplets, especially because masks used by the public do not meet basic filtration standards set for medical masks. They tend to be much too permeable, permitting virus particles relatively easy access to the wearer's face.
Even in some medical masks, pore sizes are much too big to act as a real deterrent to virus particles, though they suffice to greatly inhibit the much larger bacterium organisms.
If a tight, appropriate mask is being worn when a nearby person coughs or sneezes, the chance of inhaling the droplet -- which is much larger than an individual virus particle -- is sharply reduced. But, normally these ejecta fall to the ground fairly soon.
Technical discussion of permeability
https://textiles.ncsu.edu/tatm/wp-content/uploads/sites/4/2017/11/Ogulata_Full_199-06.pdf
According to one study of six types of medical mask, the average of the six mean mask pores was 29.67 microns. But virus particle sizes are substantially smaller, with the longest known rod-like one put at 1,000 nanometers. Globular virus particles generally have diameters that do not exceed 500 microns. A CDC report finds that coronavirus particles typically have diameters of 80 to 100 nanometers.
CDC report
https://wwwnc.cdc.gov/eid/article/27/4/20-4337_article
First, we check the longest virus length against the smallest pore width and find that
If we take the mean of each representative hole width for each of the six masks, we get about 30x10-6 meters. That width versus the longest virus particle yields:
If we reduce the virus length to 100 nanometers -- which is a typical coronavirus length -- we find that the particle is 0.33% the width of a typical fabric hole. Since that figure is substantially lower than 1%, we conclude that cloth masks provide very low levels of protection against virus particles suspended in air.
We should focus on the fact that the above figures are for medical masks. The porosity of unregulated cloth masks can be expected to be much higher.
And considering these virus-size to pore-size ratios, there is little benefit to be seen in the erstwhile federal suggestion that citizens wear two masks at a time.
A problem with N95 medical masks -- those that "filter out 95% of airborne particles" -- is that they don't work well unless fitted tightly against the face and head. That problem is far more apparent on cloth masks sold to the public.
A check of the CDC shows that N95 masks are often used for industrial purposes. Thus, the claim that a mask filters out 95% of airborne particulates may not actually mean virus particles. What does the phrase "95% of particulates" mean? Does it mean 95% of common types of particulates or most particulates of any size?
A CDC fact sheet for the public does not disclose the precise meaning of that phrase.
Types of mask
https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/types-of-masks.html
The fact sheet however does specify approved types of cloth masks. It recommends
It is noteworthy that the very people most likely to be imperiled by covid are most likely to have pre-existing breathing impediments, making the use of purportedly effective masks either impossible or at least inadvisable.
Cloth masks are unlikely to give much protection from airborne virus particles that are not clinging to sneezed or coughed moisture droplets, especially because masks used by the public do not meet basic filtration standards set for medical masks. They tend to be much too permeable, permitting virus particles relatively easy access to the wearer's face.
Even in some medical masks, pore sizes are much too big to act as a real deterrent to virus particles, though they suffice to greatly inhibit the much larger bacterium organisms.
If a tight, appropriate mask is being worn when a nearby person coughs or sneezes, the chance of inhaling the droplet -- which is much larger than an individual virus particle -- is sharply reduced. But, normally these ejecta fall to the ground fairly soon.
Technical discussion of permeability
https://textiles.ncsu.edu/tatm/wp-content/uploads/sites/4/2017/11/Ogulata_Full_199-06.pdf
According to one study of six types of medical mask, the average of the six mean mask pores was 29.67 microns. But virus particle sizes are substantially smaller, with the longest known rod-like one put at 1,000 nanometers. Globular virus particles generally have diameters that do not exceed 500 microns. A CDC report finds that coronavirus particles typically have diameters of 80 to 100 nanometers.
CDC report
https://wwwnc.cdc.gov/eid/article/27/4/20-4337_article
First, we check the longest virus length against the smallest pore width and find that
103x10-9
---------
17x10-6
= 1/17.
That is, the longest virus particle is about 6% of the width of the smallest average hole size for that mask type.If we take the mean of each representative hole width for each of the six masks, we get about 30x10-6 meters. That width versus the longest virus particle yields:
3x10-5
---------
10-6
= 3.34%
That is, the longest particle is no more than 3.34% of the width of a typical pore on one of the tested masks.If we reduce the virus length to 100 nanometers -- which is a typical coronavirus length -- we find that the particle is 0.33% the width of a typical fabric hole. Since that figure is substantially lower than 1%, we conclude that cloth masks provide very low levels of protection against virus particles suspended in air.
We should focus on the fact that the above figures are for medical masks. The porosity of unregulated cloth masks can be expected to be much higher.
And considering these virus-size to pore-size ratios, there is little benefit to be seen in the erstwhile federal suggestion that citizens wear two masks at a time.
A problem with N95 medical masks -- those that "filter out 95% of airborne particles" -- is that they don't work well unless fitted tightly against the face and head. That problem is far more apparent on cloth masks sold to the public.
A check of the CDC shows that N95 masks are often used for industrial purposes. Thus, the claim that a mask filters out 95% of airborne particulates may not actually mean virus particles. What does the phrase "95% of particulates" mean? Does it mean 95% of common types of particulates or most particulates of any size?
A CDC fact sheet for the public does not disclose the precise meaning of that phrase.
Types of mask
https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/types-of-masks.html
The fact sheet however does specify approved types of cloth masks. It recommends
A proper fit over your nose and mouth to prevent leaks; multiple layers of tightly woven, breathable fabric; a nose wire; and fabric that blocks light when held up to bright light source.Interestingly, the CDC advisories do not say what percentage of airborne virus particles are likely to be screened for either approved cloth masks or N95 medical masks (sometimes called respirators). But pre-2020 information indicates that such masks block 95% of moisture droplets, including virus-laden ones ejected via a cough or sneeze. Sneezing is not typically a symptom of covid.
It is noteworthy that the very people most likely to be imperiled by covid are most likely to have pre-existing breathing impediments, making the use of purportedly effective masks either impossible or at least inadvisable.
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