Curated by Marilyn M. Singleton, MD, JD
COVID-19 is as politically-charged as it is infectious. Early in the COVID-19 chaos, the WHO, the CDC and NIH’s Dr. Anthony Fauci discouraged wearing masks as not useful for non-health care workers.
Now they recommend wearing cloth face coverings in public settings where other social distancing measures are hard to do e.g., grocery stores and pharmacies.
The recommendation was published without a single scientific paper or other information provided to support that cloth masks actually provide any respiratory protection.
Now, let’s look at the data.
The theory behind mask wearing:
- Source control: Cloth mask can trap droplets that come out of a person’s mouth when they cough or sneeze.
- Protection: Personal Protective Equipment (PPE) – only N95 masks
Transmission of SARS-CoV-2
Note: A COVID-19 (SARS-CoV-2) particle is 0.125 micrometers/microns (μm); influenza virus size is 0.08 – 0.12 μm; a human hair is about 150 μm.
*1 nm = 0.001 micron; 1000 nm = 1 micron; Micrometer (μm) is the preferred name for micron
*1 meter is = 1,000,000,000 [trillion] nm or 1,000,000 [million] microns
- Virus is transmitted through respiratory droplets produced when an infected person coughs, sneezes, or talks.
- Larger respiratory droplets (>5 μm) remain in the air for only a short time and travel only short distances, generally <1 meter. They fall to the ground quickly.https://www.thelancet.com/journals/lanres/article/PIIS2213-2600(20)30245-9/fulltext
- This idea guides the CDC’s advice to maintain at least a 6-foot distance.
- Small (<5 μm) aerosolized droplets can remain in the air for at least 3 hours and travel long distances (up to 27 ft.).
- In an air conditioned environment these large droplets may travel farther.
- Ventilation. Even the opening of an entrance door and a small window can dilute the number of small droplets to one half after 30 seconds. (This study looked at droplets from uninfected persons). This is clinically relevant because poorly ventilated and populated spaces, like public transport and nursing homes, have high SARS-CoV-2 disease transmission despite physical distancing.
- Since 1961, experiments showed that viral-pathogen-carrying droplets were inactivated within shorter and shorter times as ambient humidity was increased. Dryness drives the small aerosol particles. See e.g., review of studies, https://aaqr.org/articles/aaqr-20-06-covid-0302
The preponderance of scientific evidence supports that aerosols play a critical role in the transmission of SARS-CoV-2. Years of dose response studies indicate that if anything gets through, you will become infected.
- Thus, any respiratory protection respirator or mask must provide a high level of filtration and fit to be highly effective in preventing the transmission of SARS-CoV-2. (Works for Mycobacterium tuberculosis (3μm)
- Public health authorities define a significant exposure to COVID-19 as face-to-face contact within 6 feet with a patient with symptomatic COVID-19 that is sustained for at least a few minutes (and some say more than 10 minutes or even 30 minutes).
- The chance of catching COVID-19 from a passing interaction in a public space is therefore minimal.
Filter Efficiency and Fit
Data from a University of Illinois at Chicago review https://www.cidrap.umn.edu/news-perspective/2020/04/commentary-masks-all-covid-19-not-based-sound-data
- HEPA (high efficiency particulate air) filters are 99.97 to 100% efficient. HEPA filters are tested with particles that are 0.125 μm (the size of SARS-CoV-2).
- Masks and respirators work by collecting particles through several physical mechanisms, including diffusion (small particles) and interception and impaction (large particles).
- Surgical masks are loose-fitting devices that were designed to be worn by medical personnel to protect accidental contamination of patient wounds, and to protect the wearer against splashes or sprays of bodily fluids. They aren’t effective at blocking particles smaller than 100 μm. https://multimedia.3m.com/mws/media/957730O/respirators-and-surgical-masks-contrast-technical-bulletin.pdf
- OSHA/CDC: A surgical mask is not a respirator. It cannot be used to protect workers who perform or assist with aerosol-generating procedures, which may create very fine aerosol sprays. A surgical mask can only be used to protect workers from contact with the large droplets made by patients when they cough, sneeze, talk or breathe. https://www.osha.gov/dts/guidance/flu/healthcare.html
- N95 filtering facepiece respirators (FFRs) are constructed from electret (a dielectric material that has a quasi-permanent electric charge.) An electret generates internal and external electric fields so the filter material has electrostatic attraction for additional collection of all particle sizes. As flow increases, particles will be collected less efficiently.
- A properly fitted N95 will block 95% of tiny air particles down to
0.3 μm from reaching the wearer’s face.
- Problem: no source control. An N95 does not filter exhaled air passing through the exhaust/exhalation valve (for easier breathing and less moisture inside the mask).
- Study measuring filter efficiency (2010)
- https://academic.oup.com/annweh/article/54/7/789/202744; https://www.cidrap.umn.edu/news-perspective/2020/04/commentary-masks-all-covid-19-not-based-sound-data; https://academic.oup.com/annweh/article/54/7/789/202744
- Filter efficiency was measured across a wide range of small particle sizes (0.02 to 1 µm) at 33 and 99 L/min.
- All the cloth masks and materials had near zero efficiency at 0.3 µm, a particle size that easily penetrates into the lung (SARS-CoV-2 is 0.125 µm)
- Efficiency for the entire range of particles
- T-shirts — 10%
- Scarves — 10% to 20%
- Cloth masks — 10% to 30%
- Sweatshirts — 20% to 40%
- Towels — 40%
- Study measuring filter efficiency (2014, Korea)
- Evaluated 44 masks, respirators, and other materials with similar methods and small aerosols (0.08 and 0.22 µm)
- N95 FFR filter — >95% efficiency
- Medical masks — 55% efficiency
- General (cloth) masks — 38% efficiency
- Handkerchiefs — 2% (one layer) to 13% (four layers) efficiency.
- Conclusion: Wearing masks (other than N95) will not be effective at preventing SARS-CoV-2 transmission, whether worn as source control or as PPE.
- N95s protect health care workers, but are not recommended for source control transmission.
- Surgical masks are better than cloth but not very efficient at preventing emissions from infected patients. Cloth masks must be 3 layers, plus adding static electricity by rubbing with rubber glove.
- The cloth that serves as the filtration for the mask is meant to trap particles being breathed in and out. But it also serves as a barrier to air movement because it forces the air to take the path of least resistance, resulting in the aerosols going in and out at the sides of the mask.
- An August 2020 UCSF study suggested that the mask would decrease the absolute volume of the inoculum. (The concentrations of bacteria upstream and downstream of the test devices were measured with an aerodynamic size spectrometer) https://ucsf.app.box.com/s/blvolkp5z0mydzd82rjks4wyleagt036
Paul Ebeling, Editor
Have a healthy week, Keep the Faith!