curated by Marilyn M. Singleton, M.D., J.D.
Transmission of SARS-CoV-2
Note: A COVID-19 (SARS-CoV-2) particle is 0.125 micrometers (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 (an older term)
1 meter is = 1,000,000,000 nm or 1,000,000 microns
- In air conditioned environment these large droplets may travel farther.
- However, 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. https://www.thelancet.com/journals/lanres/article/PIIS2213-2600(20)30245-9/fulltext
Objects and surfaces
- Person to person touching
- The CDC’s most recent statement regarding contracting COVID-19 from touching surfaces: “Based on data from lab studies on Covid-19 and what we know about similar respiratory diseases, it may be possible that a person can get Covid-19 by touching a surface or object that has the virus on it and then touching their own mouth, nose or possibly their eyes,” the agency wrote. “But this isn’t thought to be the main way the virus spreads. https://www.cdc.gov/media/releases/2020/s0522-cdc-updates-covid-transmission.html.
- Chinese study with data taken from swabs on surfaces around the hospital
- The surfaces where tested with the PCR (polymerase chain reaction) test, which greatly amplifies the viral genetic material if it is present. That material is detectable when a person is actively infected. This is thought to be the most reliable test.
- Computer mouse (ICU 6/8, 75%; General ward (GW) 1/5, 20%)
- Trash cans (ICU 3/5, 60%; GW 0/8)
- Sickbed handrails (ICU 6/14, 42.9%; GW 0/12)
- Doorknobs (GW 1/12, 8.3%)
- 81.3% of the miscellaneous personal items were positive:
- Medical equipment (spirometer, pulse oximeter, nasal cannula)
- Cellular phones (83.3% positive for viral RNA)
- Remote controls for in-room TVs (64.7% percent positive)
- Room surfaces (80.4% of all sampled)
- Bedside tables and bed rails (75.0%)
- Stainless Steel: up to 2-3 days
- Floor – gravity causes droplets to fall to the floor. Half of ICU workers all had virus on the bottoms of their shoes
Filter Efficiency and Fit
*Data from a University of Illinois at Chicago review
- HEPA (high efficiency particulate air) filters – 99.97 – 100% efficient. HEPA filters are tested with particles that are 0.125 μm.
- Masks and respirators work by collecting particles through several physical mechanisms, including diffusion (small particles) and interception and impaction (large particles)
- 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.
- N95 – A properly fitted N95 will block 95% of tiny air particles down to 0.3 μm from reaching the wearer’s face. https://www.honeywell.com/en-us/newsroom/news/2020/03/n95-masks-explained.
- But even these have problems: many have exhalation valve for easier breathing and less moisture inside the mask.
- Filter efficiency was measured across a wide range of small particle sizes (0.02 to 1 µm) at 33 and 99 L/min.
- N95 respirators had efficiencies greater than 95% (as expected).
- T-shirts had 10% efficiency,
- Sweatshirts 20% to 40%, and
- All of the cloth masks and materials had near zero efficiency at 0.3 µm, a particle size that easily penetrates into the lungs.
- Another study evaluated 44 masks, respirators, and other materials with similar methods and small aerosols (0.08 and 0.22 µm).
- N95 FFR filter efficiency was greater than 95%.
- Medical masks – 55% efficiency
- Handkerchiefs – 2% (one layer) to 13% (four layers) efficiency.
- Conclusion: Wearing masks will not reduce SARS-CoV-2.
- N95 masks 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 will be ineffective at preventing SARS-CoV-2 transmission, whether worn as source control or as personal protective equipment (PPE).
“Masks may confuse that message and give people a false sense of security. If masks had been the solution in Asia, shouldn’t they have stopped the pandemic before it spread elsewhere?”
*The first randomized controlled trial of cloth masks. https://bmjopen.bmj.com/content/5/4/e006577
- Penetration of cloth masks by particles was 97% and medical masks 44%, 3M Vflex 9105 N95 (0.1%), 3M 9320 N95 (<0.01%).
- Moisture retention, reuse of cloth masks and poor filtration may result in increased risk of infection.
- The virus may survive on the surface of the face- masks
- Self-contamination through repeated use and improper doffing is possible. A contaminated cloth