Masks, respirators, face shields, and social distancing
Updated high-quality scientific evidence for advising healthcare workers and the general public on reducing the transmission of COVID-19 comes from a recent systematic review and meta-analysis that looked at 216 individual studies which included over 25,000 patients. 1 Physical distancing is confirmed to be a key method of reducing COVID-19 transmission, with 1 meter (3.3 feet) required but with improved protection with 2 meters of separation. 1 An N95 respirator filters out 95% of aerosols with a 0.3 µm or greater aerodynamic mass median diameter. For health-care workers, N95 respirators are superior and, when available, are recommended to be used rather than medical/surgical masks in all healthcare settings, not just aerosol-generating procedure environments. This differs from the formal recommendations provided by the World Health Organization 2 and the Center for Disease Control 3 that the N95 respirators are only required for aerosol-generating procedure environments. Likewise, disposable medical/surgical masks offer better protection than non-medical masks, e.g., “fabric” masks, made of woven or non-woven fabric such as polypropylene.1 A cotton fabric mask that could favorably compare to a disposable medical mask, would need to have 12-16 layers of fabric, which would most likely impair the ability to comfortably breathe.1 The use of a face shield over an N95 or face mask is also recommended to further reduce the transmission of COVID-19, predominantly by reducing virus particles from entering the eye.1 However, the face shield can also serve as a reminder to not touch the mask and face. Using face shields alone (no mask) is considered to be an inferior alternative and should only be considered for special populations, e.g., those with mental health disorders, developmental disabilities, deaf and hard of hearing communities, and young children).2
Non-medical fabric masks are intended to be used for “source control”, preventing an infected person from transmitting COVID-19 to others, and not for protection for the person wearing the mask. However, this does not diminish the usefulness of fabric masks for healthy people in the general public but emphasizes the need to have universal mask wearing for this to be maximally effective. The WHO now recommends universal use of masks when social distancing (minimum of 6 feet) is not possible. 2
The WHO recommends a medical/surgical disposable mask (when supplies for medical personnel are adequate) rather than a fabric mask for members of the general public defined to be at high risk, e.g., individuals ≥ 60 years of age, or those with underlying comorbidities, such as cardiovascular disease or diabetes mellitus, chronic lung disease, cancer, cerebrovascular disease, and immunosuppression.2
New evidence endorsed by over 200 world-wide medical scientists and currently being reviewed by the WHO, suggests that following normal breathing and talking there can be lingering microdroplet aerosols of coronavirus in indoor environments for distances beyond 6 feet (up to tens of meters) and for prolonged periods of time, and that these are likely responsible for the superspreading COVID-19 events. 4 Mitigation measures recommended include improved ventilation (simple steps such as opening windows and doors), minimization of recirculating air, high efficiency air filtration, active exhaust, ultraviolet germicidal lighting, and avoiding overcrowding.4 If the WHO or CDC accept that microdroplet aerosols are significant means of COVID-19 transmission, the use of non-medical homemade masks could be problematic with up to 97% aerosol penetration for cloth masks compared to 44% aerosol penetration with medical/surgical disposable masks. 5
The WHO provides detailed information on filtration efficiency and breathability of various types of non-medical fabric masks.2 The filtration efficiency depends upon the tightness of the weave, fiber/threat diameter, and manufacturing process. Multiple fabric thicknesses increase the filtration efficiency but can decrease the breathability. Thus, one must find a balance between the filtration efficiency and the breathability and this balance is known as the “filter quality factor” or “Q factor”. (World Health Organization, 2020 #5) The WHO rates polypropylene (e.g., interfacing material sold at fabric stores), a nonwoven man-made fabric, as one with a high Q factor. The WHO describes a practical and ideal non-medical fabric mask as having these characteristics:
- Minimum of 3 layers
- Outer layer should be hydrophobic (resisting water and other liquids), e.g., polyester, or polypropylene,
- Inner level should be hydrophilic (retaining water and humidity), e.g. cotton fabric
- Middle layer of synthetic non-woven fabric, e.g., interfacing fabric
How to put on and take off a mask2, 3:
- Wash hands with soap and water for 20 seconds or use alcohol-based cleanser before putting on mask
- Cover mouth and nose, adjust to nose bridge, and minimize gaps between face and mask with elastic ear loops or fabric ties
- Avoid touching mask while wearing it
- Do not wear mask on forehead or below chin at any time
- When removing, do not touch front of mask but loosen ties or elastic and then carefully remove
- Fold outer corners together and either discard disposable mask or place reusable mask in sealed bag for laundering prior to wearing again.
- Wash hands with soap and water of alcohol-based had cleanser
- Replace masks when they become damp or visibly soiled or torn
- Dispose of single-use masks. (When in short supply, consider following suggested disinfecting procedure below if mask is otherwise intact.)
- Reusable masks should be laundered in hot water (60 degree C) and detergent
Reusing disposable masks when there is a shortage of PPE
The global shortage of PPE during the COVID-19 pandemic has created a need for reliable and easy method of disinfecting masks for reuse. Using soapy water, alcohol, bleach immersion, ethylene oxide, ionizing radiation, autoclave are not being recommended as methods for decontamination as these have been shown to reduce the filtration and fit properties of the masks.5 Several methods of decontamination, hydrogen peroxide vapor, ultraviolet radiation, moist heat, dry heat and ozone gas, appear promising. (Munoz-Jimenez, 2020 #9)The U.S. Food and Drug Administration (FDA) has recently approved the STERRAD Sterilization System, employing vaporized hydrogen peroxide, for healthcare facilities. 6 The only two decontamination methods that could easily be reproduced at home (but not endorsed by the CDC, FDA or WHO) includes hot air and steam. The Spanish Ministry of Labor and Social Economy and the International Medical Center of Beijing have both endorsed the dry heat method.
Researchers at Stanford University were able to show that the use of 75 °C (167 °F) dry heat for 30 minutes could be used up to 20 times and still maintain the filtration properties on N95 respirators. 5 Similarly, the Spanish Society of Preventive Medicine, Public Health and Hygiene recommend 70 °C for 30 minutes in a convection oven. 5 However, the Stanford authors caution that there are risks handling contaminated masks in the home environment and discourage the use of a home oven to decontaminate masks. Using hot water steam for 10 minutes was less efficient as this method reduced the filtration properties after 3 decontamination treatments. 5
Microwave-generated steam has also been shown in one study to effectively decontaminate disposable N95 masks. This involves placing the contaminated N95 mask on the mesh from a produce bag that has been stretched over an open glass container [e.g., cooking glass casserole dish 7” x 7” x 3” (depth) or larger] and secured with a rubber band. Add 2 oz of water to the glass dish and microwave on high in a 1100 watt or higher microwave oven for 3 minutes (See Figure 3 in article).7
There have been fewer studies on decontamination of disposable medical/surgical masks. The researchers from Columbia University’s Lamont-Doherty Earth Observatory provide a step by step method of decontaminating these disposable masks by “baking” them at 170 °F for 30 minutes. The contaminated mask is first secured inside a small paper bag and then placed inside a sealed oven roasting bag. (https://blogs.ei.columbia.edu/2020/06/15/disinfect-mask-step-by-step-guide/)
An alternative method used by many hospitals during the PPE shortage to extend the supply of N95 masks was to rotate the use of these masks as studies indicate that the COVID-19 virus particles on the contaminated masks would be incapable of transmitting infectious particles after 5 days. Each healthcare worker would be supplied with 5 individual masks and each contaminated mask would be placed in a paper bag, secured, and help for 5 days and then reused. 8A similar rotation process could be considered for the general public as long as the disposable medical or N95 masks remain dry, intact, and free of visible soiling.
1. Chu DK, Akl EA, Duda S, Solo K, Yaacoub S, Schunemann HJ, et al. Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis. Lancet. 2020;395(10242):1973-87.
2. World Health Organization. Advice on the use of masks in the context of COVID-19: Interim guidance, 5 June 2020 June 5 2020: World Health Organization; 2020 [updated June 5 2020; cited 2020 July 8 2020]. Available from: https://www.who.int/publications/i/item/advice-on-the-use-of-masks-in-the-community-during-home-care-and-in-healthcare-settings-in-the-context-of-the-novel-coronavirus-(2019-ncov)-outbreak.
3. Centers for Disease Control and Prevention. How to wear cloth face coverings 2020 [cited 2020 July 8 2020]. Available from: https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/how-to-wear-cloth-face-coverings.html.
4. Morawska L, Milton DK. It is Time to Address Airborne Transmission of COVID-19. Clin Infect Dis. 2020.
5. Munoz-Jimenez A, Rubio-Romero E, Marenco de la Fuente JL. Proposal for the use of anakinra in acute respiratory distress secondary to COVID-19. Reumatol Clin. 2020.
6. U.S. Food and Drug Administration. Use the Correct Cycle and Compatible N95 Respirators when Decontaminating Respirators with Sterrad Sterilization Systems- Letter to Health Care Providers 2020 [Available from: https://www.fda.gov/medical-devices/letters-health-care-providers/use-correct-cycle-and-compatible-n95-respirators-when-decontaminating-respirators-sterrad.
7. Zulauf KE, Green AB, Nguyen Ba AN, Jagdish T, Reif D, Seeley R, et al. Microwave-Generated Steam Decontamination of N95 Respirators Utilizing Universally Accessible Materials. mBio. 2020;11(3).
8. Centers for Disease Control and Prevention. Decontamination and Reuse of Filtering facepiece respirators 2020 [updated April 29, 2020. Available from: https://www.cdc.gov/coronavirus/2019-ncov/hcp/ppe-strategy/decontamination-reuse-respirators.html.