Is there evidence to support the use of non-medical masks?Posted on by
While non-medical masks are less effective than medical masks, there is limited indirect evidence suggesting that non-medical face masks may reduce infection risk by decreasing the release of respiratory droplets. However, there is no evidence that non-medical masks prevent COVID-19 infection for the person wearing the mask. Importantly, non-medical masks are not recommended for healthcare providers (1, 2, 5, 7, 25).
- N95 are slightly better than surgical masks, which are better than cloth masks, which themselves get better with thicker cloth or additional layers (17). All told, there is an approximately 2% absolute difference in the chance of getting a respiratory infection between mask types (22).
- Wearing a mask might prevent infected individuals, including those who are asymptomatic, from transmitting the disease to others (17).
Modelling suggests that universal community masking (even with non-medical masks), could significantly reduce COVID-19 cases (23, 24) and that the benefits of masking are multiplied when both healthy and infected individuals wear masks (23).
Evidence suggests COVID-19 could be transmitted by pre-symptomatic and asymptomatic individuals, thus, community transmission might be reduced by public masking. By extension, Health Canada advises Canadians to consider wearing a non-medical mask in the community as a complement to physical distancing, hand hygiene, and other public health measures (1, 2, 3).
II. Non-medical masks
- A recent scoping review concluded that the protection offered by non-medical masks is superior to no protection (25). However, there is a lack of evidence regarding optimal non-medical mask design and material, and whether protection is enhanced when combined with a face shield (25). Similar to medical-grade masks, fit is an important factor in the effectiveness of non-medical masks, with peripheral air leakage potentially exposing wearers to airborne viral particles (25).
Current state of evidence
- Used a model to estimate the relative risk of aerosol from normal breathing. The model indicates a multiplicative benefit when cloth masks are worn by both infected and uninfected individuals. According to the model, wearing a cloth mask reduces aerosol inhalation, fomite contamination as well as the number of large droplets released.
- Designed a mathematical model of COVID-19 transmission dynamics and control. Their analysis indicated that even non-medical masks could significantly reduce disease burden. For example, face-mask efficacy of 25% could lead to a 64% reduction in the number of confirmed cases, if 75% of the population wore face-masks in public.
- Quantified the extent to which fabrics reduce droplet dispersion from a simulated sneeze. A single-layer of fabric reduced the number of droplets dispersed and restricted dispersion to <30cm. When used as double-layers, textiles were as effective as medical masks, reducing droplet dispersion to <10cm, and the area of circumferential contamination to ~0.3%. Thus, concluded that textile masks may reduce the risk of transmitting or acquiring COVID-19(8).
- “To evaluate if masks can protect others from symptomatic COVID-19 patients, the authors conducted an experiment with 4 symptomatic, hospitalized COVID-19 patients. The subjects held a 90mm x 15mm petri dish with viral transport media 20cm from their mouths while coughing 5 times. This was performed without a mask, with a surgical mask, with a cotton mask, and again with no mask. The outer and inner surfaces of the masks were then swabbed. The author detected a similar quantity of virus from the petri dish with and without masks: median viral load (log copies/mL) = 2.56 without masks, 2.42 with a surgical mask, 1.85 with a cotton mask”.(9)
- “Conducted an experiment, using an avian influenza virus, on the comparable efficiency between N95, surgical masks, and homemade masks (made from 4 layers of “kitchen paper” plus 1 layer of polyester cloth) to block aerosols. They found that the masks blocked 99.9%, 97.1%, and 95.1% of aerosols, respectively.”(11)
- “Found that masks made from cotton t-shirts had about 50% the median-fit factor of surgical masks. Both masks blocked microorganisms expelled; however, surgical masks were three times more effective.”(12)
- Evaluation of the filtration performance of cloth masks and common fabric materials against 20–1000 nm size particles. “Certain fabrics (e.g., towels and scarves) had slightly lower penetration (around 20-80% for towels, increasing with particle diameter), which was noted by the authors to be comparable to other studies of surgical mask penetration levels (measured in cited studies ranging from 51- 89%). They conclude that fabric materials provide minimal respiratory protection to the wearer from aerosol sized particles, but that “the use of improvised fabric materials may be of some value compared to no protection at all when respirators are not available.”(13)
- “Compared homemade tea cloth masks, surgical masks, and FFP-2 (European equivalent of N95 respirators) in healthy volunteers performing various physical maneuvers and measuring quantitative differences in particles with a Portacount. They calculated median protection factors (or PFs, the ratio of particle concentrations sized 0.02-1 µm outside to inside the mask) of 2.2-3.2 for cloth masks, 4.1-5.3 for surgical masks, and 66-113 for FFP-2 respirators. Marginal protection was seen for all mask types when testing for reduction in outgoing transmission of respiratory particles.”(14)
- “Fashioned a nine-ply (one outer layer and eight inner layers) face mask out of heavy-weight cotton t-shirt material, and achieved a maximum fit factor of 67 using quantitative measurements (a Portacount Fit Tester), with minimal discomfort or difficulty breathing reported in the three test subjects. Note that National Institute for Occupational Safety and Health (NIOSH)-approved N95 respirators are required to have a fit factor of 100.”(15)
III. Medical masks for healthcare providers
- “Surgical masks and N95 respirators appear to provide similar protection against viral infections, with N95 masks having slightly lower, but not statistically different, infection rates (~1-2%). Cloth masks are less effective than surgical masks (~2-3% more respiratory infections in 4 weeks). No randomized controlled trials examined transmission from healthcare workers to others and none examined COVID-19.”(17)
- Mask conservation strategies including extended use, reuse, and decontamination of medical-grade masks result in inferior protection (25).
Current state of evidence
N95 masks compared to surgical masks in preventing a wide range of viral respiratory tract infections:
- 4 systematic reviews with 3-6 Randomized Controlled Trials (RCTs)(18, 19, 20, 21)
- Largest Meta-Analysis(18):
|Healthcare worker condition||N95 Masks||Surgical Masks||Statistical Difference|
|Confirmed Viral Respiratory Tract Infection||5.7%||8.3%||No|
MacIntyre et al. (2015) – Cloth masks versus surgical masks
- “RCT of 1607 healthcare workers in Vietnam for 4 weeks.
- Viral infections in the wearer:
- Clinical Respiratory Infections: 7.6% cloth versus 4.8% surgical masks (borderline statistically different).
- Influenza-like Illness: 2.3% cloth versus 0.2% surgical masks (statistically different).
- Laboratory confirmed viral infection: 5.5% cloth versus 3.3% surgical mask (not statistically different).”(17)
- “Penetration of cloth masks by particles was almost 97% and medical masks 44%.
- Moisture retention, reuse of cloth masks and poor filtration may result in increased risk of infection.”(7)
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- Health Canada. Considerations in the use of homemade masks to protect against COVID-19 [Internet]. 2020 April 19 [cited 2020 April 19]. Available from: https://www.canada.ca/en/health-canada/services/drugs-health-products/medical-devices/activities/announcements/covid19-notice-home-made-masks.html
- Health Canada. Council of Chief Medical Officers of Health Communication: Use of Non-Medical Masks (or Facial Coverings) by the Public [Internet]. 2020 April 7 [cited 2020 April 19]. Available from: https://www.canada.ca/en/public-health/news/2020/04/ccmoh-communication-use-of-non-medical-masks-or-facial-coverings-by-the-public.html
- Public Health Ontario. 2019-nCoV – What We Know So Far About…Wearing Masks in Public [Internet]. 2020 April 7 [cited 2020 April 19]. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/covid-wwksf/what-we-know-public-masks-apr-7-2020.pdf?la=en
- Centre for Disease Control. Use of Cloth Face Coverings to Help Slow the Spread of COVID-19 [Internet]. 2020 April 13 [cited 2020 April 19]. Available from: https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/diy-cloth-face-coverings.html
- World Health Organization. Advice on the use of Masks in the Context of COVID-19 [Internet]. 2020 April 6 [cited 2020 April 19]. Available from: https://apps.who.int/iris/rest/bitstreams/1274280/retrieve
- European Centre for Disease Prevention and Control. Using Face Masks in the Community: Reducing COVID-19 Transmission from Potentially Asymptomatic or Pre-symptomatic People Through the use of Face Masks [Internet]. 2020 April 8 [cited 2020 April 19]. Available from: https://www.ecdc.europa.eu/sites/default/files/documents/COVID-19-use-face-masks-community.pdf
- MacIntyre CR, Seale H, Dung TC, Hien NT, Nga PT, Chughtai AA, et al. A Cluster Randomised Trial of Cloth Masks Compared with Medical Masks in Healthcare Workers [Internet]. BMJ open. 2015 [cited 2020 April 19]. Available from:. https://bmjopen.bmj.com/content/5/4/e006577
- Rodriguez-Palacios A, Cominelli F, Basson A, Pizarro T, Ilic S. Textile Masks and Surface Covers: A ‘Universal Droplet Reduction Model’ Against Respiratory Pandemics [Internet]. MedRxiv. 2020 April 10 [cited 2020 April 19]. Available from: https://www.medrxiv.org/content/10.1101/2020.04.07.20045617v1.full.pdf+html
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- Public Health Ontario. Review of “Effectiveness of Surgical and Cotton Masks in Blocking SARS-CoV-2: A Controlled Comparison in 4 Patients” [Internet]. 2020 April 12 [cited 2020 April 19]. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/research/research-effectiveness-masks-blocking-sars-cov-2-comparison.pdf?la=en
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