Homemade facemask preventing SARS-CoV-2 transmission

Clinical Question

What do we know about homemade masks in the context of preventing SARS-CoV-2 transmission?

Key Findings

  • There is conflicting evidence on whether homemade masks are effective, though it is widely held belief that it prevents the wearer from infecting others more than it protects the wearer from people they come into contact with.
  • Quilting fabric appears to be the best household fabric, followed by 600-count pillowcases and flannel.
  • Cotton and bandanas have good wearability and tolerability, which is associated with increased compliance, but a decreased efficacy (about half that of a surgical mask).
  • Filters can be inserted between two layers of cotton, and many filter materials have been proposed. Vacuum cleaner filters are noted to be effective but are less tolerated. Filters not recommended include HEPA filters or vacuum cleaner bags (risk of inhalation of fiberglass particles), and coffee filters (create a large pressure drop)
  • Surgical wraps are being used at a few facilities for general mask use - the efficacy is probably closer to that of a surgical mask and this material should probably be reserved for use in health care settings.

Summary of Information

Two of the main studies looking at efficiency of home-made masks include one done in Cambridge and another in the Netherlands. The Cambridge study looked at a number of household materials and found that surgical masks had a calculated mean filtration efficiency of 90% of bacteriophage MS2 (diameter about 27nm) (used to simulate the influenza virus). Vacuum cleaner filters and tea towels were found to be the next most successful, though they caused a pressure drop that made breathing difficult, and therefore were not recommended for masks alone. When accounting for pressure drop, the authors recommended using 100% cotton t-shirts and pillow cases. These had a filtration efficiency of 51% and 57%, respectively. Linen was reported to have an efficiency of 61%, though this option was not discussed much in the paper. Doubling the material did not appear to improve efficiency.

The Netherlands study compared short term (15 minutes) and long term (3 hours) protection, as well as prevention of outgoing transmission for N95 equivalent masks, surgical masks, and home-made masks built of tea cloths. Particle analysis showed the N95 equivalent mask provided significant protection compared to surgical masks, which provided twice more protection than home-made masks. All masks reduced exposure to aerosols during various activities. Outward prevention did not differ between surgical and N95 equivalent masks, however there was marginal prevention using the home-made mask. Notably, protection was decreased for children for all masks, likely due to inferior fit of the mask on a smaller face. Each mask provided stable protection over time, and while the authors note that home-made masks of tea-cloth may not provide much protection against droplet nuclei particles, they can still reduce transmissibility and viral exposure.

A search for the use of HEPA filters revealed limited results. One source measured transmission of airborne particles using oxygen masks with HEPA filters compared to Hudson-type oxygen masks. Oxygen masks with HEPA filters had a notably higher ratio of ambient to intramask 0.02-1um particles, and about 60% of volunteers had ratios >100, indicating that oxygen masks with HEPA filters were far superior at protecting patients against airborne pathogens. Thus, it is likely that inserting HEPA filters into home-made masks would provide superior protection than materials used in home-made masks, such as cotton or tea cloth, provided the mask had a tight fit. A few web pages, though not reputable sources, report that inserting HEPA filters between layers of cotton should block 99.97% of particles whose diameter is equal to 0.3 micrometers if the filter is a certified HEPA filter.

Author Information

Authors: Tyler Kirchberg MS1 and Laurie Simon MS1, UC San Diego School of Medicine
Completed on: March 24, 2020
Last updated on: April 7, 2020

Reviewed by: Gary Smithson MD
Reviewed on: April 14, 2020

This summary was written as part of the CoRESPOND Earth 2.0 COVID-19 Rapid Response at UC San Diego. For more information about the project, please visit http://earth2-covid.ucsd.edu

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