The efficacy of cloth masks challenged
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In 2015, we conducted a randomized controlled trial to compare the efficacy of cloth masks with that of medical masks and controls (standard practice) among healthcare workers in Vietnam (4). Rates of infection were consistently higher among those in the cloth mask group than in the medical mask and control groups. This finding suggests that risk for infection was higher for those wearing cloth masks. The mask tested was a locally manufactured, double-layered cotton mask. Participants were given 5 cloth masks for a 4-week study period and were asked to wash the masks daily with soap and water (4). The poor performance may have been because the masks were not washed frequently enough or because they became moist and contaminated. Medical and cloth masks were used by some participants in the control group, but the poor performance of cloth masks persisted in post hoc analysis when we compared all participants who used medical masks (from the control and the medical mask groups) with all participants who used only a cloth mask (from the control and the cloth mask groups)(4).
We also examined the filtration ability of cloth masks by reviewing 19 studies (3). We found that the filtration effectiveness of cloth masks is generally lower than that of medical masks and respirators. Filtration effectiveness of cloth masks varies widely; some materials filter better than others (9–11). Filtration effectiveness of cloth masks depends on many factors, such as thread count, number of layers, type of fabric, and water resistance (3). One study tested medical masks and several household materials for the ability to block bacterial and viral aerosols. Participants made masks from different materials, and all masks tested showed some ability to block the microbial aerosol challenges although less than that of medical masks (11). Another study found that homemade cloth masks may also reduce aerosol exposure although less so than medical masks and respirators (12). Masks made of cotton and towel provide better protection than masks made of gauze. Although cloth masks are often not designed to fit around the face, some materials may fit snugly against the face. One study found that the use of nylon stockings around the mask improved filtration (A.V. Mueller et al., unpub. data, ). Filtration effectiveness of wet masks is reportedly lower than that of dry masks (3).
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I suspect bandanas are not that effective either. Since people have been required to wear masks I have not noticed any decrease in the spread of the disease. In fact, it seems to have accelerated although its potency appears to be decreasing. The latter may be the result of better treatment and also better protection for the most vulnerable.
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