Maximizing Fit for Cloth and Medical Procedure Masks to Improve Performance and Reduce SARS-CoV-2 Transmission and Exposure, 2021

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Universal masking is one of the prevention strategies recommended by CDC to slow the spread of SARS-CoV-2, the virus that causes coronavirus disease 2019 (COVID-19) (1). As of February 1, 2021, start highlight38end highlight states and the District of Columbia had universal masking mandates. Mask wearing has also been mandated by executive order for federal property* as well as on domestic and international transportation conveyances.^† Masks substantially reduce exhaled respiratory droplets and aerosols from infected wearers and reduce exposure of uninfected wearers to these particles. Cloth masks^§ and medical procedure masks^¶ fit more loosely than do respirators (e.g., N95 facepieces). The effectiveness of cloth and medical procedure masks can be improved by ensuring that they are well fitted to the contours of the face to prevent leakage of air around the masks’ edges. During January 2021, CDC conducted experimental simulations using pliable elastomeric source and receiver headforms to assess the extent to which two modifications to medical procedure masks, 1) wearing a cloth mask over a medical procedure mask (double masking) and 2) knotting the ear loops of a medical procedure mask where they attach to the mask’s edges and then tucking in and flattening the extra material close to the face (knotted and tucked masks), could improve the fit of these masks and reduce the receiver’s exposure to an aerosol of simulated respiratory droplet particles of the size considered most important for transmitting SARS-CoV-2. The receiver’s exposure was maximally reduced (>95%) when the source and receiver were fitted with modified medical procedure masks. These laboratory-based experiments highlight the importance of good fit to optimize mask performance. Until vaccine-induced population immunity is achieved, universal masking is a highly effective means to slow the spread of SARS-CoV-2** when combined with other protective measures, such as physical distancing, avoiding crowds and poorly ventilated indoor spaces, and good hand hygiene. Innovative efforts to improve the fit of cloth and medical procedure masks to enhance their performance merit attention.

At least two recent studies examined use of mask fitters to improve the fit of cloth and medical procedure masks. Fitters can be solid (2) or elastic (3) and are worn over the mask, secured with head ties or ear loops. The results indicated that when fitters are secured over a medical procedure mask, they can potentially increase the wearer’s protection by ≥90% for aerosols in the size range considered to be the most important for transmitting SARS-CoV-2 (generally <10 μm). Other studies found that knotting and tucking a medical procedure mask or placing a sleeve made of sheer nylon hosiery material around the neck and pulling it up over either a cloth or medical procedure mask (3,4) also significantly improved the wearer’s protection by fitting the mask more tightly to the wearer’s face and reducing edge gaps. A recent expert commentary (5) proposed double masking as another means to improve the fit of medical procedure masks and maximize the filtration properties of the materials from which they are typically constructed, such as spun-bond and melt-blown polypropylene. Based on experiments that measured the filtration efficiencies of various cloth masks and a medical procedure mask (6), it was estimated that the better fit achieved by combining these two mask types, specifically a cloth mask over a medical procedure mask, could reduce a wearer’s exposure by >90%.

During January 2021, CDC conducted various experiments to assess two methods to improve medical procedure mask performance by improving fit and, in turn, filtration: 1) double masking and 2) knotting and tucking the medical procedure mask (Figure 1). The first experiment assessed how effectively various mask combinations reduced the amount of particles emitted during a cough (i.e., source control) in terms of collection efficiency. A pliable elastomeric headform was used to simulate a person coughing by producing aerosols from a mouthpiece (0.1–7 μm potassium chloride particles) (7). The effectiveness of the following mask configurations to block these aerosols was assessed: a three-ply medical procedure mask alone, a three-ply cloth cotton mask alone, and the three-ply cloth mask covering the three-ply medical procedure mask (double masking). The second experiment assessed how effectively the two modifications to medical procedure masks reduced exposure to aerosols emitted during a period of breathing. Ten mask combinations, using various configurations of no mask, double masks, and unknotted or knotted and tucked medical procedure masks, were assessed (e.g., source with no mask and receiver with double mask or source with double mask and receiver with no mask). A knotted and tucked medical procedure mask is created by bringing together the corners and ear loops on each side, knotting the ears loops together where they attach to the mask, and then tucking in and flattening the resulting extra mask material to minimize the side gaps^†† (Figure 1). A modified simulator with two pliable elastomeric headforms (a source and a receiver) was used to simulate the receiver’s exposure to aerosols produced by the source (8). In a chamber approximately 10 ft (3.1 m) long by 10 ft wide by 7 ft (2.1 m) high, which simulated quiet breathing during moderate work, the source headform was programmed to generate the aerosol from its mouthpiece at 15 L/min (International Organization for Standardization [ISO] standard for a female performing light work), and the receiver headform’s minute ventilation was set at 27 L/min (ISO average of a male or female engaged in moderate work).^§§ For each of the 10 masking configurations, three 15-minute runs were completed.

Results from the first experiment demonstrated that the unknotted medical procedure mask alone blocked start highlight56.1%end highlight of the particles from a simulated cough (standard deviation [SD] = start highlight5.8end highlight), and the cloth mask alone blocked start highlight51.4%end highlight (SD = start highlight7.1end highlight). The combination of the cloth mask covering the medical procedure mask (double mask) blocked start highlight85.4%end highlight of the cough particles (SD = start highlight2.4end highlight), start highlightand the knotted and tucked medical procedure mask blocked 77.0% (SD = 3.1)end highlight.

In the second experiment, adding a cloth mask over the source headform’s medical procedure mask or knotting and tucking the medical procedure mask reduced the cumulative exposure of the unmasked receiver by 82.2% (SD = 0.16) and 62.9% (SD = 0.08), respectively (Figure 2). When the source was unmasked and the receiver was fitted with the double mask or the knotted and tucked medical procedure mask, the receiver’s cumulative exposure was reduced by 83.0% (SD = 0.15) and 64.5% (SD = 0.03), respectively. When the source and receiver were both fitted with double masks or knotted and tucked masks, the cumulative exposure of the receiver was reduced 96.4% (SD = 0.02) and 95.9% (SD = 0.02), respectively.

Corresponding author: John T. Brooks, zud4@cdc.gov.

¹CDC COVID-19 Emergency Response Team. ²Health Effects Laboratory Division, National Institute for Occupational Safety and Health, CDC.

References

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3. Clapp PW, Sickbert-Bennett EE, Samet JM, et al.; CDC. Evaluation of cloth masks and modified procedure masks as personal protective equipment for the public during the COVID-19 pandemic. JAMA Intern Med. Epub Dec 10, 2020. https://doi.org/10.1001/jamainternmed.2020.8168 PMID:33300948
4. Mueller AV, Eden MJ, Oakes JM, Bellini C, Fernandez LA. Quantitative method for comparative assessment of particle removal efficiency of fabric masks as alternatives to standard surgical masks for PPE. Matter 2020;3:950–62. https://doi.org/10.1016/j.matt.2020.07.006 PMID:32838296
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FIGURE 1. Masks tested, including A, unknotted medical procedure mask; B, double mask (cloth mask covering medical procedure mask); and C, knotted/tucked medical procedure mask

FIGURE 2. Mean cumulative exposure* for various combinations of no mask, double masks, and unknotted and knotted/tucked medical procedure masks^†

* To an aerosol of 0.1–7 μm potassium chloride particles (with 95% confidence intervals indicated by error bars) measured at mouthpiece of receiver headform configured face to face 6 ft from a source headform, with no ventilation and replicated 3 times. Mean improvements in cumulative exposures compared with no mask/no mask (i.e., no mask wearing, or 100% exposure) were as follows: unknotted medical procedure mask: no mask/mask = 7.5%, mask/no mask = 41.3%, mask/mask = 84.3%; double mask: no mask/mask = 83.0%, mask/no mask = 82.2%, mask/mask = 96.4%; knotted/tucked medical procedure mask: no mask/mask = 64.5%, mask/no mask = 62.9%, mask/mask = 95.9%.

^† Double mask refers to a three-ply medical procedure mask covered by a three-ply cloth cotton mask. A knotted and tucked medical procedure mask is created by bringing together the corners and ear loops on each side, knotting the ears loops together where they attach to the mask, and then tucking in and flattening the resulting extra mask material to minimize the side gaps.