This paper describes key differences between the WHO and the CDC fabric mask guidelines, including the rationale for the inclusion of hydrophobic polypropylene materials in the WHO guidance, and was initially shared on the SSRN Preprint Server on June 26, 2020.
Adopting WHO Guidance on Fabric Masks for COVID-19
Despite the many similarities between the guidance on fabric masks for COVID-19 from the World Health Organization (WHO) and those from the Centers for Disease Control (CDC), there are important differences in mask designs, materials and requirements.1,2 Both agencies describe fabric masks (aka non-medical masks or cloth face coverings) as washable multilayer face coverings that cover the mouth and nose and are intended for use as source control by the general population to reduce the spread of potentially infectious droplets in public settings where physical distancing cannot be achieved.1-3 These fabric masks are not designed to replace risk reduction strategies such as physical distancing and hand hygiene, are not recommended for use by anyone unable to remove them without assistance (e.g., children under two), are not surgical masks, and are not yet subject to the same legal, regulatory, and manufacturing controls as medical masks.1-3
Key differences between WHO and CDC fabric mask guidelines:
- The CDC recommends cotton masks with at least two layers. These recommendations are consistent with research available prior to COVID-19 on multilayer cotton masks improvised in the field from t-shirts, sweatshirts, bandanas, and tea towels.4,5
- The WHO recommends masks with at least three layers, including both hydrophilic (e.g., cotton) and hydrophobic (e.g., nonwoven polypropylene, NWPP) layers. These recommendations incorporate more recent advances in fabric technologies, using nonwovens for mechanical filtration, synthetics for electrostatic filtration, and hydrophobic materials for water-resistance.6-9
Mask Filtration and Breathability
Unlike the CDC, the WHO provides quantitative criteria to guide the selection of mask materials and layering combinations. When selecting materials for individual mask layers, the WHO recommends evaluating the filter quality factor (Q), which is a function of both filtration efficiency and breathability and remains constant for a given material regardless of the number of fabric layers evaluated. According to the guidance, a Q of 3 is the lowest acceptable value, and higher values are better.2 The highest Q materials reported by the WHO are spunbond NWPP (Q =17), cotton materials (5 < Q < 8) and polyester (Q=7).2,6 For hydrophobic materials Q can be increased by electrostatically charging the material.
When choosing layering combinations, the WHO provides a set of minimum performance standards (AFNOR SPEC S76-001).2,9 These fabric mask standards require ≥70% filtration for ≤ 3um particles or droplets (NaCl or paraffin oil), a total pressure drop of < 100 Pa across the mask to ensure breathability, and the ability to withstand ≥ 5 high temperature wash cycles (≥ 60ºC).2,9 Although fabric masks made from two spunbond NWPP layers (60 gsm each) offer adequate breathability and filtration, an innermost hydrophilic (e.g., cotton) layer can be added to absorb the users droplets without compromising breathability.2,9,10 Combinations of cotton and spunbond NWPP can also withstand high temperature washing/disinfection by boiling or steaming.2,9,10
Mask Layering Combinations
Based on available data, the WHO concludes that the ideal fabric mask requires three fabric layers and recommends a combination of hydrophilic materials (e.g. cotton) to retain droplets and hydrophobic materials (e.g. spunbond NWPP) to enhance filtration and reduce penetration of external contaminants through the mask (Table 1).
|Table 1. WHO Guidance on Ideal Fabric Mask Materials and Layers2|
|1. Innermost Layer||2. Middle Layer||3. Outermost Layer|
|Hydrophilic (e.g., cotton): retains droplets||Hydrophilic (e.g., cotton): retains droplets OR||Hydrophobic, synthetic (e.g. NWPP): reduces penetration of external contaminants through the mask|
|Hydrophobic, nonwoven (e.g. NWPP): enhances filtration|
Hydrophobic Mask Layers
The addition of hydrophobic layers to the WHO designs represents a major departure from the exclusively hydrophilic CDC designs. This difference is significant because the hydrophobic outermost layer in the WHO design allows for the possibility of an added benefit to the user beyond source control. Despite the theoretical advantages, additional research is required to determine if these benefits translate into real world advantages. In the meantime, it is important to remember that fabric masks are solely intended for use as source control and are not considered personal protective equipment in any circumstances.
Note: Cotton masks made with ≥ 2 layers according to CDC guidance can be adapted to comply with the WHO guidance for ≥ 3 layer masks with an outermost hydrophobic layer by wearing a single layer spunbond NWPP mask cover on top of the existing cotton mask.
- Centers for Disease Control and Prevention. Recommendation regarding the use of cloth face coverings, especially in areas of significant community-based transmission. 3 April 2020; accessed at https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/cloth-face-cover.html on 14 June 2020
- World Health Organization. Advice on the use of masks in the context of COVID-19: interim guidance. 5 June 2020; WHO Reference Number: WHO/2019-nCov/IPC_Masks/2020.4
- United States Food and Drug Administration. Emergency use authorization (EUA) authorizing the use of face masks for use by members of the general public. 24 April 2020; accessed at https://www.fda.gov/media/137121/download on 14 June 2020
- Dato VM, Hostler D, Hahn ME. Simple respiratory mask.Emerging infectious diseases, 2006; 12(6): 1033–1034. https://doi.org/10.3201/eid1206.051468.
- Rengasamy S, Eimer B, Shaffer RE. Simple respiratory protection–evaluation of the filtration performance of cloth masks and common fabric materials against 20-1000 nm size particles. Ann Occup Hyg, 2010; 54(7): 789–798. doi:10.1093/annhyg/meq044.
- Zhao M, Liao L, Xiao W, Yu X, Wang H, Wang Q, et al. Household materials selection for homemade cloth face coverings and their filtration efficiency enhancement with triboelectric charging. Nano Lett. 2 June 2020, XXXX, XXX, XXX-XXX. https://doi.org/10.1021/acs.nanolett.0c02211.
- Jang JY, Kim SW. Evaluation of filtration performance efficiency of commercial cloth masks. J Environ Health Sci. (한국환경보건학회지), 2015; 41(3): 203–215
- Podgórski A, Bałazy A, Gradoń L. Application of nanofibers to improve the filtration efficiency of the most penetrating aerosol particles in fibrous filters. Chem Eng Sci, 2006; 61: 6804–6815. https://doi.org/10.1016/j.ces.2006.07.022
- French Standardization Association (AFNOR). SPEC S76-001: Masque barrière. Guide d’exigence minimales, de méthode d’essais, de confection et d’usage. Accessed at https://masques-barrieres.afnor.org/home/telechargement on 14 June 2020
- Songer, J. The Big Four: Criteria for Community Mask Materials. Accessed at https://makermask.org/the-big-four-criteria-for-community-mask-materials/ on 14 June 2020