To the Editor—The importance of nonpharmaceutical interventions in preventing the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is clearly established, and their ongoing improvement must include all available expertise within science, medicine, and engineering. The nonpharmaceutical intervention of masking, specifically the potential incremental benefit of wearing 2 or more masks, is currently receiving considerable attention. ^{Reference Wu1} When asked about the possible benefit of wearing 2 masks during a January 25, 2021, Today Show interview, Dr Anthony Fauci responded, “So, if you have a physical covering with one layer, you put another layer on, it just makes common sense that it likely would be more effective.” Subsequent news stories have provided further perspectives on this concept; most create a sense of probable benefit and no potential harm. ^{Reference Godoy2} In addition, a recent limited study suggested the benefit of improving mask fit and decreasing leakage by wearing a cloth mask over a poorly fitting surgical mask based upon controlled bench tests. ^{Reference Brooks, Beezhold and Noti3} More fully understanding the potential risks and benefits of double masking is very relevant for both the general public and for providers practicing in current and future clinical environments subject to personal protective equipment scarcity.

Although the overall benefit of simple public masking is well accepted, the degree of individual benefit is determined by several variables including mask materials, design, cleanliness, fit, and the technique used for placement and removal. All cloth, surgical, and medical masks (referred to as surgical masks in this letter) are filters through which some, but not all, of a user’s respiratory airflow passes. Grinshpun et al ^{Reference Grinshpun, Haruta, Eninger, Reponen, McKay and Lee4} demonstrated that 5–6 times more contaminants reach users through leakage around surgical masks versus those which pass through the mask’s filter media. Drewnick et al ^{Reference Drewnick, Pikmann, Fachinger, Moormann, Sprang and Borrmann5} have reported additional results emphasizing the importance of leakage. The ratio of airflow leaking around versus passing through the mask is determined in part by the mask’s resistance to airflow and the related pressure difference across the mask: the higher the resistance and associated pressure difference for a given inhalational airflow, the greater the amount of air that will leak or be shunted around the mask and into the airway. The same problem could occur during exhalation and thus impair the mask’s protection of others. We recently presented a mathematical analysis of a similar potential problem when surgical masks are worn over N95 filtering facepiece respirators. ^{Reference Mueller, Karimi, Poterack, Seville and Tipton6} Unlike N95 filtering facepiece respirators, surgical masks have no intended true seal between the mask edge and the face, making shunting or leakage around the mask edges an expected design characteristic.

The incremental benefit of the increased filtration efficiency created by using multiple masks could be negated or even exceeded by the incremental harm of increased leakage around the masks. That is, additional masks might provide better filtration of a reduced fraction and cause an increase in the unfiltered fraction of total airflow (Fig. 1). Accurately determining the net protective effect of beneficial versus harmful factors in a 2-mask scenario is a significant engineering and fluid mechanics problem. Attempts to understand SARS-CoV-2 transmission problems such as this one must recognize the complex and nonintuitive nature of aerosol and airflow physics. ^{Reference Mittal, Ni and Seo7} The net effect could vary with individual mask designs, minute ventilation, airway pressures, facial anatomy, and facial movement. It is also important that empirical and analytical models recognize the cyclical, time-variable nature of respiratory airflow, and that peak impulses of pressure and flow will create the intervals of maximum leakage. These variables and possibly others will determine the concerning fraction of respiratory airflow that passes between the edges of a surgical mask and the face. Additional experimental and analytical investigations are necessary to produce an evidence-based assessment of the risks and benefits of double masking.

Fig. 1. Schematic diagram of single and double masking, displaying the possible scenario of increased respiratory airflow leakage due to increased mask resistance.