Abstract
A large variety of disposable face masks have been produced since the onset of the COVID-19 pandemic. Decreased resistance to inspiration improves adherence to the use of the mask; the so called breathability is usually estimated by the measurement of air flow across a section of the tissue under a given pressure difference. We hypothesized that the mask pressure—flow relationship studied in conditions that mimic tidal breathing could allow a more comprehensive characterization of airflow resistance, a major determinant of mask comfort. A physical analog was made of a plaster cast dummy head connected through a pneumotachograph to a series of bellows inflated/deflated by a respirator. Pressure was measured at the mock airway opening over which the mask was carefully secured. The precision of the measurement equipment was quantified using two estimates of measurement error: repeatability coefficient (RC) and within-mask coefficient of variation (CVwm). The airflow resistance of 10 surgical masks was tested on 4 different days. Resistance means did not differ significantly among four repeated measures (0.34 hPa.s.L−1; 0.37 hPa.s.L−1; 0.37 hPa.s.L−1; and 0.37 hPa.s.L−1; p = 0.08), the estimated RC was 0.08 hPa.s.L−1 [95%CI: 0.06–0.10 hPa.s.L−1], and CVwm was 8.7% [95%CI: 1.5–12.2%]. Multiple comparisons suggest the presence of a learning effect by which the operator reduced the error over the course of repetitive resistance measurements. Measurement precision improved considerably when the first set of measures was not taken into account [RC ~ 0.05 hPa.s.L−1 (95%CI: 0.03–0.06 hPa.s.L−1); CVwm~4.5% (95%CI: 1.9–6.1%)]. The testing of the face mask resistance (R) appears simple and highly repeatable in conditions that resemble tidal breathing, once operator training was assured. The procedure adds further to the current standard assessment of breathability and allows estimating the maximal added respiratory load, about 10–20% of the respiratory resistance reported in heathy adult subjects.
Highlights
The SARS-CoV-2 pandemic has led to the generalized use of face covering materials to minimize respiratory transmission of the disease
Surgical mask air flow resistance has been measured on a dummy head under conditions that simulate quiet tidal breathing in an adult subject
The model described here resembles the Sheffield dummy head that has been developed for the validation of filtering face pieces and respirators in the context of airway protection of workers in dusty environments (Mogridge et al, 2016)
Summary
The SARS-CoV-2 pandemic has led to the generalized use of face covering materials to minimize respiratory transmission of the disease. Saliva droplet projection was identified as the major route for respiratory transmission for which surgical masks appear to provide efficient protection compared to face piece respirators, with furthering the knowledge in COVID mechanisms, aerosols may represent a possible route especially at the bedside of COVID patients, where ambient air may contain high concentration of viral particles (Sommerstein et al, 2020). One attempts to determine an optimal compromise between efficient particulate filtration and ease of wear, referred to as breathability (Aydin et al, 2020; Ju et al, 2021), which is related to the added respiratory load. The end point in conceiving any protection material is optimal compromise between filtering efficiency and breathability. The fact that, in the long term, mask comfort significantly contributes to a subject’s adherence to its use justifies more detailed studies of mask mechanics under conditions that resemble tidal breathing
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