Medical face masks offer self-protection

Posted by Emam Hossain Shakil on May 12th, 2021

Since the appearance of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the question regarding the efficacy of various hygiene measures and the use of personal protective equipment (PPE) has become the focus of scientific and above all public discussion. To compare respirators, medical face mask, and cloth masks and determine if it is recommendable to wear face masks to protect the individual wearer of the mask from inhaling airborne particles, we challenged 29 different masks with aerosols and tested the pressure drop as a surrogate for breathing resistance owing to the mask material. We found that Type II medical face masks showed the lowest pressure drop (12.9±6.8 Pa/cm2) and therefore additional breathing resistance, whereas respirators such as the KN95 (32.3±7.0 Pa/cm2) and FFP2 (26.8±7.4 Pa/cm2) showed the highest pressure drops among the tested masks. The filtration efficacy of the mask material was the lowest for cloth masks (28±25%) followed by non-certified face masks (63±19%) and certified medical face masks (70±10%). The materials of the different respirators showed very high aerosol retentions (KN95 [94±4%] and FFP2 [98±1%]). For evaluating the as-worn filtration performance simulating real live conditions each mask type was also tested on a standardized dummy head. Cloth masks and non-EN-certified face masks had the worst as-worn filtration efficacies among the tested masks, filtering less than 20% of the test aerosol. Remarkably, certified type II medical face masks showed similar (p>0.5) as-worn filtration results (47±20%) than KN95 masks (41±4%) and FFP2 masks (65±27%), despite having a lower pressure drop. Face shields did not show any significant retention function against aerosols in our experiment. Our results indicate that it seems recommendable to wear face masks for providing base protection and risk reduction against inhaling airborne particles, in low-risk situations. In our study, especially EN 14683 type II certified medical face masks showed protective effectiveness against aerosols accompanied by minimal additional breathing resistance. FFP2 Respirators, on the other hand, could be useful in high-risk situations but require greater breathing effort and therefore physical stress for users.

Citation: Sterr CM, Nickel I-L, Stranzinger C, Nonnenmacher-Winter CI, Günther F (2021) Medical face masks offer self-protection against aerosols: An evaluation using a practical in vitro approach on a dummy head. PLoS ONE 16(3): e0248099. doi:10.1371/journal.pone.0248099

Editor: Amitava Mukherjee, VIT University, INDIA

Received: January 19, 2021; Accepted: February 22, 2021; Published: March 3, 2021

Copyright: © 2021 Sterr et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: The authors received no specific funding for this work.

Competing interests: The authors have declared that no competing interests exist.

Background/Introduction
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is one of seven coronaviruses known to elicit infections in humans and is very similar to the SARS virus of 2002 [1,2]. From what is known to date, its transmission mainly occurs via droplets (particles >5 μm) and aerosols (particles <5 μm) in poorly ventilated settings [3,4]. Recent findings suggest that SARS-CoV-2 can be transmitted before the symptoms occur [5].

Besides social distancing and quarantine, one of the major factors preventing the spread of the virus is the practice of face covering [6]. Nurses and doctors are unable to maintain distances from their patients and are at high risks of acquiring the infection and transmitting it to others [7]. Furthermore, in a pandemic situation, their workforce is the most essential [8]. Therefore, it is crucial to provide health care workers (HCWs) with high-quality face masks or respirators to offer protection to them and their patients [8]. In the medical field, surgical or medical face masks can be differentiated from FFP or N95 respirators [9,10]. All masks and respirators are usually tested against the European standards (EN) to ensure reliable quality [11,12]. EN 149 is used for testing respirators, and EN 14683 is used for testing medical face masks [11,12]. Respirators are usually recommended for preventing airborne diseases such as tuberculosis or measles, although they have not been tested for the passage of bioaerosols according to the EN 149 standard [10,11]. The use of medical face masks is recommended for protection from infections caused by droplets [10]. At the beginning of the SARS-CoV-2 outbreak, some experts, especially in Europe, advised against the usage of medical face masks for protection against SARS-CoV-2 [6]. This might be due to the testing method of medical face masks according to the EN 14683 standard. In that norm, only the material is tested without considering the mask fit [12]. Moreover, these tests only assess third-party protection without considering self-protection properties [12]. However, these recommendations should be questioned critically. Loeb et al. showed that the protective effects of medical face masks and respirators against influenza are not significantly different [9].

In a non-pandemic situation, it is easy to understand the recommendations to wear masks, given the supply of certified medical face masks and respirators. However, the SARS-CoV-2 outbreak has led to a shortage of personal protective equipment (PPE) such as medical face masks and respirators. Moreover, many newly produced masks seemed to be of poor quality. Soon after, there were warnings of fake masks in the global market [13]. Particularly, masks labeled as KN95 were notified to the Rapid Alert System for Dangerous Non-food Products (RAPEX), a black list of the European Union. In addition, do-it-yourself (DIY) or cloth masks produced without certificates and of variable quality were available everywhere. Because of the crucial role that HCWs play and the uncertainty of the protective function of many newly offered masks, we decided to test every mask type intended for HCWs before using them in hospital. To compare respirators, medical face masks, and cloth masks and determine if it is recommendable to wear face masks to protect the individual wearer of the mask from inhaling airborne particles, we challenged 29 different masks and 3 face shields with di-ethyl-hexyl-sebacat (DEHS) aerosols and tested pressure drops over the mask materials.