Surgical masks offer good protection against aerosols, but not cloth masks or face shields

Wearing a surgical face mask provides a high protective effect against aerosols while posing low airflow resistance, in contrast to plastic face shields which offer little to no protection, a study reveals.

Despite WHO recommendation and mounting body of evidence supporting the use of face mask, controversy has arisen with many believing that surgical face masks protect others (from the wearer who may be ill) but not the wearer. To compound the matter further, many face masks available on the market appear to be of poor quality.

Dr Christian Sterr, along with colleagues from the Philipps University of Marburg in Marburg, Germany, compared 32 types of mask, including cloth masks, surgical masks, respirators (including FFP2 and KN95), and face shields. Some of the surgical masks were EU quality standard certified (EN 14683 certification) while the others were not. [PLoS ONE 2021;doi:10.1371/journal.pone.0248099]

In the first experiment, the researchers evaluated the filtration efficacy of the material the mask was made of. Each mask was fitted to an air-collecting tube in a tank containing aerosol of the chemical di-ethyl-hexyl-sebacat (DEHS). The aerosol particles passing through the collecting tube were then measured by a particle counter.

Among the different masks tested, cloth masks showed the lowest filtration efficacy against aerosols on average (28 percent), followed by non-certified surgical masks (63 percent) and certified surgical masks (70 percent). Filtration efficacy, as expected, was highest for respirators: 94 percent for KN95 and 98 percent for FFP2.

The researchers subsequently went on to assess the resistance to breathing due to mask wearing by measuring the changes in air pressure.

Surgical masks caused the least drop in air pressure, and thus would be the least likely to impede breathing: pressure dropped by 12.9 Pa/cm² for type II surgical masks and by 16.2 Pa/cm² for non-certified surgical masks. In comparison, the pressure drop with respirators was two to three times greater (by 26.8 and 32.3 Pa/cm² for FFP2 and KN95, respectively).The findings for cloth masks varied widely from 6.9 to 149.3 Pa/cm².

To assess the mask filtration efficacy as worn, the researchers simulate real-life wearing by mounting the mask on a dummy head with artificial windpipe, where a particle counter was placed.  

Interestingly, type II surgical masks produced as-worn filtration efficacy (47 percent) that was similar to KN95 respirators (41 percent) and FFP2 respirators (65 percent). Non-certified surgical masks and cloth masks showed the worst as-worn filtration results, at 14.2 percent and 11.3 percent, respectively.

In contrast, face shields did not have any significant protective function against aerosols.

Ideally, a mask should combine effective filtration with a minimal drop in air pressure, according to the authors. “In particular, EN 14683 type II certified medical face masks can provide a high protective effect with low airflow resistance at the same time. Non-certified cloth and medical face masks provided protective effects against our test aerosol; however, these effects were very poor.”

“In our tests, respirators had two to three-fold higher airflow resistances than surgical face masks.  This might lead to lower user adherence and, consequently, to a lower overall protection rate. Therefore, it seems reasonable to widely use surgical face masks in hospitals to prevent the virus from spreading, especially if distancing and quarantining are not possible,” said Sterr.

"In situations where a patient cannot wear a mask (e.g. intubation), a surgical face mask does not seem sufficient to protect the healthcare worker from SARS-CoV-2. In such cases, respirators such as FFP2 masks should be considered. KN95 respirators should be worn only if other respirators are not available,” he added. “Face shields should only be used to keep masks and respirators dry when undertaking procedures in which there is a risk of splashes.”

“Members of the public should wear certified surgical face masks of good quality, rather than cloth masks or face shields, which performed poorly in our study, or respirators, which should be reserved for medical staff,” he advised.