Surgical Face Masks Provide Good Protection Against Aerosols – Plastic Face Shields Provide No Protection

By European Society of Clinical Microbiology and Infectious Diseases June 23, 2021

New research presented at this year’s European Congress of Clinical Microbiology and Infectious Diseases (ECCMID), held online this year (July 9-12), shows that wearing a surgical face mask can provide the same protection against aerosols as wearing a surgical facemask. wearing a gas mask. However, face shields offer little or no protection.

The efficacy of personal protective equipment (PPE), including face masks, has been at the center of scientific and public interest since the emergence of SARS-CoV-2, a virus primarily transmitted via droplets and aerosols in poorly ventilated environments. It is critical to provide healthcare professionals with high quality face masks or respirators to protect both themselves and their patients. However, at the start of the pandemic, some experts, particularly in Europe, said that while surgical face masks protect others, they do not provide significant protection to the wearer. In addition, many newly produced masks turn out to be of poor quality.

The study, by Dr. Christian Sterr and colleagues from Philipps University Marburg, Marburg, Germany, compared 32 types of masks intended for hospital use, including cloth and surgical (medical) masks, respirators and face shields. The surgical masks included some with EN 14683 certification (the EU quality standard) and others that were not certified. Both FFP2 and KN95 respirators were tested. KN95 respirators, which comply with Chinese standards, were subject to EU RAPEX safety warnings from April 2020.

In the first experiment, the filtration efficiency of the mask material was measured. Each mask was attached to an air collection tube in an airtight tank. An aerosol of the chemical diethyl-hexyl-sebacat (DEHS) was pumped into the tank and the aerosol particles in the collection tube were counted by a particle counter.

The mean filtration efficiency was lowest for the cloth masks (28%), followed by the non-certified surgical masks (63%) and the certified surgical masks (70%). The KN95 breathing material filtered 94% of the particles and the FFP2 mask material 98%.

In the second experiment, the air pressure was measured on both sides of the mask. Surgical face masks produced the lowest pressure drop and thus would have the least resistance to breathing – type II surgical masks produced a pressure drop of 12.9 Pa/cm², while non-certified surgical masks produced a pressure drop of 16.2 Pa/cm².

Respirators produced two to three times higher pressure drops (26.8 Pa/cm² for FFP2 and 32.3 Pa/cm² for KN95). The results for the cloth masks varied between 6.9 and 149.3 Pa/cm².

In the third experiment, the filtration efficiency of the masks was measured while wearing. It used a similar setup to the first experiment, but the masks were mounted on a dummy head with an artificial trachea or trachea, rather than being attached to the air-collecting tube. The artificial head was the size of the average person in the US and had a skin-like coating to give a more realistic mask fit.

The cloth masks and the non-certified surgical masks had the worst filtration efficiency when worn, filtering only 11.3% and 14.2% of the particles, respectively. Notably, the type II surgical face masks had comparable filtration results (47%) as the KN95 respirators (41%) and FFP2 respirators (65%). The face shields had no significant effect.

Experiments 1 and 3 focused on 0.5 m particles – a size considered realistic for SARS-CoV-2 particles, which clump together.

The study authors say that for optimal benefit and ease of breathing, a mask must combine good filtration with a low pressure drop. They add: “In our tests, these parameters were achieved by most FFP2 and surgical type II face masks.

“In particular, EN 14683 type II certified surgical face masks can provide high protection with low airflow resistance at the same time. Non-certified fabrics and surgical face masks provided very poor protection against our test aerosol. The cloth masks showed high variability between different mask types.

“Not surprisingly, FFP2 respirators had the best protective effect on average. KN95 respirators performed relatively poorly, with filtration efficiency ranging from 36% to 47%. These results are inferior to those of the best surgical type II face masks, whose performance ranged from 13% to 66%. The results are remarkable given the higher price, better subjective sense of protection and higher airflow resistance of the KN95 respirators. However, our findings are consistent with the RAPEX warnings.”

Surgical face masks also produce less resistance when breathing, so are more comfortable to wear, says Dr. star.

He adds: “In our tests, respirators had two to three times higher airflow resistance than surgical face masks. This can lead to a lower patient compliance and therefore a lower overall protection rate. Therefore, it seems reasonable to widely use surgical face masks in hospitals to prevent the spread of the virus, especially when distancing and quarantine are not possible.

“In situations where a patient cannot wear a mask (e.g. intubation), a surgical face mask does not appear to be sufficient to protect the healthcare provider against SARS-CoV-2. In such cases, respirators such as FFP2 masks should be considered. KN95 respirators should only be worn when no other respirators are available. Face shields should only be used to keep masks and respirators dry during procedures where there is a risk of splashing.

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

Reference: “Medical face masks provide self-protection against aerosols: an evaluation using a practical in vitro approach on a dummy head” by Christian M. Sterr, Inga-Lena Nickel, Christina Stranzinger, Claudia I. Nonnenmacher-Winter, and Frank Günther, March 3, 2021, PLOS ONE.
DOI: 10.1371/journal.pone.0248099