Plastic face shields offer “little or no protection” from Covid-19 or other airborne viruses, a new study shows.
But surgical face masks offer a “good” level of protection, say scientists who have conducted extensive tests on different types of personal protective equipment (PPE).
Their results, presented at the Online Clinical Microbiology and Infectious Diseases (ECCMID) Congress, show that wearing a surgical face mask can provide similar protection from aerosols as wearing a respirator.
The efficiency of PPE, including face masks, has been put to the test since the outbreak of the Covid pandemic.
The virus is mainly transmitted through droplets and aerosols in poorly ventilated areas.
But at the start of the pandemic, some experts – especially in Europe – said that while surgical face masks protect others, they do not offer any significant protection for the wearer.
The new study compared 32 mask types for use in hospitals, including cloth and surgical masks, respirators and face protection.
The surgical masks contained some with EN 14683 certification (the EU quality standard) and others that were not certified.
Both FFP2 and KN95 respirators were tested. KN95 respirators that meet Chinese standards will be 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 manifold in an airtight tank.
An aerosol of the chemical diethyl hexyl sebacate (DEHS) was pumped into the tank and the aerosol particles in the collecting tube were counted with a particle counter.
The average filtration efficiency was lowest for the cloth masks (28 percent), followed by the non-certified mouthguards (63 percent) and the certified mouthguards (70 percent). The respiratory protection material KN95 filtered out 94 percent of the particles, while the FFP2 mask material filtered out 98 percent.
In a second experiment, the air pressure was measured on both sides of the mask. Surgical face masks created the lowest pressure drop and would therefore offer the lowest breathing resistance.
Ventilators produced a pressure drop two to three times higher.
In a third experiment, the filter effect of the masks worn was measured. It used a setup similar to the first experiment, but the masks were mounted on an artificial head with an artificial windpipe or windpipe instead of being attached to the air collection tube.
The dummy head was the size of an average person and had a skin-like coating to ensure a more realistic mask fit.
The cloth masks and the non-certified surgical masks had the worst filter performance when they were worn and only filtered out 11.3 percent and 14.2 percent of the particles, respectively.
It is noteworthy that the Type II surgical face masks showed similar filter results when worn (47 percent) as the KN95 respirators (41 percent) and FFP2 respirators (65 percent). The face shields had no significant effect.
The research team says that for a combination of optimal benefit and easy breathing, a mask should combine good filtration with a low pressure drop.
Study director Dr. Christian Sterr from Philipps University Marburg said: “In our tests, these parameters were achieved by most FFP2 and surgical type II face masks.
“Surgical face masks certified according to EN 14683 Type II in particular can offer a high level of protection with low airflow resistance at the same time.
“Non-certified fabric and surgical face masks offered very poor protection against our test aerosol.
“The fabric masks showed a high degree of variability between different mask types.
“It is not surprising that FFP2 respirators offered the best protection on average. KN95 respirators performed relatively poorly with a filtration efficiency of 36 to 47 percent.
“These results are inferior to the best Type II surgical face masks, which performed between 13 and 66 percent.
“In view of the higher price, the better subjective feeling of protection and the higher flow resistance of the KN95 respirators, the results are remarkable. However, our results are consistent. “
He said surgical face masks also create less breathing resistance and are therefore more comfortable to wear.
Dr. Sterr added, “In our tests, respirators had airflow resistance two to three times greater than surgical face masks.
“This could result in lower user loyalty and, consequently, a lower overall protection rate.
“So it makes sense to use surgical face masks widely in hospitals to prevent the virus from spreading, especially when distancing and quarantine are not an option.
“In situations where a patient cannot wear a mask, a surgical face mask does not seem to be sufficient to protect medical personnel from SARS-CoV-2. In such cases, respirators such as FFP2 masks should be considered.
“KN95 breathing apparatus should only be worn when no other breathing apparatus is available. Face protection masks should only be used to keep masks and respirators dry when performing procedures that involve the risk of splashing.
“The public should wear good quality certified surgical face masks rather than cloth masks or face shields that performed poorly in our study, or respirators that should be reserved for medical staff.”