Face Shields Called Flawed Protection Against Pathogens

Surgical masks offer better protection against aerosolized particles than face shields or even better than face shields and surgical masks used together, study finds.

Wearing a surgical mask along with a face shield affords health care professionals only statistically insignificant better protection against aerosolized particles than wearing a surgical mask alone, according a study in the American Journal of Infection Control (AJIC).

“Surgical masks alone provided good protection, surpassing the protection provided by face shields alone,” write investigators with the Detroit Medical Center. “Both used together provided the best protection, although the combined protection was similar to surgical masks use alone.”

Infection Control Today® has extensively covered this topic and experts are in agreement that infection preventionists and health care professionals need to wear masks; preferably N95 respirators but even surgical masks—the masks used in this AJIC study—will offer better protection than just face shields alone, or the combination of face shields and surgical masks.

As the study explains: “Small respiratory particles from a patient coughing can remain airborne and spread widely through a room, leaving health care workers and others vulnerable to becoming infected through inhalation, especially if in the room for an extended period of time. The rate of airflow in a room will also affect how far droplets can travel.”

As the study states, other research also questions just how much protection a face shield affords. An article published last April in ICT® noted: “Face shields aren’t perfect…. The absence of a good seal around the face could allow aerosol penetration. Also, they might cause fogging or glare.” However, as also noted in that article, face shields were one of the means to alleviate the drastic shortage of face masks in the United States at the beginning of the pandemic.

Investigators in the AJIC study simulated what would happen if a sick person and someone not infected were to have a conversation in a room.

“To simulate a human source, an aerosol generating system was made using a bacterial suspension in a nebulizer attached to an oxygen cylinder,” the study states. “A fan connected to the nebulizer created aerosols. Transmitted aerosols were detected using blood agar plates at 0.1524 and 1.8288 meters from source, simulating exposed person. The study was performed under controlled conditions at room temperature in a biohazard hood with high-efficiency particulate air (HEPA) filter and UV light.”

They used various combinations: sick person without a surgical mask, with a surgical mask alone, with a face shield alone, and finally with both a surgical mask and a face shield. The simulated well person would do the same and just what the two “people” wore at any one time were investigated. For instance, in one simulation, the sick person would be wearing both surgical mask and face shield while the well person would be without any facial covering. There were 17 combinations in all.

“Growth score was assigned to each scenario based on colony count on the agar plate,” the study states. “Increasing the distance between the 2 subjects decreased the colony count. Highest growth score was obtained when none of the subjects used any protective measure and the lowest growth score was achieved when both subjects used mask or mask with face shield.”

After each simulation, the air inside the room was let out through a HEPA filter for 5 minutes. “Culture plates were incubated at 35°C for 18 hours to allow the growth of bacteria and formation of visible colonies,” the study states. “The number of colonies on the culture media was used to evaluate the efficiency of mask and/or face shield wear by the sick person in reducing the transmission of aerosols. In some simulations the effect of mask and/or face shield in protecting a simulated listener in close proximity to the emission source (sick patient) was evaluated by protecting the culture plates with mask and/or face shield.”

Investigators pointed to the limitations of the study. First, there were no human subjects involved. The “listener,” a petri plate could not actually inhale. Also, the simulated speech could not take into account the different speaking patterns and volumes of different people. Also, a bacterial solution was used, not a virus.

“Recent information has indicated that SARS CoV-2 is transmitted by aerosols and droplets, and we could not simulate various droplet sizes,” the study states. “Virus quantification in this type of experiment would be very cumbersome and use of SARS CoV-2 was not possible due to safety requirements. Thus, bacteria better served the purpose of this model.”

They also note that aspects of face shields make them more user friendly than surgical masks. They are cooler and less suffocating. They offer eye protection, prevent somebody from touching their face and can prolong the use of surgical masks if both are worn together.

“Professionals working in high-risk settings with exposure to aerosols and droplets, such as dentists, would benefit from the use of face shields, or perhaps goggles, which this experiment could not simulate, as added eye protection,” the study states.

That said, though, health care professionals should not assume that face shields alone will afford enough protection.

“The mask alone provides such a significant improvement that it is difficult to tell, based on this simple simulation, whether the protection afforded by a surgical mask combined with a face shield is improved,” the study states. “Face shields might provide eye protection if when worn as a supplement to the mask, but the role of the eyes in transmission of SARS CoV-2 is not well established.”