The use of personal protective equipment (PPE) is a widespread public health measure to reduce the transmission of respiratory pathogens. The two most common barriers are face masks and face shields. A face mask, such as a cloth covering or a surgical mask, fits snugly over the nose and mouth. A face shield is a transparent, curved piece of plastic extending from the forehead to below the chin, covering the entire face. The core question concerns the comparative effectiveness of these two barriers in reducing transmission.
The Mechanism of Protection
The fundamental difference between masks and shields lies in their mechanism for blocking respiratory particles. Face masks rely on filtration, where the material acts as a porous sieve to capture and trap droplets and aerosols. This filtration provides protection to the wearer and, more importantly, acts as “source control” by preventing the wearer’s expelled particles from escaping.
Face shields operate on the principle of deflection and barrier protection. The clear plastic provides a physical wall that blocks the direct trajectory of large droplets expelled during a cough or sneeze. This mechanism prevents a direct spray from reaching the wearer’s face, including the eyes.
The critical distinction is that masks capture particles, while shields deflect them. A mask must be made of materials with specific filtration efficiencies, such as non-woven polypropylene or tightly woven cotton. A shield’s performance is mainly determined by its shape, size, and how completely it wraps around the face. The primary function of a face shield is to safeguard the wearer, particularly the eyes, from incoming splashes and sprays.
Comparative Effectiveness Against Respiratory Droplets
When comparing performance against large respiratory droplets, masks demonstrate a superior ability to achieve source control. Studies have shown that a cough or sneeze launches a high-velocity jet of droplets, which a mask effectively slows and traps. Even simple cloth masks significantly reduce the forward spread and total volume of droplets released.
A face shield is effective at blocking the initial, direct impact of a droplet spray. Research suggests a face shield can reduce immediate viral exposure to a wearer by a substantial amount, sometimes as high as 96% at close range. However, the shield’s open design allows droplets to be carried by air currents around the plastic barrier.
This leakage means that while the shield protects the wearer, it does little to contain the wearer’s own expelled respiratory plume. Studies found that a face shield alone blocked only a small percentage of droplets, allowing the vast majority to escape around the sides and bottom. Therefore, for protecting others—the primary goal of public masking—a face shield used alone is significantly less effective than a mask. A well-fitting mask forces exhaled air through the filter material, greatly limiting droplet dispersion.
Aerosol Transmission and Leakage Gaps
The limitations of face shields become pronounced when considering aerosol transmission, which involves tiny particles that remain suspended in the air for extended periods. Since a face shield does not seal against the face, it cannot filter the air the wearer inhales or exhales. The large gaps at the sides and bottom allow air containing fine aerosols to move freely into and out of the space beneath the shield.
When a person wearing a face shield coughs, the air and suspended aerosols are immediately vented out through the open space beneath the chin. This results in an aerosol plume that can easily circulate and expose others nearby. Any protection offered by shields against inhaled aerosols diminishes rapidly over time and with prolonged exposure.
Masks, even those with lower filtration efficiency like surgical or cloth masks, are fundamentally better at mitigating aerosol transmission due to their close fit and filtration mechanism. However, a poor fit severely compromises a mask’s aerosol protection. If a mask does not seal tightly around the nose and mouth, unfiltered air and aerosols can bypass the material through gaps, known as “leakage jets.” A poor seal can drastically reduce the mask’s overall effectiveness against fine airborne particles.
Practical Guidance for Different Settings
The scientific consensus supports using a face mask as the primary form of respiratory source control in most public settings. In crowded indoor environments, where physical distance is difficult to maintain, a well-fitting mask (surgical, cloth, or N95/KN95) is the superior choice. This is because the mask’s filtration and snug fit directly address the risks associated with both droplets and aerosols.
A face shield is not recommended as a substitute for a mask for general community use, but it does have specific applications. It can be a beneficial supplement when worn in addition to a mask, providing extra protection for the eyes against splashes and large droplets. Face shields are also useful when visual communication is necessary, such as for people who are deaf or hard of hearing.
For individuals who cannot wear a mask due to medical contraindications, a face shield may be an acceptable alternative, but only if physical distancing is strictly maintained. Public health guidance recommends prioritizing the use of a face mask for respiratory protection. When a face shield is used, it should be designed to wrap around the face to minimize the open gaps that compromise its effectiveness.