Face shield design & structure to combat Covid-19

Face shields are personal protective equipment (PPE) devices that are used by many workers (e.g., medical, dental, veterinary) for protection of the facial area and associated mucous membranes (eyes, nose, mouth) from splashes, sprays, and spatter of body fluids. Face shields are generally not used alone, but in conjunction with other protective equipment and are therefore classified as adjunctive PPE. Face shields protect medical and other emergency service personnel from biological splatter and are thought to extend the usefulness of face masks. Commercially known as infectious disease control shields and colloquially as “spit shields,” the worldwide demand for them has spiked with the recent viral pandemic.

As existing manufacturers try to meet the demand, some companies in other businesses have begun producing face shields to donate to hospitals to make up for the shortage. With cutting implements and assembly areas already available, sail makers, upholstery manufactures, etc. are able to quickly gear up for manual production. Healthcare workers’ faces have been reported to be the body part most commonly contaminated by splashes, sprays and spatter of body fluids. A face shield is classified as PPE that provides barrier protection to the facial area and related mucous membranes (eyes, nose, lips).

A face shield offers a number of potential advantages, as well as some disadvantages, compared with other forms of face/eye protection used in healthcare and related fields shown in Table 1. The millions of potential users of face shields include healthcare workers, dental providers, veterinary care personnel, laboratory workers, pre-hospital emergency medical providers, police, firefighters, and custodial staff dealing with spills and contaminated waste. Infectious diseases (e.g., severe acute respiratory syndrome (SARS), avian influenza, etc.) and severe infectious agents associated with the potential for body fluid exposures (e.g., Ebola virus), have resulted in increased attention to face/eye protection.

Face shield design and structure

Visor: Visors, also referred to as lenses or windows, are manufactured from any of several types of materials that include polycarbonate, propionate, acetate, polyvinyl chloride, and polyethylene terephthalate glycol (PETG) and come in disposable, reusable, and replaceable models acetate provides the best clarity and PETG tends to be the most economical, but polycarbonate is one of the most widely used. Polycarbonate and propionate offer better, although still somewhat imperfect, optical quality that aids in reducing eye strain

Frame: Face shield frames used in healthcare are generally made of lightweight plastic. There are a variety of frame styles, including adjustable and nonadjustable frames that fully or partially encircle the circumference of the skull or those with eyeglass-type temple bars that are worn like standard eyewear. There are also metal clip-on frames available that are designed to attach face shield visors to prescription eye- wear, and some frames allow for the visor to be flipped up when not in use.

Suspension systems: There are a variety of face shield suspension systems on the market that offer either fully or partially circumferential attachment features. Fully circumferential suspension systems include plastic headbands that are adjustable for comfort by a ratchet mechanism, pin lock systems, or velcro nonadjustable systems employ elastic straps some models utilise eyeglass type temple bars for suspension with or without eyewear-like nose pads and bridge assemblies to assist in maintaining face shield position and stability on the face. A top band that is adjusted for depth is found on some models.

Lens plate polycarbonate is the material of choice for many helmet-style face shields because of its optical clarity and impact resistance. Where the goal is mainly to protect the face and eyes against contact with bodily fluids, the material used for the lens plate can be less resistant to impact than polycarbonate as long as it maintains some optical properties. Some less costly materials include polyester (PET). Sail works of Hood River, Oregon, is using seven million thick mono film, a polyester film used by sail makers that is both clear and reasonably strong, to manufacture its emergency lens plate. The film is thin enough to cut with scissors yet sufficiently rigid to hold a shape. It probably would not protect against rioters throwing bricks, but certainly should manage to keep blood spatter at bay in an operating room.

Headband: Headbands for most PPE such as welding helmets and medical face shields are generally injection molded affairs with adjustments for head size as well as helmet height. Again, for emergency service personnel who require durable protection, this design makes sense. For medical teams, a simple foam band that rides on the forehead and holds the lens plate, coupled with an adjustable length of 1-in. wide elastic webbing, is the approach that sail works takes. The resulting product is inexpensive and disposable. Medical personnel prefer the entire assembly to be as light as possible.

Benefits
The idea behind face shields is also to extend the useful life of face masks, which too are now in great demand. Hospital workers are being asked to use the masks all day rather than disposing of them after each patient visit. The face shields in theory protect the masks from the full onslaught of viral particles encountered throughout the course of the worker’s day, enabling the masks to be used longer.

Face shield materials

Three of the materials used in face shields: acetate, PETG, and polycarbonate. The strengths of each material should be taken into account when making a selection, in order to provide the best overall solution for different applications workers encounter. There are reasons why each of these face shield materials exist and are in use: MCR Safety wants to provide you the exact PPE for your particular needs.

Acetate: Acetate face shields are used in chemical applications where optical clarity is needed. Cellulose acetate is a plant-based plastic that is hypoallergenic. Acetate was first used for eyewear in the late 1940s because of the brittleness and other problems encountered with previously used plastics.

Acetate face shields offer the following benefits:

It is well-suited for applications requiring good optical clarity and rigidity

It does not become limp or distorted under normal temperatures

It offers high impact resistance and withstands crazing or cracking over a wide range of temperatures under normal stress

It is an excellent shield for grease and oils

It maintains flexibility over long periods

PETG: PETG is thermoplastic polyester that provides excellent toughness, chemical resistance, and is easily die-cut for face shield use. PETG offers excellent protection at a competitive price point. PETG is great for face shields because:

It incredibly durable

It is highly impact-resistant

It can be sterilized

Polycarbonate: Polycarbonate is everywhere. It is a naturally transparent, amorphous thermoplastic with applications beyond eyewear lenses. However, polycarbonate has numerous benefits that make it a particularly great choice for safety eyewear:

It is able to internally transmit light almost as effectively as glass

It is lighter than glass

It possesses a natural UV filter

It can withstand impacts far greater than many other commonly used plastics

Selection of face shields
Face shields are meant to be used as barrier protection for the facial area and associated mucous membranes from airborne body fluids (blood, saliva, bronchial secretions, vomit, urine, etc.) expelled as a result of various physiological processes (vomiting, coughing, sneezing, etc.) and medical, dental, and veterinary procedures (suctioning the airway, placing nasogastric tubes, obstetrical procedures, surgery, dental procedures, etc.).

In as much as there are currently no standards for face/eye protection against biological hazards, and research data is scant, recommendations for the proper selection of face shields for infection control must rely on currently avail- able knowledge, the task to be performed and the anticipated risk associated with the procedure. The selection of the most appropriate face shield model(s) will depend on the circumstances of exposure, other PPE used concurrently, and personal vision needs.

Face shields with single velcro or elastic straps tend to be easiest to don and doff; doffing can be accomplished with a single hand. In order to be efficacious, face shields must fit snugly to afford a good seal to the forehead area and also to prevent slippage of the device. Visors manufactured from acetate, propionate, and polycarbonate offer improved visual clarity and optical quality with the potential for less eye strain. Visors that offer protection from UV light would be an important feature for individuals utilising UV light sources (e.g., dental personnel). Face shields should be selected that have visors treated for anti-glare, anti-static, and anti-fogging properties. For improved protection from infectious agents, face shields should be, at a minimum, full face length with outer edges of the face shield reaching at least to the point of the ear, include chin and forehead protectors, and cover the forehead.

Brow caps or forehead cushions should be of sufficient dimensions to ensure that there is adequate space between the wearer’s face and the inner surface of the visor to allow for the use of ancillary equipment (medical/surgical mask, respirator, eyewear, etc.). Cost effective considerations include disposable face shields v/s reusable models and those that offer replaceable parts. Although some models of industrial face shields could be used for infection control purposes (e.g., in the event of face shield shortages), they generally tend to be more expensive, heavier and bulkier than face shields used for infection control purposes.

Conclusions
Face shields are PPE that are commonly used as barrier protection for infection control purposes by numerous workers. There is no standard regarding face/eye protection from biological hazards and this deficit needs to be remedied as quickly as possible. Due to the lack of a good facial seal peripherally that can allow for aerosol penetration, face shields should not be used as solitary face/eye protection, but rather as adjunctive to other PPE.

Acknowledgement
The author would like to thank the management of GRG Institutions for their constant encouragement and motivation to carry out this work.

References
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[2] International Safety Equipment Association (ISEA):“Draft ISEA 119: Standard for eye and Face Protection against Biological Hazards.”
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[4] U.S. Patent Office: “Ellen Dempsey. Patent 737, 591. Sanitary Face Shield. 1903.”
[5] U.S. Patent Office: “James H. Bolker. Patent 3,943,575. Comfortable Surgical Hood. 1976.”
[6] U.S. Patent Office: “Darryl Dial, John M Geesbreght. Patent 4,805,639. Medical Cap with Face Shield.1989.”
[7] Occupational Health and Safety Administration (OSHA): “Blood Borne Pathogens Standard 1910.1030.”.
[8] Grainger. Quick Tips Technical Resources: “Quick Tip #373. Face shield Protection.”
[9] Beckerdite, K.: “Eye & Face Protection: Overcome the Challenges to Using Face shields. Indust. Hyg. & Safety.News. 2012.”
[10] Nighswonger, T.: “Face Up to Proper Protection. Environ Health Safety. 2000.”

Footnote:
The article is authored by Jeyaraman Anandha Kumar, who is a lecturer with the Department of Textile Processing, G.R.G.Polytechnic College, Kuppepalayam, Sarkar Samakulam, Coimbatore, Maharashtra. He can be contacted on: Email: anna_781@rediffmail.com.