Surgical mask

Although the material of which surgical masks are made will filter out some viruses and bacteria by trapping the aerosol suspended in breathed air, they only provide partial protection from airborne diseases because of the typically loose fit between the mask edges and the wearer's face. Surgical masks are distinct from filtering respirators, such as those made to the American N95 standard, which are more airtight and purposefully designed to protect against finer airborne particles.

Evidence from randomized controlled trials that surgical masks reduce infection from diseases such as influenza is weak. Although a recent very large (over 300,000 people) study found some evidence that they reduced transmission in the community, surgical masks can vary greatly in quality which may make these studies less useful. The effect of surgical masks is partially attributed to filtering out some of aerosol particles that are how airborne diseases are transmitted. Surgical masks are highly variable but the material of which they are made typically filter out more aerosol particles than do cloth masks but much less than does the material of which N95, FFP2 and similar masks, are made. This combined with the poor fit suggests that surgical masks offer some protection to airborne diseases such as COVID-19 but less than do N95, FFP2 and similar masks.

There are standards for the materials masks are made from. For example, the European EN 14683 Type II standard requires the material of the mask to filter particles (mean diameter close to 3 micrometres) containing the bacterium Staphylococcus aureus. The bacterial filtration efficiency of the mask material is the fractional reduction in the number of colony-forming units (CFUs) when the aerosol is passed through the material. For a Type II mask under this standard, the material must filter enough of the aerosol particles containing the bacteria to achieve a CFU reduction of at least 98%.

ASTM International has an F2100 standard with similar bacterial filtering standard to the European Type II standard but in addition uses a test aerosol of 0.1 micrometre particles. The Level 3 standard F2100 standard requires that these particles must be filtered out with at least 98% efficiency. Euro II and F2100 tests measure filter efficiency in a sealed environment - failing to account for air leakage when worn. Personal protection equipment standards such as the FFP test both ideal and worn conditions to account for air leakage.

Surgical masks are made of a nonwoven fabric created using a melt blowing process. They came into use in the 1960s and largely replaced cloth facemasks in developed countries. The colored (usually dark blue, green, or occasionally yellow) side of the mask (fluid-repellant layer) is to be worn outwards, and the white side (absorbent layer) inwards.

In some East Asian countries, masks have often customarily been worn by people who are sick in order to avoid spreading it, to protect against air pollution or allergens, as a fashion statement, or to deter social interaction. The use of surgical masks during the COVID-19 pandemic was a subject of debate, as mask shortage was a central issue.

Function

A surgical mask serves as a mechanical barrier that interferes with direct airflow in and out of respiratory orifices (i.e. nose and mouth). Most commonly used surgical masks are designed to only trap respiratory droplets, and therefore do not filter or block fine airborne particles that are smaller than the designed filtration ratings, which may be transmitted by coughs, sneezes, unintentional spitting during talking, or certain aerosol-generating medical procedures (e.g. bronchoscopy, laryngoscopy or dental procedures). Surgical masks also cannot provide complete protection from germs and other contaminants because of the often loose fit between the mask edges and the wearer's face, especially when the mask is worn outright incorrectly (e.g. low with the nose and/or mouth exposed).

A surgical mask is a disposable device that creates a physical barrier between the respiratory tract openings (nose and mouth) of the wearer and potentially pathogenic contaminants in the immediate environment. If worn properly, surgical masks are meant to help block out most (if not all) large-particle droplets, splashes, sprays, or splatter that may contain viruses and bacteria, keeping them from entering the wearer's nose and mouth., and conversely are also effective barriers for retaining large droplets released from the wearer's the mouth and nose. Surgical masks help reduce exposure of the wearer's saliva and respiratory secretions to others that could otherwise travel up to 7.9 metres (26 ft). Surgical mask also remind wearers not to touch their mouth or nose, which could otherwise transfer viruses and bacteria after having touched a contaminated surface.

A surgical mask is not to be confused with a respirator (which is specifically rated for sub-micron particles) and is not certified as such. Surgical masks are not designed to protect the wearer from inhaling airborne bacteria or virus particles and are less effective than respirators, which are designed for this purpose. Collection efficiency of surgical mask filters can range from less than 10% to nearly 90% for different manufacturers' masks when measured using the test parameters for NIOSH certification. However, a study found that even for surgical masks with "good" filters, 80–100% of subjects failed an OSHA-accepted qualitative fit test, and a quantitative test showed 12–25% leakage.

Modern surgical masks are made from paper or other non-woven material and should be discarded after each use.

Physical form

The design of the surgical masks depends on the intended usages. Usually, the masks are rectangular shaped with pleats to allow the wearer to expand and curve the mask so it can better cover the entirety of the area from the nose to around the chin. The outward-facing side of the mask is typically colored (usually blue, green, or yellow) and made thicker, tougher, and water impermeable. The inner layers of the mask are made of three-ply (three layers) melt-blown polymer (most commonly polypropylene) placed between non-woven fabric. The melt-blown material acts as the filter that stops microbes from penetrating and exiting the mask. Some masks have an attached thin polyethylene faceshield (known as a "splash shield") to provide additional spray protection over the eye area.

A different type of mask, known as "duckbill" masks, uses a trapezoid pouch-like design that has significantly shortened side edges — sometimes none at all — to minimize loose gaps that pathogens can leak past. These masks are typically made to the N95/P2 standards, and are commonly used for clinical situations that demand fine particulates protection, such as tuberculosis.

Small strips of foam or thickened fabric are often sewn along the top edge of the mask to help better seal away exhaled water vapors (which can fog up eyewears and faceshields) and soak up excess perspiration dripping from above. Small bendable metal strips are frequently added to the top edge to better fit over the nasal bridge. Occasionally adhesive tapes can also be added to secure the seal and prevent the mask from slipping up and down.

The masks are typically fastened to the head with straps or elastic bands that are attached to the mask's four corners. Straps come in four free-hanging ribbons that are manually tied in two pairs horizontally around the back of the head, and are most frequently used in surgical operations due to the ability to customize the strap length and tension comfortably to the wearer's face shape and head movements. Elastic bands come in a pair of loops that can either be horizontally or vertically attached. Horizontal loops go around the head like tied straps, designed to exert tension on the top and bottom edges of the mask for firmer contact seal, and are usually seen on duckbill masks; while vertical loops hook around the ears with less tension (due to the weaker rigidity of the elastocartilaginous auricles compared to the bony skull) and thus less firmly secured to the face, but are more popular in non-procedural usages due to the ease of putting on and taking off.

Filter material in the middle layer may be made of microfibers with an electrostatic charge; that is, the fibers are electrets. An electret filter increases the chances that smaller particles will veer and hit a fiber, rather than going straight through (electrostatic capture). While there is some development work on making electret filtering materials that can stand being washed and reused, current commercially produced electret filters are ruined by many forms of disinfection, including washing with soap and water or alcohol, which destroys the electric charge. During the COVID-19 pandemic, public health authorities issued guidelines on how to save, disinfect and reuse electret-filter masks without damaging the filtration efficiency. Standard disposible surgical masks are not designed to be washed.

Physical properties and quality

Performance of surgical masks is evaluated based on such parameters as filtration (mask capture of exhaled aerosols), exposure (transfer of aerosols from outside), mask airflow resistance (pressure difference during breathing, ΔP, also known as breathability), liquid penetration resistance, air and water vapor permeability, water repellency (for outer and inner surfaces).

Filtration and exposure is typically measured in bacterial filtration efficiency (BFE) using particles of size 3.0 μm. Particulate filtration efficiency (PFE) using particles of size 0.3 μm is only measured in China.