Updated date:

Small Things Considered: Benchmarking of Face Masks and Face Shield for the New Normal

Madison Pascual Munar has a Doctorate degree in Molecular Biotechnology.

SARS-CoV-2 (Yellow), Human Apoptotic Cell (Green)

SARS-CoV-2 (Yellow), Human Apoptotic Cell (Green)

Size Does Matter

The SARS-CoV-2 viral particle, the causative agent of COVID-19, has a diameter of about 100 nanometers (nm) or 0.1 micrometers (μm). SARS-CoV-2 virion can stay suspended in the air for approximately 1 hour due to their minute sizes.

Aerosols which may carry viral particles vary from 10 nm to 10 μm in sizes. Therefore, filtration efficiency of masks should be considered in choosing the kind of face mask for use in specific needs.

small-things-considered-benchmarking-face-mask-and-face-shield-for-the-new-normal

Face Masks Are Effective As Source Control

COVID-19 is transmitted primarily through respiratory droplets. Thus, wearing face masks is a viable strategy to control the spread of this disease.

Respiratory droplets may carry viral particles as far as 3 feet or approximately 1 meter. Social distancing, keeping a safe distance of at least 2 meters, may also reduce the tendency of contracting the virus.

Benchmarking Masks Filtration Efficiency

SARS-CoV-2 Viral Particle Size (nm)N95 Respirators Size Filtration (nm)Surgical Mask Size Filtration (nm)Cloth Mask Size Filtration (nm)

100

300

100 - 3000

300

Face Masks

Face Masks

Face Masks prevent the transfer of bodily fluids, microorganisms such as bacteria or viruses, and particulate matters (dusts) that may serve as vectors for aerosol-transmissible diseases like COVID-19.

Fitting and Reusability

AttributesN95 RespiratorsSurgical MaskCloth Mask

Fitting

Tight

Loose

Loose

Reusability

No

No

Yes

Decontamination

For Emergency Only

No

Yes

N95 Masks

N95 masks or respirator masks are effective in preventing COVID-19 transmission. However, N95 Masks are not advisable for public use to prevent resource scarcity for health workers who are attending to COVID-19 patients in the hospitals.

Respirator masks such as the N95 mask manufactured in the USA and China has 95%-99% filtration efficiency for 300 nm airborne particles, while respirator masks manufactured in Europe has lower filtration efficiency about 80%-99% for 300 nm-sized particles.

A paper published in Physics of Fluids revealed how gaps may reduce filtration efficiency of N95 masks as a consequence of insufficient face fitting.

The Centers for Disease Control and Prevention (CDC) does not recommend that the general public wear N95 respirators to protect themselves from respiratory diseases, including coronavirus (COVID-19).

Surgical Face Masks

Studies have found out that surgical masks have a comparable filtration efficiency with N95 masks in preventing airborne diseases.

Filtration efficiency of masks varies depending on the region where it is manufactured. For instance, surgical masks produced in the USA have a 95%-98% filtration efficiency for up to 100 nm airborne particles, whereas surgical masks produced in Europe has 95%-98% filtration efficiency for 3000 nm-sized airborne particles.

According to CDC, COVID-19 can be spread by people who do not have symptoms and do not know that they are infected.

Cloth Masks

The Centers for Disease Control and Prevention (CDC) recommends the use of cloth face masks for general public use.

Cloth masks offer the same protection comparable to a surgical face mask when used properly. Cloth masks filtration efficiency was attributed to the mechanical and electrostatic-based filtration offered by the fibers.

Cloth masks made of cotton, and multi-layered masks consisting of different fabrics such as cotton, satin, chiffon, or flannel, can provide effective protection against airborne-diseases such as COVID-19.

Surprisingly, hybrid mask made of cotton and chiffon has the highest filtration efficiency for airborne particles with less than 300 nm in sizes. This is slightly better than the N95 mask and surgical face mask.

It is noteworthy that gaps in cloth masks due to insufficient face fitting may reduce its filtration efficiency by 60%.

Cloth Masks offer significant protection from contracting COVID-19 disease. It also provides an efficient source control method to prevent the spread of the virus from symptomatic and asymptomatic infected individuals.

Benchmarking Filtration Efficiency of N95 Mask, Surgical Mask, and Different Cloth Masks

TPI- Threads Per Inch

Mask/FabricPercent Filtration Efficiency For < 300 nm ParticlesPercent Filtration Efficiency For > 300 nm Particles

N95 Mask (No Gaps)

85

99

N95 Mask (With Gaps)

34

12

Surgical Mask (No Gaps)

76

99

Surgical Mask (With Gaps)

50

44

Cotton Quilt

96

96

Cotton Quilt (80 TPI, 1 Layer)

9

14

Cotton Quilt (80 TPI, 2 Layers)

38

49

Cotton (600 TPI, 1 Layer)

79

98

Cotton (600 TPI, 2 Layers)

82

99

Flannel

57

44

Chiffon (1 Layer)

67

73

Chiffon (2 Layers)

83

90

Silk (1 Layer)

54

56

Silk (2 Layers)

65

65

Silk (4 Layers)

86

88

Hybrid 1 (Cotton/Chiffon)*

97*

99*

Hybrid 2 (Cotton/Silk, No Gaps)

94

98

Hybrid 2 (Cotton/Silk, With Gaps)

37

32

Hybrid 3 (Cotton/Flannel)

95

96

The CDC recommends that members of the public use simple cloth face coverings when in a public setting to slow the spread of the virus.

Valve Masks Are Ineffective In Preventing COVID-19 Transmission

Face masks with valves does not prevent the spread of respiratory droplets from an infected person.

Respiratory droplets from a COVID-19 positive or even asymptomatic person may escape the masks through the valves and enable the spread of the viral particles.

CDC does not recommend the usage of face masks with valves.

Centers for Disease Control and Prevention (CDC) Guidelines on Masks Usage

Recommendations

Wear in Public Settings

Should Not Be Worn By Children Under Age 2

Should Not Be Worn By A Person With Difficulty In Breathing

Masks With Valves Should Not Be Worn By People Infected or Suspected With COVID-19

Face Shields As Additional Layer of Personal Protective Equipment (PPE)

According to CDC, face shield should not be used as a substitute for face mask.

However, there are some instances where face shields can be used in special cases such as when a person has difficulty in breathing while using face masks or for those people who have hearing impairment and relies on face and mouth expressions to communicate.

CDC also recommends to disinfect face shields right after use.

Face Shield

Face Shield

Plastic face shields for newborns and infants are NOT recommended by the CDC.

Face Shields Alone Are Less Effective In Preventing COVID-19 Transmission

A visualization experiment on the use of face shields on preventing respiratory droplet transmission has shown that face shields when used alone may not effectively suppress respiratory droplets especially aerosols which could possibly contain viral particles.

Using lasers, the simulated sneeze was initially blocked, however, aerosols eventually escaped and was carried by the ambient air.

Aerosols may reach as far as 3 feet or approximately 1 meter from the source origin.

Smaller aerosols remain suspended in the air for a longer period of time and tends to flow around the face shield rendering higher chances to get inhaled.

Study on Influenza-Laden Aerosols Revealed Inefficacy of Face Shields Against Small Aerosols

A paper published in 2014 tested the efficacy of face shields against influenza-laden cough aerosols. The study revealed that face shields can reduce inhalation exposure of virus-laden aerosols with larger sizes about 8500 nm by 96%. However, face shields were shown to be ineffective in reducing exposure to smaller aerosols about 3400 nm. Face shields offer only 68% reduction of exposure from smaller cough aerosols.

The protection further decreases by 23% when exposed to cough aerosols continuously for about 30 minutes. This is true because smaller aerosols tend to settle longer than larger aerosol particles.

CDC does not currently recommend the use of face shields as a substitute for face masks.

References