Which face mask  is more effective against COVID-19  transmission?

By

Dr. Sharad N. Pachpute

 

Flow Physics of Breathing

• The major reason for a pandemic of viruses is due to airborne disease transmission. Hence, airborne disease transmission has been an important topic of interest in many fields for several decades
• Fluid dynamics plays a significant role to understand the flow physics of breathing
• This article presents the flow dynamics of breathing and COVID-19 transmission. The parameters of breathing like flow rate, the direction of breathing jet, area of nose or mouth opening, and breathing or respiratory rate are assessed for selection of masks and ventilation
• Breathing comprises two parts which are inhalation and exhalation
• During the process of inhalation (inspiration), the air is taken into the lungs from the external environment. This is achieved by creating negative pressure with the contraction of respiratory muscles and diaphragm
• Exhalation (expiration) is also one of the parts of breathing and during this process, the air is taken out from the lungs with the relaxation of respiratory muscles. In this process, the pressure of the air created inside the lungs is much than that of the external atmosphere

• The normal breathing flow rate is roughly represented by a sine wave for all the subjects. As per respiration, the time of exhalation is longer than that of inhalation. The following figure shows a typical flow rate variation over time

• The functional form of the flow rate during breathing is expressed as

Flow rate for breathing =a sin (βt)

Where β can be calculated by following function

Where respiration frequency (RF), minute volume (MV), and tidal volume (TV) depend on the subject and body surface area (BSA) of humans.

• The mean angle of air jet with respect to the horizontal line is roughly same for most of subjects, θ_m = 60° ± 6°. The mean spreading angle of air jet from the nose, θ_s = 23° ± 14°.

• The change in flow rate of air from the nose with time in a breathing period is given below

 

• Change in minute volume (MV) with the body surface area (BSA) for male and female subjects are given below (Gupta et al., 2009)

 Flow Dynamics of Cough

• Cough is found to be one of the major sources of airborne diseases as a large number of droplets are expelled from the mouth with high velocity. Cough flow is a multiphase flow. There is an interaction of two phases of fluid (water droplet and air).
• In general, cough is defined as a sudden expulsion of air with fluids, mucus, foreign particles and microbes through the large breathing passages (nose or mouth)
• Understanding flow dynamics of cough is paramount to control the airborne diseases like COVID-19 transmission
• The size of droplets expelled from the mouth varies as their distance increases. This depends on the evaporation of droplets, aerosols and coalescence of droplets

• The flow rate of fluid generated from a typical cough over a short time depends on a subject (male/female)
• Based on experimental studies, It is observed that the cough begins with a very short period of inhalation (<1% of the total exhaled air volume)
• High acceleration of droplets is observed afterwards in the exhalation and subsequently, it reduces drastically. The inhalation volume is very small and can be neglected

• For a sequential cough, the peak values of cough flow reduce with time as shown below. For a first cough, the volume of air expelled is higher and lower for the next cough. The first high cough can lead to transmission of droplets through masks if high-velocity droplets impinge on the inner side of the mask as well as side leakage. This is true for a single layer mask.

• Flow visualization of the cough process is reported with a frequency of 120 Hz. After 10 Hz, the volume of cough expelled increases drastically

• The effect of the cough on the lungs and airways is shown below. Due to continuous and prolonged cough, mucus deposits at the inner side of airways and reduces airflow rate. Further blocking of air can lead to a critical stage of patient or putting on ventilation mode

Flow Physics for COVID-19 Transmission

• For COVID-19, the transmission of respiratory infections is mainly by virus-laden fluid particles carried with droplets from infected persons
• These fluid particles are expelled from the mouth and nose during breathing, speaking, talking, coughing and sneezing. However, the physical properties of these droplets such as inertia, gravity and evaporation determine how long they survive in the environment
• Large droplets settle faster and take more time to evaporate. They contaminate surrounding surfaces. However smaller droplets evaporate faster before they settle, so they form droplet nuclei that can stay airborne for some hours. Their nuclei may be transmitted over long distances
• For COVID-19, human-to-human (H2H) transmission occurs by three major routes:
  • large droplets which are expelled with sufficient momentum can reach directly to the recipients’ mouth
  • Physical contact with droplets deposited on a surface
  • Airborne’ transmission route: Inhalation contaminated air (aerosolized droplet nuclei) by the recipient

• The key difference in transmission by droplets and airborne is given below
  • Droplets: cough and sneezing from the mouth can spread droplets of saliva and mucus
  • Airborne: Tiny particles produced by dispersion or evaporation  of  droplets with air formed during talking can travel longer distances

• Each stage of COVID-19 transmission process is a complex flow phenomenon such as air–mucous interaction, turbulent jets, liquid sheet fragmentation, and droplet evaporation and deposition, flow-induced particle dispersion and sedimentation

• A variety of methods have been adopted to reduce the transmission of virus such as hand washing, sanitization and wearing face masks, fogging machines, ventilation, social distancing
• Despite of the long history of medical research and experience in the transmission of respiratory infections, it is quite hard to say one of the methods is enough effective to protect of these viruses. Based on some of the case studies we can confirm the efficacy of the above method like ventilation, wearing masks, sneezing etc.
• Before going to adapt this method, we have to understand the flow physics of breathing and exhaling the air or cough
• Hence, understanding the effect of the mask on flow physics is essential to determine key factors for the transmission of COVID-19
• The flow phenomena to the transmission process are determined by experimental studies and CFD analysis.

Scope of CFD modeling for cough Flow

• CFD analysis helps to understand the flow pattern of water (mucus) particles ejected in an open atmosphere
• Based on CFD analysis of cough carried out  using  CFD software (ANSYS) shows  how an aerosol cloud travels in the air for longer distances 
• Particles during sneezing can travel a much longer distances than we see from the naked eyes

Effect of Mask on Corona Transmission

• We see a variety of masks used to protect the viral particles of COVID-19. These face masks are classified based on breathing rate, filtration efficiency and fabric.
• Higher filtration mask has multiple layers of fabrics. Hence it can result in lower breathing. These masks are not healthy during walking and running when our bodies require more oxygen levels. An example of a high filtration mask is N95. A high filtration mask creates very low pressure on the mouth side and the air is drawn from the sides of the mask.

• Low filtration mask consists of single or double-layer fabric. They can result in the breathing of viral particles. It is against COVID-19 protection. An example of a low filtration mask is a homemade mask. For a low filtration mask, the air passes through the layer of mask due to lower filtration efficiency

• The cases studies carried by Verma et al. (2020) for handkerchief, homemade mask, shell-cone style mask, surgical and N-95 masks are presented below.

 Handkerchief

• A handkerchief mask comprises one layer of fabric. It has less side leakage but more leakage through the cloth near the mouth region. This is due to lower porosity of fabric and high pressure of exhaling droplet
• This mask is not recommended for COVID-19

Homemade cotton mask

• This mask comprises folded layers of cotton fabrics. It reduces leakage from the front side but side leakages are more
• Hence, this mask is not protective against COVID-19

 Shelf cone style mask

• Based on experimental analysis of flow dynamics using visualization technique, the filtration efficiency and side leakage is studied for shell cone style mask
• By wearing the shelf cone, significant of air leakage is observed from sides of the facemask as shown in below

• The average distance covered by the droplets is up to eight feet for the uncovered face. This distance reduces as the number layer over the mask increases
• For commercial masks, the average distance of droplets is around 8 inch

Surgical Mask

• When a mask is faulty or worn out, some droplets can travel a longer distance (up to 1 meter) during the periods of coughing
• Without a mask, droplets can travel double the distance of that by with the mask. Hence, wearing a surgical mask will reduce the traveling distance of particles
• The shape and size of droplets change due to their interaction with the mask. These droplets escape from the mask and enter into the external environment
• The filtration efficiency of the surgical masks can range from less than 10% to 90% for different manufacturers
• Effect of a surgical mask to know the qualitative assessment of airborne droplet transmission is shown below based on CFD analysis of airflow from the nose

 N95 Mask

• N95 masks have high filtration efficiency. Hence, they are widely used against COVI-19 transmission
• These masks filter at least 95% of particles smaller than 300 nm. Hence, it is widely used face mask to block viral particle
• Most of N95 masks use membranes to reduce the droplet transmission through it

• The fitted N95 is one of the most effective for blocking droplets. They have a very less transmission rate of droplets ( 0.1% ) through the mask. This is a more suitable mask for healthcare workers who have more exposure to the coronavirus.

• The disadvantage of N95 Mask: after some time, the tight-fitting of the mask reduces the breathing rate and the amount of available oxygen can be reduced up to 20 percent.
• The following N95 respirator comprises the filtration layer (made from electrically charged fibres) which is covered by supportive layer on its both side

• A comparison of masks is shown below, the fitted N-95 mask is safest among all commercial mask due to lowest droplet transmission rate

 Important Points for Selection of Mask

The selection of mask depends on many factors such as material composition, brand, product, and batch, material type (knit, woven, or non-woven, woven fabric, thread count, and weave). To select a suitable mask, the following points are essential:

• The mask made from non-woven and microfiber materials has higher filtration efficiency ( >80%) than the masks of oven materials
• The hydrophobic mask can be useful for blocking droplets
• For most of fabric masks, filtration efficiency is lower than microfiber and non-woven materials
• The masks made from natural fibers and synthetic knit have very low filtration efficiency (particles 300 nm in diameter). This filtration efficiency of the woven mask is primarily dependent on the method of weave how the air gaps in fibers are made
• Using multiple layers of the same material on the mask may improve filtration efficiency. However, each layer on a face mask also increases the pressure drop of air and reduces breathability. The reduced breathing makes it uncomfortable during speaking, walking, and running
• The masks made from non-woven materials should be cleaned with detergent and water, but the washing of masks increases air openings and reduces their filtration efficiency
• By using the electrostatic charge materials by rubbing them with latex, the filtration efficiency is improved within 30-150 min
• Wet masks have nearly the same filtration efficiency compared to dry masks
• The mask materials made from harmful chemical or mechanical toxins or should be avoided

Filtration Efficiency of Mask

• Filtration efficiency one of the important parameters is used to check against the transmission of bacteria, particle and virus. This effect gives three filtration efficiency as BFE, BPE and VFE
• Most of the experimental mask filters in the market have been reported to have bacterial or particle filter efficiency of more than 96%. Kindly note this point while buying the mask

• Possibility of COVID-19 transmission from one person to other depends on health condition of corona carrier such cough, sneezing as well as wearing the mask
• Compared to other factors, the selection, and fitting of a right mask play a crucial role to control the virus transmission with droplets and aerosols

Disinfection and reuse of Mask

• By washing, and heating can disinfect masks. However, washing the mask and rubbing with alcohol reduce filtration efficiency. Hence, this method is no longer used for the reuse of masks. If the filtration efficiency is not affected for some fabrics of masks such masks can be washed and dried with heated air in order to reuse the same mask
• Alcohol disinfection is also a better option, but dipping and rubbing the mask in alcohol reduces particle capture by around 40%
• Hence, the following there methods are widely adapted to sterilize the infected area
  • Vaporized hydrogen peroxide (H₂O₂) sterilization process,
  • Heating  masks more than 150 degrees Fahrenheit for 30 minutes is be the safest way to reuse the same mask
  • UV light irradiation is considered to be a safe and cost-effective way to sanitize masks,

• Watch the following video, how to disinfect the mask

 

Summary

• The flow physics of breathing is affected by various parameters like the filtration efficiency, rate of cough flow during sneezing, coverage of face mask over the mouth. CFD analysis helps to understand flow patterns with and without a facemask.
• The N-95 mask has better filtration efficiency but it may not be comfortable
•  Heating and UV light disinfect the facemask.

References

1. J.K.Gupta, C. H.Lin, Q.Chen, Characterizing exhaled airflow from breathing and talking, Indoor Air (2010)20: 31–39
2. .S, Verma, M. Dhanak, J. Frankfield, Visualizing the effectiveness of face masks in obstructing respiratory jets, Phys. Fluids 32, 083305 (2020)
3. J.K.Gupta, C. H.Lin, Q.Chen,  Flow Dynamics and Characterization of a Cough, Indoor Air, 19(2009) 517-525.

How to Buy Best Masks

• Many healthy face masks are available online, you can buy the following N-95 face masks by clicking on the images

 

• Reusable N-95 face masks with a multilayer of fabrics, are economical rather than buying a new face mask all time. After using three to four-time, we should wear a new face mask