Health authorities believe COVID-19 spreads by the transmission of respiratory droplets, and the Centers for Disease Control and Prevention recommends homemade cloth face coverings for use in public spaces. Starting today, Illinois joins many other states in requiring people to wear masks while out.
However, initial uncertainty regarding the masks’ effectiveness in reducing exhaled droplets leaves some people unsure or skeptical of their usefulness during the current COVID-19 pandemic.
Mechanical science and engineering professor Taher Saif spoke with News Bureau physical sciences editor Lois Yoksoulian about a study that he and his graduate students, Onur Aydin and Bashar Emon, performed on the effectiveness of common household fabrics for use in homemade masks.
Physically speaking, are the respiratory droplets produced by talking and breathing the same as those that come from a cough or a sneeze?
The droplets released during sneezing and coughing are larger than those released while speaking and breathing, and any of these droplets may carry many virus particles.
The larger droplets tend to fall nearby due to gravity, but the smaller ones can go far, with the majority of them remaining within six feet of the infected individual.
Unfortunately, because symptomatic, presymptomatic and asymptomatic carriers can shed the coronavirus, we cannot tell without testing which individuals are the sources of infection. Hence, a physical barrier, such as a mask, can prevent the spreading.
How does droplet type influence the fabric choice and the number of layers used to make a mask?
The droplets released during coughing and sneezing come in various sizes and velocities. The fabric for any mask should be breathable and impermeable to high- and low-velocity droplets.
It is important to realize that a highly impermeable fabric is likely to be less breathable. Low breathability will force airflow through the sides and will defeat the purpose of the mask. In that case, the mask simply gives a false sense of safety. The choice of fabric and the number of layers is a matter of compromise between breathability and droplet resistance. We need to maximize both.
What fabric properties did you test?
We tested the breathability and droplet blocking ability of common household fabrics. To quantify breathability, we simply measure the airflow velocity through the material. Measuring droplet blocking is a more complicated process that uses an inhaler to generate high-velocity droplets.
We fill the nozzle of an inhaler with distilled water mixed with 100-nanometer fluorescent particles, which mimic the coronavirus in size. When puffed, the inhaler forces the water through the nozzle and generates high-momentum droplets that we collect on a plastic dish placed vertically in front of the inhaler.
We then repeat the process with the fabric we are testing over the collection dish. We measure how much water landed on the dish in both cases by counting the nanoparticles using a microscope. We can then use the ratio of the volume collected with and without the fabric to give us a measure of droplet-blocking efficiency.
What types of fabric did you examine?
We considered a set of 10 common household fabrics and compared their properties with those of a medical/dental-quality mask as a benchmark.
The fabrics had different combinations of cotton, polyester and silk. We also measured the breathability and resistance for two- and three-layered T-shirt fabric. Our work produced two key findings.
Second, most common fabrics are hydrophilic, meaning they up soak water, whereas medical masks are hydrophobic, meaning they repel water. This tells us that common household fabrics use an alternative mechanism to hold droplets by retaining them.
What type of fabric do you recommend for the home mask maker?
We found that very breathable fabrics are a good choice, like common t-shirt materials. They tend to have low droplet resistance, and their efficiency increases when used in a two-layer mask.
The net breathability of the two layers is much higher than the medical masks, too. In fact, the breathability of cotton T-shirt fabrics remains higher than a medical mask, even with three layers.
Homemade masks can be an effective tool for the public, together with testing and contact tracing, social distancing and other interventions to mitigate disease transmission.
Wearing a face mask in public areas may impede the spread of an infectious disease by preventing both the inhalation of infectious droplets and their subsequent exhalation and dissemination.
In the event of a pandemic involving an airborne-transmissible agent, the general public will have limited access to the type of high- level respiratory protection worn by health care workers, such as N95 respirators.
Images of members of the public wearing surgical masks were often used to illustrate the 2009 H1N1 flu pandemic. However, the evidence of proportionate benefit from widespread use of face masks is unclear.
A recent prospective cluster-randomized trial compar- ing surgical masks and non-fit-tested P2 masks (filters at least 94% of airborne particles) with no mask use in the prevention of influenza-like illness.
The findings of the study found that adherence to mask use significantly reduced (95% CI, 0.09-0.77; P 5 .015) the risk for infection associated with influenza-like illness, but that less than 50% of participants wore masks most of the time.1
Facemasks may prevent contamination of the work space during the outbreak of influenza or other droplet-spread communicable disease by reducing aerosol transmission. They may also be used to reduce the risk of body fluids, including blood, secretions, and excretions, from reaching the wearer’s mouth and nose.
To date, studies on the efficacy and reliability of face masks have concentrated on their use by health care workers. Although health care workers are likely to be one of the highest risk groups in terms of exposure, they are also more likely to be trained in the use of masks and fit tested than the general public.
Should the supply of standard commercial face masks not meet demand, it would be useful to know whether improvised masks could provide any protection to others from those who are infected.
METHODS AND MATERIALS
In this study, common household materials (see Table 1) were challenged with high concentrations of bacterial and viral aerosols to assess their filtration efficiencies. Surgical masks have been considered the type of mask most likely to be used by the general public, and these were used as a control.
The pressure drop across each of the materials was measured to determine the comfort and fit between face and mask that would be needed to make the material useable in mask form. We devised a protocol for constructing a ‘‘homemade’’ mask, based on the design of a surgical mask, and volunteers were invited to make their own masks.
These were then quantitatively fit tested. To determine the effect of homemade and surgical masks in preventing the dispersal of droplets and aerosol particles produced by the wearer, the total bacterial count was measured when the volunteers coughed wearing their homemade mask, a surgical mask, and no mask.
Testing the Filtration Efficiency
A range of common household materials were tested, together with the material from a surgical mask (Mo¨lnlycke Health Care Barrier face mask 4239, EN14683 class I), for comparison. Circular cutouts of the tested materials were placed without tension in airtight casings, creating a ‘‘filter’’ in which the material provided the only barrier to the transport of the aerosol.
A Henderson apparatus allows closed-circuit generation of microbial aerosols from a Collison nebulizer at a controlled relative humidity. This instrument was used to deliver the challenge aerosol across each material at 30 L/min using the method of Wilkes et al,2 which is about 3 to 6 times per minute the ventilation of a human at rest or doing light work, but is less than 0.1 the flow of an average cough.
Downstream air was sampled simultaneously for 1 minute into 10 ml of phosphate buffer manucol antifoam using 2 all-glass impingers. One impinger sampled the microorganisms that had penetrated through the material filter, while the other sampled the control (no filter).
The collecting fluid was removed from the impingers and assayed for microorganisms. This test was performed 9 times for each material. The filtration efficiency (FE) of the fabric was calculated using the following formula (cfu indicate colony-forming units):
The pressure drop across the fabric was measured using a manometer (P200UL, Digitron), with sensors placed on either side of the filter casing, while it was challenged with a clean aerosol at the same flow rate.
Two microorganisms were used to simulate particle challenge: Bacillus atrophaeus is a rod-shaped spore-forming bacterium (0.95-1.25 mm) known to survive the stresses caused by aerosolization.3
The suspension was prepared from batches previously prepared by the Health Protection Agency, Centre for Emergency Preparedness and Response Production Division.4 Each material was challenged with approximately 107 cfu B atrophaeus.
Bacteriophage MS2 (MCIMB10108) is a nonenveloped single-stranded RNA coliphage, 23 nm in diameter, known to survive the stresses of aerosolization.5 Each material was challenged with approximately 109 plaque-forming units (pfu) of bacteriophage MS2.
The two test organisms can be compared in size to influenza virus, which is pleomorphic and ranges from 60 to 100 nm; Yersinia pestis, which is 0.75 mm; B anthracis, which is 1 to 1.3 mm; Francisella tularensis, which is 0.2 mm; and Mycobacterium tuberculosis, which is 0.2 to 0.5 mm.6 Bacteriophage MS2 and
B atrophaeus were chosen as the test organisms to represent influenza virus. This decision was made not only because of the lower risks of associated infection but also because the work would be technically easier to carry out using an Advisory Committee on Dangerous Pathogens (ACDP) class 1 organism versus an ACDP class 2 organism influenza.
Making the Face Mask
For this study, 21 healthy volunteers were recruited, 12 men and 9 women. The participants were aged between 20 and 44 years; the majority was in the 20- to 30-year age range. Each volunteer made a homemade face mask following a protocol devised by the authors. All face masks were made with 100% cotton t-shirt fabric using sewing machines to speed construction. A surgical mask (Mo¨ lnlycke Health Care Barrier face mask 4239, EN14683 class I) was used as a control. Also, all volunteers completed a questionnaire indicating their opinions of mask wearing.
Determining the Fit Factor of the Mask
A commercial fit test system (TSI PortaCount Plus Respirator Fit Tester and N95- Companion Module model 8095) was used to measure respirator fit by comparing the concentration of microscopic particles outside the respirator with the concentration of particles that have leaked into the respirator. The ratio of these 2 concentrations is known as the fit factor. To conduct the fit test, the apparatus was set up and operated according to the manufacturer’s instructions.
Volunteers were instructed to fit their surgical and homemade face masks with no help or guidance from the operator; to ensure that the mask was comfortable for 2 minutes; the participants were given time to purge any particles trapped inside the mask. The fit test was then conducted with volunteers performing the following consecutive exercises, each lasting 96 seconds: (1) normal breathing, (2) deep breathing,7 (3) head moving side to side, (4) head moving up and down, (5) talking aloud (reading a prepared paragraph),
- bending at the waist as if touching their toes, and
- normal breathing.
Determining the Effect of Masks in Preventing the Dispersal of Droplets and Aerosol
An enclosed 0.5-m3 mobile sampling chamber, or cough box, which was constructed for the purpose of sampling aerosols and droplets from healthy volunteers (PFI Systems Ltd, Milton Keynes), was placed in a 22.5-m3 high-frequency particulate air-filtered environmental room. Four settle plates were placed in the cough box to sample for droplets, together with a 6-stage Andersen sampler to sample and separate small particles.8 A Casella slit-air sampler9 was also attached to the cough box. Tryptose soya agar was used as the culture medium. Volunteers wearing protective clothing (Tyvek suits) coughed twice into the box, and the air inside was sampled for 5 minutes. Each volunteer was sampled 3 times: wearing the homemade mask, the surgical mask, and no mask. The air within the cough box was high-frequency particulate air filtered for 5 minutes between each sample to prevent cross-contamination between samples. The plates were incubated for a minimum of 48 hours at 378C before counting.
To evaluate the face mask fit, the median and interquartile range were calculated for each exercise and face mask for the 21 individuals. Wilcoxon sign rank tests were used to compare the masks. The same approach was used to determine differences between the different mask types and their efficacy in preventing dissemination of droplets and particles
All the materials tested showed some capability to block the microbial aerosol challenges. In general, the filtration efficiency for bacteriophage MS2 was 10% lower than for B atrophaeus (Table 1).
The surgical mask had the highest filtration efficiency when challenged with bacteriophage MS2, followed by the vacuum cleaner bag, but the bag’s stiffness and thickness created a high pressure drop across the material, rendering it unsuitable for a face mask.
Similarly, the tea towel, which is a strong fabric with a thick weave, showed relatively high filtration efficiency with both B atrophaeus and bacteriophage MS2, but a high pressure drop was also measured.
The surgical mask (control) showed the highest filtration efficiency with B atrophaeus. Also, as expected, its measured low pressure drop showed it to be the most suitable material among those tested for use as a face mask. The pillowcase and the 100% cotton t-shirt were found to be the most suitable household materials for an improvised face mask. The slightly stretchy quality of the t-shirt made it the more preferable choice for a face mask as it was considered likely to provide a better fit.
Although doubling the layers of fabric did significantly increase the pressure drop measured across all 3 materials (P , .01 using Wilcoxon sign rank test), only the 2 layers of tea towel material demonstrated a significant increase in filtration efficiency that was marginally greater than that of the face mask.
In the questionnaire on mask use during a pandemic, 6 participants said they would wear a mask some of the time, 6 said they would never wear a mask, and 9 either did not know or were undecided.
None of the participants said that they would wear a mask all of the time. With 1 exception, all participants reported that their face mask was comfortable. However, the length of time each participant kept their mask on during testing was minimal (15 min), and with long-term wear, comfort might decrease.
Facemask Fit Testing
A Wilcoxon sign rank test showed a significant difference between the homemade and surgical mask for each exercise and in total (all tests showed P , .001). The median and interquartile range for each mask and exercise are given in Table 2.
Prevention of Droplet and Particle Dissemination When Coughing
Results from the cough box experiments showed that both the surgical mask and the homemade mask reduced the total number of microorganisms expelled when coughing (P , .001 and P 5 .004, respectively; see Table 3).
On analyzing the effect of mask wearing in reducing the number of microorganisms isolated from the Anderson air sampler (Table 4), the surgical mask was found to be generally more effective in reducing the number of microorganisms expelled than the homemade mask, particularly at the lowest particle sizes.
The number of microorganisms isolated from the coughs of healthy volunteers was generally low, although this varied according to the individual sampled (Table 3). It is possible, therefore, that the sampling limitations negatively affected the statistical analysis.
Pearson x2 tests comparing the proportion of particles greater than 4.7 mm in diameter and particles less than 4.7 mm in diameter found that the homemade mask did not significantly reduce the number of particles emitted (P 5 .106). In contrast, the surgical mask did have a significant effect (P , .001).
A protective mask may reduce the likelihood of infection, but it will not eliminate the risk, particularly when a disease has more than 1 route of transmission. Thus any mask, no matter how efficient at filtration or how good the seal, will have minimal effect if it is not used in conjunction with other preventative measures, such as isolation of infected cases, immunization, good respiratory etiquette, and regular hand hygiene.
An improvised face mask should be viewed as the last possible alternative if a supply of commercial face masks is not available, irrespective of the disease against which it may be required for protection.
Improvised homemade face masks may be used to help protect those who could potentially, for example, be at occupational risk from close or frequent contact with symptomatic patients.
However, these masks would provide the wearers little protection from microorgan- isms from others persons who are infected with respiratory diseases. As a result, we would not recommend the use of homemade face masks as a method of reducing transmission of infection from aerosols.
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University of Illinois