While COVID-19 has been ravaging lives across the world for more than a year, there is still an ongoing debate about which mask is more effective in avoiding transmission of the virus. Now, a new study shows that by testing the fabric of the mask under realistic environmental conditions—that mimic the humidity of a person’s breath—an accurate analysis can be conducted about the effectiveness of the masks.
"Cotton fabrics are still a great choice," said Christopher Zangmeister, a research scientist at the National Institute of Standards and Technology, United States. "But this new study shows that cotton fabrics perform better in masks than we thought."
The study found that filtration efficiency—a measure of how well a material captures particles—grew by an average of 33% in cotton fabrics under humid conditions. Relatively, synthetic fabrics like nylon, polyester, and rayon performed worse and their efficiency did not increase with humidity. While the material used in medical-procedure masks performed similarly to cotton, it did not improve under humid conditions.
The results also highlight that there was no difference in the breathability of the masks under humid conditions. This study provides essential evidence to critics of mask-wearing measures who have long demanded that a randomized controlled clinical study needs to be conducted for establishing the effectiveness of masks.
Moreover, in comparison to other studies that used mannequins or performed the trials under idealistic conditions, this study stands out as it made all of its observations in a realistic environment.
For this study, the researchers tested fabric swatches instead of actual masks. They placed dual-layer fabric swatches inside a box where the humidity was maintained at 99%, mimicking a person’s breath. To compare, the second batch of swatches was kept at 55% humidity. Air containing salt particles—similar in size to the droplets exhaled during breathing, coughing, or speaking—was passed through the swatches.
By calculating the number of particles present in the air before and after it passed through the fabric, the researchers estimated the filtration efficiency of the different materials. Moreover, they could measure the breathability by observing the air pressure felt on both sides of the fabric as air passed through them.
Through these methods, the researchers tested nine types of cotton fabric, which improved their filtration efficiency from 12% to 45% under humid conditions. This jump in filtration efficiency of cotton fabrics under humid conditions can be attributed to its property of being hydrophilic—having an affinity for water. Humidity in human breath creates a moist environment within the fabric, leading to better absorption of the microscopic particles.
However, it is important to note that cotton fabrics can only absorb about 150 milligrams of water from the human breath, the equivalent of just one or two drops. Therefore, if a cotton mask gets wet due to any other reason like weather, it is not advisable to wear it as it might be difficult to breathe through its fabric.
This study, conducted by scientists at the National Institute of Standards and Technology (NIST) and the Smithsonian's Museum Conservation Institute, was published in ACS Applied Nano Materials on Monday, March 8, and can be accessed here.
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