The coronavirus disease 2019 (COVID-19) pandemic has led to a bumper crop of home-made and commercially manufactured face masks. Almost daily, new ‘improved’ designs appear in the market, or online.
In the absence of easily adaptable methods to assess how well different materials filter out particles in the air, it is difficult for those who make masks at home, in non-profit organizations, or small-scale businesses, to compare them and choose the best mask materials.
Face Masks. Image Credit: InkheartX/Shutterstock.com
Now a new paper published in the preprint server medRxiv* presents an original method called the Qualitative Filtration Efficiency Assessment (QFEA). Hopefully, this will help to define what materials are suitable for this purpose, in an inexpensive and easily available manner.
While there are some tools available to measure the efficiency of filtration, these are too expensive to be used by small-scale mask designers and makers, including those who make them at home, or those who depend on fabric masks bought online or from the stores. Those most affected by this inability to define a good mask are the disadvantaged sections of the population, including most people living in developing countries.
For mask designers to work well, they must have access to simple and inexpensive tools to choose their materials at the very beginning of the design phase.
Prototyping “cheaply and early” has been found to have a strong correlation with positive design outcomes.”
The qualitative filtration efficiency assessment
To meet this need, the current study describes QFEA. This method can sort potential mask materials by filtration efficiency, to pick out the promising ones for further assessment. It can also provide a comparison of relative filtration efficiencies of various filter materials.
The researchers worked out two QFEA methods. One requires the use of a nebulizer, which is available online or from medical stores, in most areas. The second uses an aroma diffuser, exploiting knowledge gained from earlier research. Aroma diffusers are available for about $18.
In principle, the QFEA method builds on the Occupational Safety and Health Administration (OSHA)-approved qualitative mask fit testing procedures. These, in turn, use the fact that materials with high filtration efficiencies can prevent small particles from crossing them and thereby causing a taste or smell to arise in the person wearing the mask.
Usually, sodium saccharin and bitrex are used, to generate a sweet and bitter taste, respectively. With a well-fitting N95 mask, neither sweet nor bitter tastes will occur. If the fitting leaves gaps, some particles will enter the nose or mouth, and a taste will result.
The same method was used by the current researchers, who postulated that this could tell the ability of the material to block small materials. However, to achieve this outcome, the fit part of the test was removed, as this tests the presence of gaps rather than the filtration efficiency of the material itself.
The material in an N95 mask was taken as a standard since it filters out all taste molecules. The study was based on the hypothesis that the degree to which the substance tested produces taste in the mouth is proportional to the filtration efficiency.
To perform the test, short pieces of PVC pipe were used, an inch in diameter closed at one end with the material to be tested. The tubes were held close to the mouth, with the material on the other end. The participants were required to breathe normally through the tube, without inhaling through the nose until the test was completed.
Five samples were tested, sample A containing N95 mask material, sample B being material from a surgical mask, and the other three being materials already tested for their filtration properties. Seven individuals participated, both male and female. Two of them were tested with both sweet and bitter tastes. One of the participants was found to be unable to detect either taste.
The method of testing was to inhale the empty sample tube and remember the sensation as “Very Strong Taste”. After that they breathed through the five samples of materials, and were asked to describe the resulting taste as No Taste; Barely Taste; Some Taste; or Strong Taste, ordering the samples accordingly.
Repeat sampling was encouraged to arrive at a firm conclusion regarding their ordering. Finally, the rank-order was compared with the filtration efficiency of the materials, as already established by earlier research.
Correct ranking in most cases
In 7/8 tests, sample A was correctly ranked as the most effective (No Taste or Barely Taste). In 5/8 cases, B was ranked similarly. Again, in 5/8 cases, A and B were ranked as equally effective. In one test, sample B was ranked higher than A, and in another, C as more effective than B.
This shows that ordinary individuals were able to rank filtration efficiency using the QFEA, whether fabric or commercial filter-grade surgical-mask or N95-mask material. The incorrect ranking occurred either because of residual taste in the mouth – either the participant failed to rinse out the mouth or to wait for the required 30 seconds between tastes.
The two highly efficient filter materials were ranked equally in most cases, but with the others, that were less efficient, correct sorting was universal. Thus, this crude method can separate materials with high filter efficiency from those of lower efficiency. If materials with similar efficiency (like A and B in this study) are tested, quantitative testing will be suitable. However, the QFEA can be used to select the best filters from a choice of materials.
A limitation of the study is its dependence on the ability of the participant to taste the substance and the sensitivity of perception. Successive tests should be separated in time to prevent the build-up of the substance, altering the subsequent taste perception. Taking a short break or rinsing the mouth between samples is a good way to prevent such confusing aftertastes.
Utilizing the QFEA provides an affordable method for anyone contemplating designing, purchasing, or wearing a fabric face mask to gain insight into which of the various options available affords the highest filtration.”
While not claiming laboratory-grade specificity or accuracy, it can help to accurately select the best of available materials. It could be useful as a complementary assessment tool during the prototyping phase of mask design, or to measure the utility of a mask intended to protect against the virus causing the current COVID-19 pandemic.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.