Optical microscopic study of surface morphology and filtering efficiency of face masks

Abstract

BackgroundLow-cost face masks made from different cloth materials are very common in developing countries. The cloth masks (CM) are usually double layered with stretchable ear loops. It is common practice to use such masks for months after multiple washing and drying cycles. If a CM is used for long time, the ear loops become stretched. The loop needs to be knotted to make the mask loop fit better on the face. It is not clear how washing and drying and stretching practices change the quality of a CM. The particulate matter (PM) filtering efficiency of a mask depends on multiple parameters, such as pore size, shape, clearance, and pore number density. It is important to understand the effect of these parameters on the filtering efficiency.MethodsWe characterized the surface of twenty different types of CMs using optical image analysis method. The filtering efficiency of selected cloth face masks was measured using the particle counting method. We also studied the effects of washing and drying and stretching on the quality of a mask.ResultsThe pore size of masks ranged from 80 to 500 μm, which was much bigger than particular matter having diameter of 2.5 μm or less (PM2.5) and 10 μm or less (PM10) size. The PM10filtering efficiency of four of the selected masks ranged from 63% to 84%. The poor filtering efficiency may have arisen from larger and open pores present in the masks. Interestingly, we found that efficiency dropped by 20% after the 4th washing and drying cycle. We observed a change in pore size and shape and a decrease in microfibers within the pores after washing. Stretching of CM surface also altered the pore size and potentially decreased the filtering efficiency. As compared to CMs, the less frequently used surgical/paper masks had complicated networks of fibers and much smaller pores in multiple layers in comparison to CMs, and therefore had better filtering efficiency. This study showed that the filtering efficiency of cloth face masks were relatively lower, and washing and drying practices deteriorated the efficiency. We believe that the findings of this study will be very helpful for increasing public awareness and help governmental agencies to make proper guidelines and policies for use of face mask.

Ability of fabric face mask materials to filter ultrafine particles at coughing velocity

“…One study on one fabric face mask showed a decrease in filtration efficiency with washing. 11 All fabric materials were tested after one wash and dry in a home machine.…”

Current understanding on the Effectiveness of Face Masks and Respirators to Prevent the Spread of Respiratory Viruses

“…In such emergency situations, locally made cloth mask are widely used due to low cost and added advantage of decontamination and washing between uses [31] . The particulate matter filtering performance of cloth mask have been found lower than commercially available surgical masks and respirators [52][53][54] . Davies et al [71] reported the filtering efficiency of two layered cloth mask made from commonly available fabrics using bacteriophase A cluster randomized trial of cloth masks in healthcare workers in hospital settings reported that influenza like illness was higher in health care workers who wore cloth mask than those who wore surgical mask [28] .…”

“…Also, in cloth face mask the pores are much bigger (figures 2c and d). This arrangement does not provide gradient filtering and explains why cloth face masks have filtering efficiency than surgical face mask [52][53][54] . [52] .…”

Cloth masks as respiratory protections in the COVID-19 pandemic period: evidence gaps

“…The effectiveness of a mask depends on the type of material (14) , the particle size and aerosol charge (15) and can be measured according to its filtering efficiency. A mask is considered effective if its filtering capacity is greater than 95%.…”

“…In the case of cotton masks, WHO recommends washing them with household detergent (24) . Furthermore, use of masks by the population conveys a false sense of security, inducing the individual to devalue the set of preventive measures against the pandemic by COVID-19 (14)(15)22) .…”