The CDC and WHO recommends washing fabric masks after each use. Recommended methods include: 1) washer and dryer with high heat, 2) soapy detergent soak/rinse and air dry, or 3) bleach soak/rinse and air dry.
To study the effects of laundering on the filtration performance and breathability (inverse of pressure drop, Pa) of different fabric materials, we collaborated with the UBC Aerosol Lab to test a number of common fabric materials before and after laundering. All test fabric materials were laundered 10 times in a protective laundry bag, which was placed into the washing machine and dryer along with other clothes. High heat was used for both washing and drying.
Prior to laundering, the fabrics displayed a large range of different filtration efficiencies (PFE 8-60% at 3 microns) and breathing resistance (5-44Pa). Quilting cotton performed poorly, achieving <10% filtration efficiency at 3microns. Fleece and double-knit cotton jersey performed very well, achieving almost 60% filtration efficiency at 3 microns, while having low breathing resistance.
Laundering with high heat seem to improve the filtration efficiency of some fabrics (eg. quilting cotton), and degrade the filtration efficiency of others fabrics (knit cotton jersey, fleece). The improvement in filtration efficiency of some fabrics can be attributed to fabric shrinkage that is common with woven and knit materials. After laundering, the fabrics displayed a smaller range of filtration efficiencies (PFE 10-40% at 3 microns) and breathing resistance (5-25 Pa).
Laundering with high heat seem to affect mainly cotton material. Polyester, nylon, spandex, wool and silk seem less affected by laundering.
Using single-layer filtration (particle filtration efficiency of NaCl aerosols with aerodynamic sizes of 0.5-6microns) and breathability (Pressure drop, Pa) data, measured at a flow rate of 30L/min (equivalent to normal-moderate work of breathing), we estimated the filtration performance and breathability for the typical cotton mask made of quilting cotton with and without the use of biocompatible filters such as dried baby wipe, paper towel, and Halyard sterilization wrapping. We tested the NIOSH standard N95 mask, ASTM Level 2 surgical mask and single-use non-medical mask from the local drug store for comparison.
Laundering dried baby wipe and Halyard sterilization wrapping 10 times in a laundry bag did not significantly degrade their effectiveness at filtering particles at or greater than the 3 micron size. Therefore, both dried baby wipes and Halyard sterilization wrappings (H100-400) can be safely laundered and reused along with the fabric mask for at least 10 times. Even after laundering the filters and fabrics 10 times in the washer and dryer, the reused cotton mask with the reused filters still performed 50%-120% better than the 2-layered cotton mask with no filter, for filtration performance at the 3 micron particle size.
Take away points:
- Laundering can drastically affect the filtration performance of fabric materials by either improving or reducing the fabric's filtration performance.
- The filtration performance of quilting cotton improved after laundering it 10 times in the washer and dryer.
- Filters like dried baby wipe and Halyard sterilization wrapping can be laundered and reused for more than 10 times without significantly affecting its filtration performance.