Thread by @tifrarnab

As promised a detailed thread on washing surgical masks:
I've HAND washed several surgical, and "cloth with polymer interlayer" masks to test their particle filtration efficiency at 0.3um. There are both high quality and cheaper masks, sourced from India and Bangladesh. (1/n)

New masks have hydrophobic outer layer (see water balling up?) Rub a bar of detergent and it gets wet. Scrub 2 min, rinse well. Hang to dry. First few washes, will recover hydrophobic nature, after lots of scrubbing it gets rough. (2/n)

Since most high-quality masks depend on a charged layer of meltblown polypropylene fibers for filtration (same as in N95s) one expects a sharp drop on washing once the charge is removed. So we measured particle filtration efficiency (PFE) at 0.3um after 1, 10, 20 washes. (3/n)

And this is how it looks. For the high-quality masks the PFE typically drops a lot on 1st wash. But then it stays almost flat and remains about 65-70% even after 20 washes! Now 65% PFE at 0.3um is still better than a typical cloth mask! (4/n)

Mostly one is limited by material fraying on repeated washing, seams coming apart etc before PFE gives up. I've repeated expts w/ powder detergent, similar trends. Do NOT machine wash! Intense churning leads to clumping/shredding. Nose clip distortion often gives modern art (5/n)

Hand washing 20X & PFE measurements are tedious. I did only 1 mask of each type, this isn't statistically significant data etc, but hopefully still useful/thought provoking! Many vendors on @amazon & @Flipkart sell "washable" masks. I haven't seen any PFE data after washing!(6/n)

I'm sure this isn't recommended practice by regulatory authorities in most countries. But given the amount of non-biodegradable medical waste we generate by trashing single-use masks, we should indeed look at mask decontamination and reuse, even in times of plenty! (7/n)

Thanks to a lot of help and ideas from folks @N95decon @cspramesh @TataMemorial @PrakashLab @Kwong_Laura @AStycz @poverty_action and others, and of course many @TIFRScience. PFE measurements were done using our home made setup github.com/shescitech/TIF… (8/n)