COVID-19 virus can be spread by asymptomatic, pre-symptomatic, and symptomatic carriers. When sneezing, coughing, or speaking, high-velocity droplets containing virus particles are released. Social distancing or wearing a mask can help prevent exposure to these droplet particles.
As of May 1, Illinois now requires people to wear protective face masks in cases where people cannot separate themselves by six feet. Commercial masks such as FM-EL medical masks can be difficult to source, so the CDC recommends wearing homemade cloth masks. This raises the question, how effective will cloth masks be compared to FM-EL medical masks for blocking droplets?
A recent study, conducted by Mechanical engineering Professor Taher Saif from the University of Illinois at Urbana-Champaign, alongside his graduate students Onur Aydin and Bashar Emon, quantified the efficacy of home materials as masks against high-velocity droplets. Saif’s work shows that a mask needs to be both droplet impermeable and breathable. If a mask is permeable to high-velocity droplets, like those from a sneeze, then droplets carrying germs will be able to pass through the mask material. If a mask is not breathable, then air will flow through the gaps between the mask and the user’s face instead of through the mask material itself, which will negate the blockage efficacy of the mask.
Breathability and droplet permeability of masks have a direct relationship. This means that a completely breathable mask (no mask at all) will be completely permeable, while a completely impermeable mask (solid lead mask) will not be breathable. Saif’s research tested ten FM-EL alternative materials on both breathability and droplet permeability. Saif’s research tests ten FM-EL alternative materials on both breathability and droplet permeability. All materials tested are shown on the table below.
| Material | Impermeability (%) | Breathability (mm / Pa × s) |
|---|---|---|
| FM-EL medical mask | 96.3 | 2.5 |
| Used shirt: 100% cotton | 91.1 | 1.8 |
| New quilt cloth: 100% cotton | 60.1 | 10.6 |
| Used t-shirt: 75% cotton, 25% polyester | 42.6 | 15.3 |
| Used shirt: 70% cotton, 30% polyester | 90.1 | 2.4 |
| New t-shirt (1 layer): 60% cotton, 40% polyester | 43.3 | 7.8 |
| New t-shirt (2 layers): 60% cotton, 40% polyester | 98.6 | 4.6 |
| New t-shirt (3 layers): 60% cotton, 40% polyester | 99.98 | 3.1 |
| New quilt cloth: 35% cotton, 65% polyester | 71.8 | 6.2 |
| New bed sheet: 100% polyester | 83.1 | 5 |
| Used dishcloth: 85% polyester, 15% nylon | 97.9 | 4.9 |
| Used shirt: 100% silk | 91.3 | 5.6 |
| Used shirt: 100% silk | 92.3 | 3.9 |
Droplet permeability was tested by spraying the fabric with a droplet blast from 25 mm away. The percentage of droplets blocked is recorded. Breathability was tested by attaching the fabric onto the end of a tube, while the other end was attached to a pump. The team increased the pressure in the tube then measured the flow of air escaping the fabric to determine breathability.
Their results showed that a 60% cotton, 40% polyester t-shirt blocks droplets with about 40% efficacy, though it is very breathable. But if you stack it so it becomes a two-layer mask, then the blocking efficacy increases to 98%. Compare this to an FM-EL medical mask, which has 96% efficacy. Even better, a two-layered t-shirt is doubly breathable compared to the FM-EL mask. However, adding a third layer to the mask adds negligible impermeability while decreasing the breathability, though is is still more breathable than a medical mask. It seems that the best mask is one composed of 2–3 layers of breathable semi-permeable material to create one highly effective but still highly breathable mask.
Updated 2021-02-12 (updated link to latest version of Saif et al. study)