You’ve probably gotten used to wearing a face mask in public. And you are probably wearing a cloth mask as we have been urged to save N95, FFP3 and other clinical grade masks forhealth care workers. This is despite the fact that science does not know how well cloth masks work.
To remedy this, a team I am part of at the University of Cambridge decided to test different substances to see how well they would protect the wearer and the public when used in face masks. One element of the effectiveness of cloth masks can be discovered by observing how well different materials block virus-sized particles (from 0.2 to 1.0 micron).
While someResearch had already investigated the ability of fabric to act as a filter, this earlier work looked at only a small selection of fabrics and household materials, such as tea towels, scarves and T-shirts. However, these early studies showed that fabrics can show promise as mask materials. For example, a 2013 study found that a cotton T-shirt could filter 69% of the particles during normal breathing.
Nevertheless, these studies left only limited guidance to those who make and buy cloth masks. Fabric comes in all kinds of fibers and types. Which is Best for Face Masks? If you put two substances on top of each other, each filtering with an activity of 40%, does this protect you against 80% of the virus particles?
These studies also could not assess whether a tissue has virus-blocking properties in the situations where viruses are most likely to be spread, such as coughing. Although a sick person may transmit a virus while breathing normally, he is likely to expel a much higher number of particles when coughing or sneezing, where the air travels at a much higher speed. If a cloth mask is to effectively protect you or others, it should block particles at much faster speeds than previously tested.
So our team devised an experiment to investigate how well commonly available substances and household materials can filter virus-sized particles at a cough rate. We have designed a device to hold a tissue sample with a diameter of 2.5 cm. Air was then passed through this sample at a speed of approximately 16.5 meters per second – the average speed of an adult cough when exiting the mouth.
Two particle counters then measured the concentration of small particles in the air before and after they crossed the fabric. We then compared these concentrations to derive the filtration efficiency of the material. We’ve done this ten times for each fabric. For comparison, an N95 mask and a surgical mask were also tested.
We also ran an additional test to see if fabrics would lose performance when wet. Seven milliliters of water, about the amount exhaled in an hour, was sprayed on some dust samples. These samples were then wet tested five more times.
While it is useful to know the filtration efficiency of individual fabric types, most dust masks are made from at least two layers of fabric. We also wanted to know how layered fabric affects filtration. So after individual layers of fabric were tested, we layered the fabric into possible mask combinations for further testing.
Our research showed that each mask, be it N95 mask or cloth mask, filters less fine particles when exposed to high velocity air (coughing / sneezing) compared to normal velocity air (breathing regularly) . During the high cough rates, even N95 masks only filtered 53% of the fine particles. Surgical masks filtered less, about 48%.
Heavy fabrics with low permeability, such as denim, provided the best protection; however, they have been found to be difficult to breathe through and are therefore not ideal for masks. Our tests indicated that more suitable fabrics were felted wool, with 36% filtration efficiency, and quilting cotton, with 35% efficiency. Fabrics that did not perform well included cotton jersey knit (25% efficiency), stretch Lycra (21%) and lightweight T-shirts (11%).
We also tested vacuum cleaner bags, which are designed to trap small particles and are readily available during the pandemic. The ones we tested were very effective at filtering out particles. A disposable HEPA filter The bag was 61% efficient and a washable filter bag 44% efficient. While these are promising, it is not yet known whether it is safe to cover your face with these materials.
When wet, N95 and surgical masks suffered a notable decrease in filtration efficiency – seven and five percentage points, respectively. The fabrics and vacuum bags did not show such a large drop, and some materials even improved their performance when wet, possibly due to fiber shrinkage or a change in electrostatic properties.
Finally, we found that we achieved much higher filtration efficiency by layering substances. Fusible interfacing – material primed with glue that is glued to other fabrics to reinforce them, such as in collars and cuffs – proved to be a useful addition. For example, a mask with one layer of quilting cotton, one layer of thick quilt batting, and two layers of fusible interfacing could filter out particles with an efficiency of about 60%. That was more than the N95 mask!
But remember: just because a substance or combination of substances can filter particles well doesn’t mean it will work for use in a mask. The fabrics and dust layers tended to be more difficult to breathe through than N95 mask material, and this can make a mask made from them more likely to leak and / or be more difficult to apply. In addition, all of these percentages only represent the fabric’s ability to filter out particles, and do not take into account the effect of a mask’s fit, which is essential to properly protect yourself and others.