Why vacuum cleaner bags make the most effective face masks

You have probably become used to wearing a face mask in public. And you probably wear a fabric one, as we’ve been urged to save N95, FFP3, and other “clinical grade” masks for healthcare workers. This is despite science not knowing how well fabric masks work.

To overcome this, a team that I am part of at the University of Cambridge decided to test various fabrics to see how well they would protect the wearer and the public when used in face masks. One element of fabric mask efficacy can be discovered by looking at how well various materials block virus-sized particles (from 0.2 to 1.0 micrometers).

While some research had already investigated the ability of fabric to act as a filter, this previous work looked at only a small selection of fabrics and household materials, such as dish towels, scarves, and T-shirts. However, these early studies showed that fabrics could be promising as mask materials. For example, a 2013 study found that a cotton T-shirt was able to filter 69% of particles during normal breathing.

Nonetheless, these studies left those making and buying fabric masks with only limited guidance. Fabric comes in all sorts of fibers and types. Which is best for face masks? If you layer two fabrics that each filter with 40% efficacy, will this protect you from 80% of virus particles?

These studies also failed to assess a fabric’s virus-blocking ability in the types of situation where viruses are most likely to be spread, such as coughing. While a sick individual may emit some virus while breathing normally, they’re likely to expel a much higher number of particles when coughing or sneezing, where air travels at a much higher speed. If a fabric mask is to effectively protect you or others, it needs to block particles at much higher velocities than previously tested.

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Simulating coughing

So, our team devised an experiment to examine how well 20 commonly available fabrics and household materials could filter virus-sized particles at coughing speed. We designed an apparatus to hold a fabric sample that was one inch in diameter. Air was then passed through this sample at approximately 16.5 meters per second – the average speed of an adult cough as it leaves the mouth.

Two particle counters then measured the concentration of small particles in the air before and after it crossed the fabric. We then compared these concentrations to derive the filtration efficiency of the material. We did this ten times for each fabric. For the sake of comparison, an N95 mask and a surgical mask were also tested.

A man wearing a mask coughing