This article accompanies our Ultimate Guide to Homemade Face Masks for Coronavirus article, and covers our testing method and provides the original data.
WATCH: Smart Air’s Paddy Explain How We Ran These Tests
The Cambridge researchers used a fancy piece of kit called a Henderson apparatus to run their tests. This allows for the controlled generation of microbial aerosols and a close control of flow rate and relative humidity. In our tests, we chose to mimic this testing method as closely as possible, but with some noticeable differences. Here’s our setup:
First off, we used ambient air pollution for our tests. The air we breathe contains thousands of tiny particles, some of which are the same size as viruses. We did this so as not to have to work with any nasty viruses or bacteria. We lined two fans up in series to generate a strong airflow, enough to blow through each material at 0.3m/s. That’s similar to speed of the air when exhaling through your mouth.
At the end of our test tunnel, we placed a 10cm x 10cm specimen of each material on the end of the tube, and adjusted the fan to measure 0.3m/s on our anemometer. For some thick materials, our two-fan setup wasn’t powerful enough to reach this speed. Our results reflect which materials these were.
After setting up the material and the airflow, we then proceeded to use our Met One GT521 laser particle counter to measure the number of particles the materials could capture. Met One is the company that makes the big BAM monitors that most governments use to measure air pollution, so we’re in good hands here.
We tested the particle capture effectiveness of each material at capturing 1.0 micron and 0.3 micron particles. The 1.0 micron size mimics the Bacillus atrophaeus bacteria used by the Cambridge researchers (0.93-1.25 microns in size). It’s also a size that can be considered similar in range to coronavirus droplets (5-10µm). 0.3 micron particles are typically considered the most difficult to capture, and so was chosen here as a ‘worst case scenario’. It can give a reasonable estimate for effectiveness of each material at capturing 0.1 micron particles – the size of the coronavirus when not in droplet form.
For each material, downstream air was samples for 30s with and without the test specimen in place, and repeated 3 times. Averaging these values gave us our 0.3 and 1.0 micron capture effectiveness for each material.
Open Data On Best Materials for Covonavirus Face Masks
As part of Smart Air’s open-data policy, we are providing all the data in our experiments available for free for people to download and analyse. The data can be downloaded here:
Paddy is the CEO of Smart Air, running operations from Beijing. He has a Masters in aeronautical engineering from Bristol University, UK having specialised in aerodynamics. An advocate for open data, free information and transparent business, he spends his spare time promoting honest business and social enterprise.