Science Buddies: "Ask an Expert"

Hello,
I was considering a science fair project that tests the efficiency of mask filtration due to the relevancy of COVID-19. However, for my project I am not allowed to involve humans nor bacteria due to guidelines. I was wondering if there was a quantitatively measurable way to test different face masks?
Thank you

Hi!

This sounds like an interesting project idea! Since having people test masks is not an option for your research, it would be necessary to find a system that simulates one or some of the common actions people take while wearing a mask (e.g. sneezing, coughing, talking or breathing behind a mask). One potential way to simulate the aerosol formation produced would be to aerosolize a liquid. Using a spray bottle could be a possibility, although a spray with a finer mist would be better for this experiment. One important thing to consider is preventing the mask from becoming soaked with the spray; research shows that the effectiveness of masks decreases when wet.

Many of the ways to quantify mask filtration require sophisticated equipment. However, one method that is relatively simple would be to illuminate the droplets with light from a laser, using a lens to alter the beam into a light sheet, then use a camera, even a phone camera would be acceptable, to see how many droplets make it through the mask. Image processing software, such as ImageJ, can be used to objectively measure the number of droplets (you can even take a section of the whole image to count the droplets as long as it is consistent across all of your experiments). This paper by Fischer et al. published last month has an example of this setup being used to test mask efficiency: https://advances.sciencemag.org/content/6/36/eabd3083.

Hope this helps and let me know if you have any further questions!

Thank you! Do you know if there is a specific laser that would need to be used? The article mentioned a green laser and I just want to clarify.

Hi - just wanted to chime in that we have a video about a similar activity. Another possible way to measure would be to see how far the droplets spread on a surface that makes them easily visible, like the newspaper in this video: https://youtu.be/ZoBslwE2luI. Many newer smartphones also have a slow-motion video feature that can make it easier to see the droplets on a video. I'm not sure if you'll need a laser, you might be able to do it with proper lighting. As you can see in the video, the droplets are easier to see against a black background. Hope that helps!

Hi again,

As far as I know, it doesn't need to be a specific laser or anything as long as the droplets are illuminated. Granted, I am not certain which exact properties of the laser made them decide to use a green laser as opposed to a different color. Additionally, the exact laser they use in their experiment is likely high powered and therefore dangerous to use without training. So if you go the illuminating the droplets route, I would experiment with some different light sources you have around the house to see what works better for you. Since in this case you would essentially be testing the relative efficiency of the masks instead of the absolute efficiency of the masks, you don't need to illuminate every single droplet in the frame like they tried to do in the article.

Another idea that may work but might require some sensitive scales is placing lightweight filter paper of some sort some distance away from the mask and comparing its weight before and after spraying the mask. My only concern with this idea is that a good mask likely will only allow very few droplets through so you won't really see a difference with the better masks, but it doesn't mean the efficiency is actually zero.