Physics of Fluids: Visualizing the Effectiveness of Face Masks
At the beginning of the coronavirus pandemic, medical experts told us that we didn’t need to wear face masks and that anything other than an N95 medical mask would do little to prevent infection. But now public health professionals are leaning toward the notion that wearing something is better than nothing.
Grocery stores and other essential businesses were the first places to supply masks for their employees. Costco has required all shoppers to wear face masks or full-face coverings since May 4th. Other stores like Target, Walmart and Kroger have followed suit. Now in areas where new coronavirus cases are surging, face coverings are becoming mandatory to wear in all public places.
Face masks can prevent people from spreading germs to others, and they can also protect the wearer from airborne germs. However, there is a big difference in effectiveness between the wide range of face masks that are available. Although medical-grade N95 masks and respirators filter out 95% of the particles that pass through it, they are in short supply and reserved for healthcare workers. So for the rest of us, which types of masks offer the most protection?
Researchers at Florida Atlantic University (FAU) experimented with different materials and styles of non-medical masks that are readily accessible to the general public, comparing bandana style face coverings and loosely folded handkerchiefs with a well-fitted quilted cotton mask and cone style masks.
Using mannequins, air machines and lasers, researchers mimicked human coughs and sneezes to see which masks were most effective at stopping the spread of airborne respiratory droplets that are expelled by infected individuals while coughing, sneezing, or even talking and breathing.
Here is what they found…
Loose-fitting single layer bandanas are the least effective; the droplets traveled three feet, seven inches.
Tutorial: DIY Bandana Facemask
With a face mask constructed out of a folded cotton handkerchief or bandana, the droplets traveled one foot, three inches.
Surprisingly, even with the cone-style masks available at pharmacies, droplets traveled about 8 inches due to leakage around the sides.
The best protection is provided by a well-fitted stitched and layered quilted cotton mask, with the droplets only traveling about 2.5 inches.
The FAU researchers also tested how far droplets traveled with no masks. They found that an emulated heavy cough travels an average of 8 feet and as far as 12 feet, which is twice the CDC’s recommended social distancing guideline of 6 feet. Although a large majority of ejected droplets will fall to the ground by that point, smaller droplets can remain suspended in the air indefinitely, until carried away by a light breeze or ventilation airflow.
Both the number and concentration of droplets will decrease with increasing distance and time, which is the fundamental rationale behind social distancing. The New England Journal of Medicine states, “Public health authorities define a significant exposure to Covid-19 as face-to-face contact within 6 feet with a patient with symptomatic Covid-19 that is sustained for at least a few minutes (and some say more than 10 minutes or even 30 minutes).”
So even though the chance of catching anything from a passing interaction in a public space is probably minimal, it’s a good idea to practice social distancing even while wearing a face mask. In addition, FAU’s research study on the physics of airborne fluids demonstrates that the right kind of mask really does make a difference in helping protect against spreading the coronavirus. The same would be true for normal colds and flu, too.
Some people complain that being forced to wear a face covering infringes on their rights. But even if you don’t believe in wearing them, if it makes other people around you feel less anxious, just be nice and consider their feelings. It won’t hurt you to wear a face mask for the short time you’re out in public. Besides, you never know, someone you come into contact with may be immunocompromised or have other health issues that puts them at elevated risk, and they will appreciate your effort in helping reduce their chance of being infected. Like the old proverb says, it’s better to be safe than sorry!
Read the full study here: https://aip.scitation.org/doi/10.1063/5.0016018