Initially, experts contended that only the immune-compromised required face coverings while traveling. Then, it was only those infected with the novel coronavirus who were advised to do so. And yesterday–in new guidance published by the CDC–researchers officially took the stance that wearing masks during a pandemic is good for everyone.
“The prevention benefit of masking is derived from the combination of source control and personal protection for the mask wearer,” the authors wrote.
“Multi-layer cloth masks can both block up to 50-70% of fine droplets and particles and limit the forward spread of those that are not captured. Upwards of 80% blockage has been achieved in human experiments that have measured blocking of all respiratory droplets,4 with cloth masks in some studies performing on par with surgical masks as barriers for source control.”
Unfortunately, not all masks offer the same degree of protection. On balance, double layer cotton masks, N-95, and K-95 masks are regarded as the best, so long as they’re efficiently made.
To receive a US safety certification, a mask needs to be able to block 95% of viral particles dispelled from an infected subject.
Are N95 masks really the best?
According to a new paper published in the journal Physics of Fluids, N95 masks with valves don’t actually offer much protection against viral particles. In fact, the comfort afforded by mask valves comes at the expense of functionality.
“It’s basically a little flap that opens up when exhaling, which lets air out without being filtered through the mask material,” Matthew Staymates, a mechanical engineer and fluid dynamicist who contributed to the new paper, explained in a media release. “I was able to visually show how this exhalation valve operates and compared it to an N95 that doesn’t have this valve.”
Staymates did so with the help of slow-motion video footage and Schlieren imaging, which is designed to visualize fluid flow as it leaves the surface of an object.
Although the schlieren imaging technique showed the flow of warm air passing through the various masks tested by the researchers, it did not provide any data about droplet penetration.
“For this, strategic lighting of fog droplets was used with a mannequin head to visualize the penetration of droplets through both masks. The mannequin exhaled with a realistic flow rate and velocity that matched an adult male,” the authors explained in the new report. “Results of these qualitative experiments show that an N95 respirator without an exhalation valve is effective at blocking most droplets from penetrating through the mask material. Results also suggest that N95 respirators with exhalation valves are not appropriate as a source control strategy for reducing the proliferation of infectious diseases that spread via respiratory droplets.”
The CDC confirmed in their recent briefing that properly made cloth and N95 masks block active viral particles more than 70% of the time.
Be sure to review the Centers for Disease Control and Prevention’s database of counterfeit masks and fraudulent manufacturers to avoid purchasing counterfeit masks.
“Our current understanding is that COVID-19 is transmitted in part via respiratory droplets, so N95s with valves are not good for source control during this pandemic,” Staymates concluded. “My hope is that this work will help inform a large audience that valves on a mask are not helpful as we fight this global pandemic together.”