N95 masks with valves ‘not good’ at filtering droplets during coronavirus pandemic

WASHINGTON — Since the start of the coronavirus pandemic, no face mask has been as popular as the N95. Despite their appeal, a study finds all these masks are not created equal. A researcher from the National Institute of Standards and Technology reveals N95 masks with valves actually do a poor job of carrying out a main need during the pandemic — keeping users from spreading potentially infectious particles.

Matthew Staymates, a mechanical engineer and fluid dynamicist, has been examining different types of masks during the COVID-19 pandemic. When it comes to which are most effective at lowering virus transmission, Staymates finds a serious flaw in masks with valves. The researcher says N95s have an exhalation valve to improve a user’s comfort, not to make them safer.

N95 Face Mask
Photo by visuals on Unsplash

“It’s basically a little flap that opens up when exhaling, which lets air out without being filtered through the mask material,” Staymates says 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.”

N95 face masks without valve still effective

The mechanical engineer generated slow-motion videos of his tests using Schlieren imaging, which helps visualize fluid flow as it leaves the surface of an object and scatters light. Staymates designed an elaborate lab in his home woodshop to view several types of face coverings. Along with testing the masks himself, he also built an artificial exhalation system to recreate human breathing patterns.

“I also designed and built a custom fog generator inside a mannequin head, which provides fog droplets that resemble closely the droplets that humans expire,” Staymates details. “The mannequin head exhales like me, and the fog generator essentially makes it look like it just took a cigarette puff.”

The results reveal N95 respirators with the exhalation valve are not the right choice for filtering a user’s respiratory droplets.

“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 explains.

N95s without a valve however are an effective choice to block droplets, the study finds. Most of the respiratory droplets leaving a wearer fail to penetrate the mask’s material.

The engineer believes Schlieren imaging and measuring flow “will continue to assist our efforts to better understand how masks and face coverings play a role in this disease transmission.”

“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.”

The study appears in the journal Physics of Fluids.