CDC says masks with exhalation valves do not prevent spread of COVID-19

Until a coronavirus vaccine is made widely available, social distancing measures like avoiding large crowds, proper hand-washing techniques and wearing facial coverings are the best defense against COVID-19. 

But experts say the type of face mask you wear matters. 

The U.S. Centers for Disease Control and Prevention updated its guidance on face masks last week, cautioning against wearing masks with exhalation valves or vents. 

Valved or vented masks are commonly used in construction sites or wherever debris or dust may hinder someone’s ability to breathe, and while they may appear efficient in protecting the wearer from unwanted particles, the CDC said they don’t protect others. 

“Masks with one-way valves or vents allow air to be exhaled through a hole in the material, which can result in expelled respiratory droplets that can reach others,” the CDC said.

“This type of mask does not prevent the person wearing the mask from transmitting COVID-19 to others. Therefore, CDC *does not recommend* using masks for source control if they have an exhalation valve or vent,” according to the CDC. 

FILE - View of a respirator from the American company 3M.

“The purpose of masks is to keep respiratory droplets from reaching others,” the health agency said. 

The CDC recommends cloth face coverings as the best line of defense against spreading the novel coronavirus. 

“We are not defenseless against COVID-19,” said CDC Director Dr. Robert Redfield. “Cloth face coverings are one of the most powerful weapons we have to slow and stop the spread of the virus – particularly when used universally within a community setting. All Americans have a responsibility to protect themselves, their families, and their communities.”

Another popular style of facial covering researchers say does not provide adequate protection for the wearer or others from the coronavirus is the popular polyester-spandex mix neck gaiter commonly used by runners and hikers. 

RELATED: Study: Wearing neck gaiters as mask may increase risk of COVID-19 spread

A proof-of-concept study published by Duke University researchers in the journal Science Advances measured 14 different types of masks — including two types of N95 masks, several cotton-based masks, a neck fleece and a bandana — and how they transmit respiratory droplets during regular speech.

The researchers determined that an N95 mask without valves, the same type used by many front-line health care workers, was most effective, while several cotton-based masks also provided good coverage.

“On the other hand, bandanas and neck fleeces such as balaclavas didn’t block the droplets much at all,” according to a news release from Duke University.

When speaking about the study’s findings during an Aug. 13 media briefing, co-author Martin Fischer discussed the thin, polyester-spandex mix neck gaiter that researchers tested.

“If you pull that and hold it up to the light, you actually see light through it, it’s very easy to breathe through, which of course then necessarily would mean more breathability, less protection,” Fischer said.

Other medical experts have used their own personal social media platforms to demonstrate the effectiveness of facial coverings that are known to stop the spread of COVID-19, which typically transmits through respiratory droplets. 

RELATED: Doctor demonstrates how face mask blocks respiratory droplets from spreading

Dr. Richard Davis, who is the clinical microbiology lab director at Providence Sacred Heart Medical Center in Spokane, Washington, tweeted a series of photos that showcased two demonstrations aimed at understanding how effective face masks are at blocking respiratory droplets from an individual’s mouth, while also illustrating the importance of social distancing.

Using a standard triple-layer hospital issue surgical mask, Davis sneezed, sang, talked and coughed toward an agar culture plate with and without a mask. Agar culture plates are Petri dishes filled with agar, a gelatinous substance obtained from red algae to culture, or help multiply, microorganisms. After performing each action, Davis said bacteria colonies formed in the dishes where the respiratory droplets emitted from his mouth landed.
 
“Bacteria colonies show where droplets landed. A mask blocks virtually all of them,” Davis wrote in a caption for the post.
 

The need for face masks has also created its own economy, with thousands of individuals and organizations answering the call to produce homemade masks during shortages that occurred when the coronavirus outbreak first occurred. 

While masks made of various types of materials are now readily available from most local stores and street vendors, researchers found evidence of which materials are the most effective. 

Scientists at the Wake Forest Baptist Medical Center evaluated approximately 400 masks made by community volunteers in order to find which, if any, masks worked as well as N95 respirators or any other medical-grade surgical masks.

“We saw the possibility that we could face a shortage of surgical masks in the hospital and wanted to investigate the possibility of using cloth masks as an alternative as long as they worked and provided good protection for our doctors, nurses and patients,” said Scott Segal, M.D., chair of anesthesiology at Wake Forest Baptist, who conceived of the idea.

RELATED: If you’re going to make your own face mask, the material matters, researchers say

After careful analysis of each mask, researchers found that the best homemade masks achieved 79 percent filtration, compared to N95 masks, which achieve 97 percent. 

But some homemade masks demonstrated as little as 1 percent filtration, according to researchers. 

According to researchers, the best design was that of a double-layer mask made of heavyweight “quilter’s cotton,” consisting of a thread count of 180 or more.
 
Segal said a double-layer mask with a simple cotton outer layer and inner layer of flannel also performed well.

The inferior masks consisted of single-layer or double-layer designs made of lightweight cotton.
 
“As important as this information is for hospitals, it is also important for people who want to make masks for their own use,” Segal said. “We don’t want people to think that just any piece of cloth is good enough and have a false sense of security.”

Justin Sedgwick contributed to this story.