Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

Federal health officials now recommend people cover their mouths and noses with cloth face masks when in public to help prevent the spread of coronavirus.

Friday afternoon, the Centers for Disease Control and Prevention updated its guidance on the matter, recommending individuals use cloth face coverings "in public settings where other social distancing measures are difficult to maintain," such as grocery stores and pharmacies. The guidance recommends people use fabric coverings, not surgical masks or specialized N95 masks, which should be reserved for health care providers.

If you are making your own covering, new research finds that some fabrics are better than others at filtering out viral particles.

"You have to use relatively high-quality cloth," Dr. Scott Segal, chair of anesthesiology at Wake Forest Baptist Health in Winston-Salem, North Carolina, said.

Full coverage of the coronavirus outbreak

Face coverings made of fabric, public health experts note, aren't intended to protect wearers from getting sick, but rather, to prevent them from spreading the virus to others. And the guidance will still exclude using surgical or medical grade masks, which experts say should be reserved for people who are sick and for the health care workers who care for them.

Segal came up with the idea to study which fabrics would work best.

In partnership with the Wake Forest Institute for Regenerative Medicine, he tested a variety of cloth materials to see which ones not only allowed for breathability, but also filtered small particles — such as viruses. The research from Wake Forest has not been submitted for publication and has not been peer-reviewed.

On the left, researchers used a special filtration test to determine how many small particles were blocked with a variety of masks and fabrics. On the right, a piece of cloth lets light shine through easily, signalling a lower quality fabric. Courtesy of Wake Forest Baptist Health

To test various masks and fabrics, the team pumped air through both types of face coverings.

"Our instruments could read down to 0.3 microns, which is about the size of a big virus," Segal said.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Regular surgical masks filtered out 62 percent to 65 percent of particles. For comparison, N95 masks filter 95 percent of those particles.

But the fabrics led to a variety of results. One piece of cloth filtered just 1 percent of particles, rendering it virtually useless, while others were found to perform even better than the surgical masks.

"We had some that performed at 79 percent," Segal told NBC News.

The best masks were constructed of two layers of heavyweight "quilters cotton" with a thread count of at least 180, and had thicker and tighter weave.

Lesser quality fabrics also worked well, as long as they had an internal layer of flannel.

"You do want to use a woven fabric, like batik," Segal said, "but you don't want to use a knit fabric, because the holes between the knit stitches are bigger."

In other words, if the fabric allows for a substantial amount of light to shine through, it's probably going to allow tiny viral particles through, as well.

Other hospitals, such as Vanderbilt University Medical Center in Nashville, have also suggested using tightly woven fabrics for do-it-yourself facial coverings. Kaiser Permanente has also published instructions on DIY masks, also calling for two layers of cotton fabrics.

Cotton T-shirts, however, may not be as useful. A 2013 study published in the journal Disaster Medicine and Public Health Preparedness compared medical grade surgical masks with homemade facial coverings using plain cotton T-shirts. The surgical masks were found to be three times more effective in blocking small particles than the T-shirts.

Not a substitute for social distancing

If you decide to use homemade face coverings, public health officials offered caution. Dr. Deborah Birx, the White House coronavirus task force response coordinator, said masks should be used only as an "additive" to social distancing, not a substitute.

During a news conference Thursday, Birx said people often feel "an artificial sense of protection because they are behind a mask," adding, "Don't get a false sense of security."

Download the NBC News app for full coverage of the coronavirus outbreak

A few tips for best practices when it comes to DIY facial coverings:

Opt for masks that tie around the ear, rather than ones that have a standard elastic band. The ties can be adjusted to fit each face better than the elastic band.

Make sure to use dry masks. When masks get wet, even from the moisture emitted when a person exhales, the fabric could be more likely to transmit virus.

Wash masks regularly, with regular detergent and in regular washing machine cycles.

Segal suggested avoiding "bleach or other harsher chemicals until we know the effect on the fabric's effectiveness."

He added additional studies will be needed to determine whether masks retain their filtering ability after multiple cycles through the washing machine.

Follow NBC HEALTH on Twitter & Facebook.