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.

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.

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 The research from Wake Forest has not been submitted for publication and has not been peer-reviewed.

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

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.

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.

“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.

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.”

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.