What’s the most effective face mask to prevent coronavirus?

Face it, masks are now front and center in the country’s united fight against coronavirus, as evidenced by Walmart’s new mandate that all customers cover up.

Beginning July 20, shoppers won’t be allowed to enter the nation’s number one retailer without wearing a face covering. The superstore chain — with more than 4,500 stores nationwide — joins Costco, Starbucks, and a number of other business behemoths that have enacted similar mask rules.

“While we’re certainly not the first business to require face coverings, we know this is a simple step everyone can take for their safety and the safety of others in our facilities,” Walmart Chief Operating Officer Dacona Smith shared in a release Wednesday.

New data continues to emerge about which masks are most effective at keeping COVID-19 from spreading. The Herald asked medical experts to weigh in on the different options available for residents in Massachusetts, and nationwide:

Respirators like N95 masks are the gold standard of personal protection, but at this stage in the pandemic, they remain in short supply for front-line workers in many areas.

“They’re very effective at preventing inhalation of viruses, but I think most of the time, that’s probably overkill,” said Dr. David Hamer, professor of global health and medicine at Boston University and a physician at Boston Medical Center.

That means in most cases, N95s should be treated as medical resources reserved for hospital staff. The Herald has previously reported on the differences between available respirators.

Surgical Masks have become more widely available across the commonwealth, but present some issues for use in the COVID-19 pandemic. Like their name implies, they’re made for surgery — not daily use. That means that they’re disposable and slightly flimsy. As they begin to wear out, they lose their effectiveness.

“You definitely don’t want any mask or face covering to be wet, soiled, or torn, because that does affect its function,” said Dr. Shira Doron, Infectious Disease Physician and Hospital Epidemiologist at Tufts Medical Center.

They also can’t be washed, making them less environmentally-friendly than some other options.

Cloth Masks are an excellent choice for the general public, Dr. Hamer and Dr. Doron agree.

An experiment conducted in June by the National Institute of Health found that wearing a “face mask stitched using two-layers of cotton quilting fabric” limited the distance germs from a cough could travel to just 2.5 inches. Without any mask, a person coughing could spread their germs up to 8 feet away.

When it comes to choosing a cloth mask, fabric and fit are key.

A recent study from the Center for Disease Control concluded “current evidence suggests that multilayered masks with water-resistant fabric, high number of threads, and finer weave may be more protective.” So consider picking out a mask that uses a few different kinds of cloth to filter virus particles.

A cloth mask should sit snugly against a person’s face without gaps on the sides of the covering.

“You want it to fit so you’re breathing through the mask, and not the edges of the mask,” said Hamer.

Cloth masks are also best for exercising, Hamer said, because they can wick away moisture and keep the covering effective.

Bandanas and other single-layer cloth coverings should really only be used as a last resort, experts say.

“They’re a looser fit, and if it’s a single-ply fabric, it’s going to have less filtration capacity,”  Hamer said. “Something like a bandana is more for show than personal protection.

Bandanas proved to limit the spread of germs from a cough or sneeze to a 3-foot, 7-inch spray — much less effective than cloth masks.

As the CDC says, “we are not defenseless” against coronavirus — if you don a mask.