Federal health officials have now recommended that we cover our faces with fabric during the coronavirus pandemic. But what material offers the most protection?

The Centers for Disease Control and Prevention has posted a no-sew mask pattern using a bandanna and a coffee filter as well as a video on making masks using rubber bands and folded fabrics found at home.

While a simple face covering can reduce the spread of coronavirus by blocking outgoing germs from coughs or sneezes of an infected person, experts say there is more variation in how much homemade masks might protect the wearer from incoming germs, depending on the fit and quality of the material used.

Scientists around the country have taken it upon themselves to identify everyday materials that do a better job of filtering microscopic particles. In recent tests, HEPA furnace filters scored well, as did vacuum cleaner bags, layers of 600-count pillowcases and fabric similar to flannel pajamas. Stacked coffee filters had medium scores. Scarves and bandanna material had the lowest scores, but still captured a small percentage of particles.

If you don’t have any of the materials that were tested, a simple light test can help you decide whether a fabric is a good candidate for a mask.

“Hold it up to a bright light,” said Dr Scott Segal, chairman of anaesthesiology at Wake Forest Baptist Health who recently studied homemade masks. “If light passes really easily through the fibres and you can almost see the fibers, it’s not a good fabric. If it’s a denser weave of thicker material and light doesn’t pass through it as much, that’s the material you want to use.”

Researchers say it’s important to remember that lab studies are conducted under perfect conditions with no leaks or gaps in the mask, but the test methods give us a way to compare materials. And while the degree of filtration for some homemade masks seems low, most of us — who are staying home and practicing social distancing in public — don’t need the high level of protection required for medical workers. More important, any face covering is better than none, especially if worn by a person who has the virus but doesn’t know it.

The biggest challenge of choosing a homemade mask material is to find a fabric that is dense enough to capture viral particles, but breathable enough that we can actually wear it. Some items being touted online promise high filtration scores, but the material would be unwearable.

Yang Wang, an assistant professor of environmental engineering at Missouri University of Science and Technology, worked with his graduate students to study various combinations of layered materials — including both air filters and fabric. “You need something that is efficient for removing particles, but you also need to breathe,” said Dr Wang, who last fall won an international award for aerosol research.

To test everyday materials, scientists are using methods similar to those used to test medical masks, which everybody agrees should be saved for medical workers.

The best medical mask — called the N95 respirator — filters out at least 95 per cent of particles as small as 0.3 microns. By comparison, a typical surgical mask — made using a rectangular piece of pleated fabric with elastic ear loops — has a filtration efficiency ranging from 60 to 80 per cent.

Dr Wang’s group tested two types of air filters. An allergy-reduction HVAC filter worked the best, capturing 89 percent of particles with one layer and 94 per cent with two layers. A furnace filter captured 75 per cent with two layers, but required six layers to achieve 95 per cent. To find a filter similar to those tested, look for a minimum efficiency reporting value rating of 12 or higher.

The problem with air filters is that they potentially could shed small fibres that would be risky to inhale. So if you want to use a filter, you need to sandwich the filter between two layers of cotton fabric.