New research suggests homemade masks combining cotton with natural silk or chiffon offer the best protection against virus-carrying aerosol particles.

Though public health experts offered conflicting guidance on mask-wearing at the outset of the COVID-19 pandemic, both the U.S. Centers for Disease Control and Prevention and the World Health Organization are now recommending that people wear masks when out in public.

With N95 masks still in short supply, officials recommend the masks be reserved for healthcare professionals and others on the front-lines of the battle against the coronavirus.

Those without an N95 mask must opt for an alternative, whether bought online or made at home. To see which materials best filter out tiny airborne particles called aerosols, scientists set up controlled experiments in the lab, the results of which were published Friday in the journal ACS Nano.

Using an aerosol mixing chamber, scientists were able to generate particles ranging in diameter from 10 nanometers to 6 micrometers. A fan in the chamber blew the aerosols across various cloth samples.

"We tried to keep the flow rates of air (measured in cubic feet per minute), about the same as the respiratory rates during inhalation at rest and we have kept the fabric areas approximately the same as the mask area," study co-author Supratik Guha, molecular engineer at the University of Chicago, told UPI in an email.

Particle counters revealed how many particles made it across the different combinations of fabrics. The combination of a tightly woven cotton sheet and two layers of polyester-spandex chiffon, a sheer fabric used to make evening gowns, filtered out between 80 and 99 percent of aerosol particles, depending on the size. Filters using cotton combined with natural silk or flannel produced similarly impressive results.

"In our measurements they performed very well, and close to the N95s," Guha said of the cotton-chiffon masks. "However keep in mind that the N95s have been thoroughly tested over the past several decades. The cloth hybrid masks we are investigating need further examination in terms of their performance when there is humidity, say when humidity builds up on the inside."

What's so special about cotton and chiffon?

"We believe one provides more physical filtering, the other provides more electrostatic filtering -- particles bind to it driven by electrical attraction," Guha said.

Researchers plan to conduct followup tests to determine how filtering performance of different fabrics is affected by humidity and washing. Scientists are also keen to see if different designs can achieve better fits.

"Keep in mind that 'fit' is critically important," Guha said. "You can have the best filtration material but if the fit is not good, air will leak around and effectiveness will be severely compromised."

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