Duke physician Eric Westman was an early champion of using masks to curtail the spread of coronavirus, working with a local non-profit in North Carolina to provide free masks to at-risk and under-served populations. He needed to know, however, if the virus-blocking claims that mask suppliers made were correct. He turned to colleagues in the Duke Department of Physics for assistance.

Martin Fischer, Ph.D., a chemist and physicist, stepped up. As director of the Advanced Light Imaging and Spectroscopy facility, he normally focuses on exploring new optical contrast mechanisms for molecular imaging, but for this task, he put together a relatively inexpensive apparatus from common lab materials. The setup consisted of a box, a laser, a lens, and a cell phone camera.

In a proof-of-concept study, Fischer, Westman, and their colleagues report that the simple, low-cost technique provided visual proof that face masks are effective in reducing droplet emissions during normal wear. “We confirmed that when people speak, small droplets get expelled, so disease can be spread by talking, without coughing or sneezing,” Fischer said. “We could also see that some face coverings performed much better than others in blocking expelled particles.”

Notably, the researchers report, the best face coverings were N95 masks without valves – the hospital-grade coverings that are used by front-line health care workers. Surgical or polypropylene masks also performed well. But hand-made cotton face coverings provided good coverage, eliminating a substantial amount of spray from normal speech.

On the other hand, bandanas and neck fleeces, such as balaclavas, didn’t block the droplets much at all.