Scientists develop a simple approach to visually evaluate mask effectiveness

Using inexpensive and widely available tools, scientists have developed a simple approach to visually evaluate how effectively different types of masks prevent the spread of droplets that could contain SARS-CoV-2 virus particles, according to a new study. While the authors note their work is still in the early stages, and their method has so far only been tested in a small group of people, these preliminary, proof-of-principle findings suggest that professional-grade N95 masks, surgical or polypropylene masks, and handmade cotton masks may all block much of the spray produced when wearers speak.

Bandanas and neck fleeces, however, likely provide little protection, as the researchers observed that more droplets are expelled through these materials - likely because the materials break up larger droplets as they pass through the material, the researchers speculate. Emma Fischer and colleagues suggest that non-experts could easily build and operate this setup to evaluate masks at the community level, such that mask producers could use the method to optimize mask designs, and educational and community outreach organizations could demonstrate proper mask fitting procedures.

As nations worldwide have instituted mask mandates to curb the spread of COVID-19, global commercial suppliers have encountered shortages in the face of unprecedented demand. To compensate, many people have turned to homemade masks and mask alternatives, but these do-it-yourself versions have not been tested systematically.

To evaluate the effectiveness of 14 different types of masks and other frequently substituted face coverings, Fischer et al. designed a simple approach in which either one male speaker or, in some cases, four speakers wore each mask while standing in a dark enclosure. The speaker would say the phrase "stay healthy, people" five times in the direction of a laser beam, which scattered light from the droplets released during speech. A cell phone camera recorded the droplets and a simple computer algorithm counted them. The setup was intentionally designed to be simple and inexpensive so it could be replicated by non-experts - the hardware it requires, including laser equipment, is commonly available and can be purchased for less than $200. While the researchers acknowledge the need for further testing, Fischer et al.'s findings suggest that N95 masks without valves blocked droplet spread best, but surgical or polypropylene masks and handmade cotton face coverings were also effective. The early findings suggest that bandanas and neck fleeces (like balaclavas), however, likely provide little protection.

Our work was a demonstration of a simple measurement method, not a systematic mask study. More work is required to investigate variations in masks, speakers, and how people wear them. We also want to extend our method to other droplet-generating actions, like coughing and sneezing. Further, we want to explore effects of incorrect placement and moisture saturation."

Martin Fischer, study's corresponding author