Simple Method to Test the Efficacy of Face Masks

The COVID-19 pandemic has led to recommendations or requirements for the use of face masks in many public places. Masks can help to reduce the spread of potentially infectious respiratory droplets that are created when people sneeze, cough, or speak. Medical-grade masks are well-studied, but often in limited supply. This has led to the widespread use of textile masks and other face coverings, such as neck fleeces or bandanas. However, the effectiveness of these masks, which vary widely in materials, layering, and form factor, is generally unknown.

Martin C. Fischer, Duke University, Durham, NC, USA, and colleagues have developed a simple, low-cost setup to test the droplet retention efficacy of different types of face masks. A test subject speaks through a face mask into a box through which a laser beam shines. The emitted droplets scatter the laser light, which is recorded with a cell phone camera. A simple program then counts the droplets in the recorded video to estimate the efficacy of the mask. Besides camera phones, which are widely available, the required hardware is fairly low-cost: suitable lasers and optical components can be purchased for less than USD 200. The setup is easy to build and operate, according to the researchers.

To demonstrate the usefulness of the test method, the team evaluated 14 commonly available masks or masks alternatives and compared them with a professionally fit-tested, valveless N95-type mask and with a control without a mask. They recorded video before, during, and after speaking, with a total length of 40 s for each test, and repeated this multiple times for each mask.

The team found that, unsurprisingly, the valveless N95 mask was most effective, closely followed by a surgical mask. Several types of homemade cloth masks also performed very well. N95 masks with a valve performed worse than some homemade textile masks due to the exhalation valve, which can let through droplets, and thus, decrease the protection of persons around the wearer. Bandanas and neck fleeces provide very little protection. Interestingly, neck fleeces caused higher droplet counts than the no-mask control. The team assumes this is due to the dispersal of larger droplets into many smaller droplets. Such smaller droplets hang around in the air for longer, which might contribute to virus spread.