Exposure to respiratory droplets contributes greatly to the spread of SARS-CoV-2 virus during the COVID-19 pandemic. A new study from UCLA School of Public Health investigated the effectiveness of various face coverings to reduce cough-generated airborne particle concentrations at 0.3, 0.9, and 1.8 m away from the source in an indoor environment. Researchers measured the particle number concentration (PNC) and particle size distribution under seven different conditions: (1) no face covering; (2) face shield only; (3) cloth mask; (4) face shield + cloth mask; (5) surgical mask; (6) face shield + surgical mask; (7) N95 respirator or equivalent (i.e., KN95 mask).
The researchers found that there were significantly more droplets at 0.3 m under conditions #1-4 and a trend toward an increase at 1.8 m, compared to the background. The face shield by itself provided little protection with a particle reduction of 4% relative to no face covering, while the cloth masks reduced the particles by 77%. Surgical and N95/KN95 masks performed well and substantially reduced the cough droplets to less than 6% at 0.3 m. In this study, most cough-generated particles were less than 2.5 µm diameter with an average diameter of ∼0.6 µm at 0.3 meters away. Approximately 80% of the particles that were smaller than 2.5 µm were able to travel to 0.9 meters away, and 10% of the particles smaller than 1.1 µm likely reached 1.8 m. Based on these results, face coverings, especially surgical and N95/KN95 masks, should be recommended as effective preventive measures to reduce outward transport of respiratory droplets during the COVID-19 pandemic.
- Li, L., Niu M., & Zhu, Y. (2021). Assessing the effectiveness of using various face coverings to mitigate the transport of airborne particles produced by coughing indoors. Aerosol Science and Technology, 55(3), 332-339.