Human expiration by means of breathing, speaking, sneezing, or coughing produces a range of droplets which serve as one of the primary ways of airborne disease transmission, including COVID-19. Micron size droplets or droplet nuclei have been shown to be effective carriers of the virions as they follow the air flow, can travel over the long distances, and accumulate in indoor spaces. A direct exposure to a high concentration of such virus-laden droplets exhaled by an infected host, either during face-to-face conversation or through the accumulation of the droplets suspended in the indoor environment, poses a serious threat to the occupants. To mitigate these risks, face masks and ventilation are often employed for controlling and containing the aerosol concentrations in the indoor space.

This project, motivated by the COVID-19 pandemic, aims to enhance our understanding of aerosol production, accumulation, and control. The recently completed series of experiments give quantitative measures for the effectiveness of both face masks and ventilation on the dispersion and accumulation of the aerosol droplets. Flow visualization (Video 1) and velocity measurement using Particle Image Velocimetry (Video 2) conducted in the immediate vicinity of a manikin, which simulates the breathing of a COVID-19 infected host, illustrate that all masks considered in the study both filter and redirect the exhaled aerosol flow through the leakage points around the mask, thereby protecting the person directly in front of the host. In addition, the videos show that the filtered flow coming out through the mask has a very low velocity which confirms that masks provide significant benefits in close proximity, particularly in face-to-face conversations.

Video 1: Exhalation of aerosol-laden flow in (a) no-mask, (b) KN95, and (c) surgical (blue) mask

Video 2: Phase-averaged exhalation velocity field through an exhaust cycle (a) no-mask, (b) KN95, and (c) surgical (blue) mask

Understanding of the accumulation of the aerosol in a large room is critical for long-term occupancy of indoor spaces, for which the study considered particle dispersion measurements at the recommended social distancing guideline of 2 m from the manikin via high resolution imaging and particle identification techniques. It was shown that the most widespread 3-ply cloth and disposable (blue) masks filtered only about 10% of the exhaled aerosol droplets as a result of the relatively large openings at the bridge of the nose and the cheeks. In contrast, higher efficiency masks such as N95 and KN95 filtered approximately 60% and 46% of the exhaled droplets, respectively, due to both better filtration efficiency of the material and a better fit to the face. However, a loosely fit high-efficiency respirator, such as KN95, was shown to have the lowest filtration efficiency for the exhaled flow, illustrating the need to ensure proper fit to the face, particularly for high-efficiency masks.

Furthermore, application of ventilation/air-purification using HEPA filters provided substantial improvements in the control of aerosol accumulation that were greater than the best performing mask in an unventilated scenario. In equivalent efficiency terms, the ventilation rates considered between 1.7 ACH (air exchanges per hour) and 3.2 ACH decreased the aerosol accumulation in the range 70% to 85% at the two-meter social distance from a single host.