There is an ongoing debate about whether COVID-19 is transmitted primarily via aerosols or droplets ( 6, 7). Furthermore, relative humidity (RH) and temperature influence the drying and settling of wet particles by gravity when exhaled into the ambient (see refs. Infectious respiratory particles may contain single or multiple copies of pathogens when exhaled by the infectious, and, when inhaled by the susceptible, there is a risk of infection given the absorbed infectious dose ( 3). Vocalization-related respiratory activities, that is, sound pressure, peak airflow frequency, and articulated consonants, strongly influence particle emission ( 4, 5). Concentrations of exhaled particles and their size have been found to strongly depend on the type of respiratory activities, for example, speaking, or singing as compared to breathing. Human respiratory particles greatly vary in composition and size, and span several decades in length scale (e.g., see refs. We conclude that wearing appropriate masks in the community provides excellent protection for others and oneself, and makes social distancing less important. When both wear a surgical mask, while the infectious is speaking, the very conservative upper bound remains below 30% after 1 h, but, when both wear a well-fitting FFP2 mask, it is 0.4%. If only the susceptible wears a face mask with infectious speaking at a distance of 1.5 m, the upper bound drops very significantly that is, with a surgical mask, the upper bound reaches 90% after 30 min, and, with an FFP2 mask, it remains at about 20% even after 1 h. We find, for a typical SARS-CoV-2 viral load and infectious dose, that social distancing alone, even at 3.0 m between two speaking individuals, leads to an upper bound of 90% for risk of infection after a few minutes.

To calculate exposure and infection risk, we use a comprehensive database on respiratory particle size distribution exhalation flow physics leakage from face masks of various types and fits measured on human subjects consideration of ambient particle shrinkage due to evaporation and rehydration, inhalability, and deposition in the susceptible airways. Here, we introduce the concept of an upper bound for one-to-one exposure to infectious human respiratory particles and apply it to SARS-CoV-2. However, due to the complexity of airborne disease transmission, it is difficult to quantify their effectiveness, especially in the case of one-to-one exposure. There is ample evidence that masking and social distancing are effective in reducing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission.

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