Small particles that can be airborne are defined as aerosol particles [
10]. Several experimental results support the view that respiratory particles are sufficiently small for airborne transmission of microorganisms inside the particles. In a study by Johnson et al. (2011), it was reported that healthy subjects (8–15 humans) generate various particles—including respiratory droplets—of three size modes during speech and voluntary coughing (1.6, 2.5 and 145 μm, and 1.6, 1.7 and 123 μm, respectively) [
11]. These particles contained very large respiratory droplets with sizes exceeding 100 μm, which fell to the ground within a few seconds. However, in the experimental results, small particles of approximately 2 μm were generated simultaneously [
11] and they could remain airborne for dozens of minutes.
In another study by Lindsley et al. (2012), the sizes of aerosol particles generated by patients (9 subjects) who were infected by the influenza virus were measured [
12]. The size of the generated particles ranged from 0.35 to 9 μm. Among the particles generated by the influenza-infected patients, particles with a size range of 0.35–2.5 μm were of higher number concentration.
...
In a study by Wölfel et al. (2020), it was reported that the average ratio of viruses in the oral fluid of COVID-19 patients was 7.00 × 106 copies per mL and the maximum ratio was 2.35 × 109 copies per mL [
25].
These experimental results can be converted to demonstrate that, on average, 2.67 × 10−7% of a respiratory fluid particle of COVID-19 patients is occupied by SARS-CoV-2 and then the minimum size of a respiratory particle that can contain SARS-CoV-2 is approximately 65 μm.
