What was done was to review the literature on the association of the use of public transport with acute respiratory diseases in general. It started in May and the first thing that was found was that frequent use of public transport is related to a higher risk and chances of presenting respiratory diseases; Based on these findings, some recommendation studies with mathematical modeling were found that also suggest this association.
In general, people's attention has been directed to sneezing or coughing, which have greater droplet dispersal power. However, talking can also emit particles in sufficient quantity to transmit respiratory diseases.
Experimental studies conducted before the pandemic showed that the transmission of respiratory particles can be similar to speaking as it is to coughing. A group of researchers identified that people emit 108 droplets when they cough 20 times or when counting from 1 to 100. Likewise, the amount of fluids collected in surgical masks was 22.9 mg when coughing and 18.7 mg when counting from 1 to 100. When collecting the fluids in plastic bags, they obtained 85 mg when coughing and 79.4 mg when count from 1 to 100, which implies that the mask captured 27% of secretions when coughing and 24% when speaking.
In one study, the velocity and angle of exhaled air were measured during coughing and speaking. It was found that the exhalation speed when speaking was between 22% and the exhalation speed when coughing was 27%, which would imply that the droplets generated when speaking would reach a shorter distance. However, the angle of exhalation is greater when speaking than when coughing, as people tend to lean over when coughing
COVID-19 "Silence Tactic" Not Received Enough Attention - Medscape - Oct 6, 2020.