Hypothesis: Transmission of COVID-19 through the Air from Asymptomatic Individuals by Speaking.

COVID-19 mortality seems to be higher in high-income, hygienic countries, but this is contrary to conventional communicable diseases, which usually occur in low-income countries. Another characteristic of COVID-19 is that a proportion of infected individuals had no symptoms but transmitted the virus to others. In this paper, the author proposes a hypothesis that infected but asymptomatic individuals produce two kinds of infectious respiratory particles by speaking: large droplets and small aerosols. The large droplets are produced from the saliva by pronouncing a plosive, /p/ or /t/, whereas the small aerosols are produced from the laryngeal secretion by the vibration of vocal cords during the voicing of vowels. The heavy droplets collide with other people's faces in conversation, resulting in droplet infection. In contrast, the lighter aerosols float in the air, accumulate inside a non-ventilated room after prolonged conversation, and cause airborne infection in other people in the same room. Then, the author discusses the global epidemiology of COVID-19, which may have depended on 1) climate and housing structure, 2) cultural behaviors, and 3) languages among different countries.