Hypothesis: Transmission of COVID-19 Virus through the Air from Asymptomatic Persons by Speaking

The COVID-19 mortality seemed to be higher in high-income/hygienic countries; this is contrary to conventional communicable diseases which usually occur in low-income countries. Another characteristic of COVID-19 is that some proportions of the infected persons had no symptoms but transmitted the virus to others. In this paper, the author proposes a hypothesis that infected but asymptomatic persons produce two kinds of infectious respiratory particles (IRP) by speaking: large droplets and small aerosols. The large droplets are produced from the saliva by pronouncing a plosive, /p/ or /t/; and the small aerosols are produced from the laryngeal exudate by vibration of vocal cords for voicing of vowels. The heavy droplets collide with the face of other persons in conversation, resulting in droplet infection. In contrast, the lighter aerosols float in the air, accumulate inside an airtight room after a longer talking, and cause airborne infection in other persons staying in the same room. Then, the author discusses on the COVID-19 global epidemiology, which may have depended on 1) climate and housing structure, 2) cultural behaviors, and 3) languages among different countries.