  Pathogens like the SARS-CoV-2 are transmitted not only through violent
expiratory events like coughing, but also through routine activities like
breathing/speaking/singing. We perform direct numerical simulations of the
turbulent transport of potentially infectious aerosols in short conversations.
It is shown that a two-way conversation significantly reduces the aerosol
exposure compared to a relative monologue by one person and relative silence of
the other. This is because the interaction of the jets ejected from the mouth
of each speaker produce a "canceling" effect. Unequal conversation is shown to
significantly increase the risk of infection to the person who talks less.
Interestingly, a small height difference is worse for infection spread, due to
reduced interference between the two speech jets, than two faces at the same
level! For small axial separation, speech jets show large oscillations and
reach the other person intermittently. We suggest a range of lateral
separations between two people to minimize transmission risk. A realistic
estimate of the infection probability is provided by including exposure through
eyes and mouth, in addition to the more common method of using inhaled virions
alone. We expect that our results will provide useful inputs to epidemiological
models and to disease management.
